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Soil Microbial EcologySoil microbial ecology is a fascinating field that delves into the intricate relationships between microorganisms and their environment. These tiny organisms play a crucial role in nutrient cycling, decomposition, and overall soil health. One of the key aspects of soil microbial ecology is understanding the diversity and abundance of microbial communities in different soil types and how they are influenced by various factors such as soil pH, moisture levels, and nutrient availability. Microorganisms in the soil are incredibly diverse, with thousands of different species coexisting in a single gram of soil. These microorganisms include bacteria, fungi, archaea, and protozoa, each playing a unique role in the soil ecosystem. For example, bacteria are important for breaking down organic matter and releasing nutrients for plant uptake, while fungi are crucial for decomposing complex organic compounds like lignin and cellulose. The composition of soil microbial communities can vary greatly depending on environmental conditions. For example, in acidic soils, acidophilic microorganisms thrive, while alkaline soils may support a different set of microbial species. Additionally, human activities such as agriculture and land use change can also impact soil microbial communities. Pesticides, fertilizers, and other chemicals can alter the composition of soil microorganisms, leading to potential imbalances in the soil ecosystem. Understanding soil microbial ecology is essential for sustainable agriculture and environmental management. By studying the interactions between microorganisms and their environment, researchers can develop strategies to improve soil health and fertility. For example, using organic farming practices that promote the growth of beneficial soil microorganisms can help reduce the reliance on synthetic fertilizers and pesticides, leading to healthier soils and crops. Moreover, soil microbial ecology plays a crucial role in climate change mitigation. Soil microorganisms are involved in processes such as carbon sequestration and greenhouse gas emissions. By studying how different microbial communities respond to changes in temperature and precipitation, scientists can better predict the impact of climate change on soil ecosystems and develop strategies to mitigate its effects. In conclusion, soil microbial ecology is a complex and dynamic field that offers valuable insights into the functioning ofsoil ecosystems. By studying the diversity and abundance of soil microorganisms and their interactions with the environment, researchers can improve soil health, enhance agricultural productivity, and mitigate the effects of climate change. It is essential to continue researching and understanding soil microbial ecology to ensure the sustainability of our planet's ecosystems for future generations.。
Microbial ecology of aquatic systems The microbial ecology of aquatic systems is a fascinating and complex field that plays a crucial role in the health and functioning of these environments. Aquatic systems, including oceans, rivers, lakes, and wetlands, are home to a diverse array of microorganisms that contribute to nutrient cycling, food webs, and the overall stability of these ecosystems. Understanding the interactions and dynamics of microbial communities in aquatic systems is essential for conservation and management efforts, as well as for gaining insights into the broader implications for global biogeochemical cycles. One of the key aspects of microbial ecology in aquatic systems is the diversity of microorganisms present. These microorganisms, including bacteria, archaea, fungi, and protists, exhibit a wide range of metabolic capabilities and play various roles in the ecosystem. For example, some microorganisms are responsible for the breakdown of organic matter, while others are involved in nitrogen fixation or sulfur cycling. The diversity of microbial communities in aquatic systems is influenced by factors such as water chemistry, temperature, and the availability of nutrients, and understanding these dynamics is essential for predicting how these ecosystems will respond to environmental changes. Another important aspect of microbial ecology in aquatic systems is the interactions between microorganisms and other organisms in the ecosystem. For example, microorganisms form symbiotic relationships with plants and animals, providing essential nutrients or aiding in the digestion of food. Additionally, microorganisms can also compete with each other for resources, leading to complex and dynamic interactions within the microbial community. These interactions have implications for the overall functioning and stability of aquatic ecosystems, and studying them can provide valuable insights into how these systems respond to disturbances or perturbations. Furthermore, the role of microorganisms in nutrient cycling and biogeochemical processes in aquatic systems cannot be understated. Microorganisms are key players in processes such as the decomposition of organic matter, the cycling of carbon and nitrogen, and the removal of pollutants from the environment. Understanding the factors that regulate these processes, as well as the resilience of microbial communities to environmental changes, is essential for predicting the responses of aquaticecosystems to human activities such as pollution and climate change. In addition to their ecological roles, microorganisms in aquatic systems also have practical implications for human society. For example, some microorganisms are used in bioremediation efforts to clean up contaminated water bodies, while others are utilized in the production of biofuels or pharmaceuticals. Understanding the diversity and functions of microbial communities in aquatic systems can thus have direct applications for human well-being and sustainable development. In conclusion, the microbial ecology of aquatic systems is a complex and dynamic field with far-reaching implications for ecosystem functioning, biogeochemical cycling, and human well-being. By studying the diversity, interactions, and functions of microorganisms in these environments, researchers can gain valuable insights into the resilience and responses of aquatic ecosystems to environmental changes. This knowledge is essential for informing conservation and management efforts, as well as for developing innovative solutions to environmental challenges.。
Soil Microbial Biogeography Soil Microbial Biogeography Soil microbial biogeography refers to the studyof the spatial distribution of microbial communities in soil ecosystems. Thisfield of research has gained increasing attention in recent years due to its importance in understanding the ecological and functional roles of soil microbes. In this essay, we will explore the concept of soil microbial biogeography, its significance, and the methods used to study it. To begin with, soil microbial biogeography is crucial for understanding the diversity and distribution of soil microbial communities. Microorganisms play a vital role in nutrient cycling, decomposition, and soil formation, making them essential for ecosystem functioning. By studying the biogeography of soil microbes, scientists can gain insights into the factors that shape microbial community composition, such as soil type, climate, and land use practices. Furthermore, soil microbial biogeography has significant implications for agriculture and environmental management. Understanding thespatial distribution of beneficial and pathogenic microbes in soil can help improve agricultural practices, such as the selection of microbial inoculants for crop production or the management of soil-borne diseases. Additionally, knowledge of soil microbial biogeography can inform conservation efforts and land use planning to protect microbial diversity and ecosystem services. In order to study soil microbial biogeography, researchers employ various molecular andbioinformatic techniques. These include high-throughput sequencing of microbial DNA, metagenomic analysis, and the use of geographic information systems (GIS) to map microbial diversity across landscapes. These methods allow scientists to characterize microbial communities at different spatial scales, from local to global, and to identify patterns of microbial diversity and distribution. In conclusion, soil microbial biogeography is a critical area of research that provides valuable insights into the spatial distribution and diversity of soil microbial communities. This knowledge has important implications for ecosystem functioning, agriculture, and environmental management. By employing advanced molecular and bioinformatic techniques, scientists are able to unravel the complexities of soil microbial biogeography and its role in shaping soil ecosystems. As our understanding of soil microbial biogeography continues toadvance, we can expect to gain new perspectives on the ecological and functional significance of soil microbes in the future.。
Soil Microbial Biogeochemistry Soil microbial biogeochemistry is a fascinating field that explores the intricate relationships between microorganisms, soil, and the environment. These tiny organisms play a crucial role in nutrient cycling, decomposition, and overall soil health. Their activities have a profound impact on the ecosystem as a whole, influencing plant growth, carbon storage, and greenhouse gas emissions. One of the key aspects of soil microbial biogeochemistry is the diversity of microbial communities present in the soil. These communities consist of bacteria, fungi, archaea, and other microorganisms, each with their own unique functions and roles. The interactions between these different groups of microorganisms are complex and dynamic, shaping the overall biogeochemical processes occurring in the soil. Microbial activity in the soil is influenced by a variety of factors, including temperature, moisture, pH, and nutrient availability. Changes in these environmental conditions can have significant effects on microbial communities and their functions. For example, an increase in temperature may lead to changes in microbial diversity and activity, impacting nutrient cycling and soil fertility. The study of soil microbial biogeochemistry is essential for understanding and managing soil health and fertility. By unraveling the intricate processes occurring in the soil microbiome, researchers can develop strategies to enhance soil productivity, mitigate climate change, and promote sustainable agriculture practices. This knowledge can also help in the development of novel biotechnologies for soil remediation and environmental conservation. However, despite the importance of soil microbial biogeochemistry, this field faces numerous challenges and limitations. One of the major hurdles is the complexity of microbial communities and their interactions, making it difficult to fully understand and predict their responses to environmental changes. Additionally, the lack of standardized methodologies and tools for studying soil microbiomes hinders progress in this field. As a researcher in soil microbial biogeochemistry, I am constantly amazed by the diversity and resilience of microorganisms in the soil. Their ability to adapt to changing environmental conditions and perform essential functions for ecosystem health is truly remarkable. I am passionate about unraveling the mysteries of the soil microbiome and contributing to thedevelopment of sustainable solutions for soil management and environmental conservation. In conclusion, soil microbial biogeochemistry is a dynamic and complex field that holds great potential for advancing our understanding of soil ecosystems and their interactions with the environment. By studying the diverse and intricate relationships between microorganisms and soil, researchers can unlock new insights into nutrient cycling, carbon sequestration, and ecosystem resilience. This knowledge is essential for developing sustainable agricultural practices, combating climate change, and preserving the health of our planet for future generations.。
托福阅读TPO20(试题+答案+译文)第3篇:FossilPreservationTPO是我们常用的托福模考工具，对我们的备考很有价值，下面小编给大家带来托福阅读TPO20(试题+答案+译文)第3篇:Fossil Preservation。
托福阅读原文【1】When one considers the many ways by which organisms are completely destroyed after death, it is remarkable that fossils are as common as they are. Attack by scavengers and bacteria, chemical decay, and destruction by erosion and other geologic agencies make the odds against preservation very high. However, the chances of escaping complete destruction are vastly improved if the organism happens to have a mineralized skeleton and dies in a place where it can be quickly buried by sediment. Both of these conditions are often found on the ocean floors, where shelled invertebrates (organisms without spines) flourish and are covered by the continuous rain of sedimentary particles. Although most fossils are found in marine sedimentary rocks, they also are found in terrestrial deposits left by streams and lakes. On occasion, animals and plants have been preserved after becoming immersed in tar or quicksand, trapped in ice or lava flows, or engulfed by rapid falls of volcanic ash.【2】The term "fossil" often implies petrifaction, literally a transformation into stone. After the death of an organism, the soft tissue is ordinarily consumed by scavengers and bacteria. The empty shell of a snail or clam may be left behind, and if it is sufficiently durable and resistant to dissolution, it may remain basically unchanged for a long period of time. Indeed, unaltered shells of marine invertebrates are known from deposits over 100million years old. In many marine creatures, however, the skeleton is composed of a mineral variety of calcium carbonate called aragonite. Although aragonite has the same composition as the more familiar mineral known as calcite, it has a different crystal form, is relatively unstable, and in time changes to the more stable calcite.【3】Many other processes may alter the shell of a clam or snail and enhance its chances for preservation. Water containing dissolved silica, calcium carbonate, or iron may circulate through the enclosing sediment and be deposited in cavities such as marrow cavities and canals in bone once occupied by blood vessels and nerves. In such cases, the original composition of the bone or shell remains, but the fossil is made harder and more durable. This addition of a chemically precipitated substance into pore spaces is termed "permineralization."【4】Petrifaction may also involve a simultaneous exchange of the original substance of a dead plant or animal with mineral matter of a different composition. This process is termed " replacement" because solutions have dissolved the original material and replaced it with an equal volume of the new substance. Replacement can be a marvelously precise process, so that details of shell ornamentation, tree rings in wood, and delicate structures in bone are accurately preserved.【5】Another type of fossilization, known as carbonization, occurs when soft tissues are preserved as thin films of carbon. Leaves and tissue of soft-bodied organisms such as jellyfish or worms may accumulate, become buried and compressed, and lose their volatile constituents. The carbon often remains behind as a blackened silhouette.【6】Although it is certainly true that the possession of hardparts enhances the prospect of preservation, organisms having soft tissues and organs are also occasionally preserved. Insects and even small invertebrates have been found preserved in the hardened resins of conifers and certain other trees. X-ray examination of thin slabs of rock sometimes reveals the ghostly outlines of tentacles, digestive tracts, and visual organs of a variety of marine creatures. Soft parts, including skin, hair, and viscera of ice age mammoths, have been preserved in frozen soil or in the oozing tar of oil seeps.【7】The probability that actual remains of soft tissue will be preserved is improved if the organism dies in an environment of rapid deposition and oxygen deprivation. Under such conditions, the destructive effects of bacteria are diminished. The Middle Eocene Messel Shale (from about 48 million years ago) of Germany accumulated in such an environment. The shale was deposited in an oxygen-deficient lake where lethal gases sometimes bubbled up and killed animals. Their remains accumulated on the floor of the lake and were then covered by clay and silt. Among the superbly preserved Messel fossils are insects with iridescent exoskeletons (hard outer coverings), frogs with skin and blood vessels intact, and even entire small mammals with preserved fur and soft tissue.托福阅读试题1.The word "agencies" in the passage (paragraph 1) is closest in meaning tobinations.B.problems.C.forces.D.changes.2.In paragraph 1, what is the author's purpose in providingexamples of how organisms are destroyed?A.To emphasize how surprising it is that so many fossils exist.B.To introduce a new geologic theory of fossil preservation.C.To explain why the fossil record until now has remained incomplete.D.To compare how fossils form on land and in water.3.The word "terrestrial" in the passage (paragraph 1) is closest in meaning tond.B.protected.C.alternative.D.similar.4.Which of the sentences below best expresses the essential information in the highlighted sentence in the passage (paragraph 2)? Incorrect choices change the meaning in important ways or leave out essential information.A.When snail or clam shells are left behind, they must be empty in order to remain durable and resist dissolution.B.Although snail and clam shells are durable and resist dissolving, over time they slowly begin to change.C.Although the soft parts of snails or clams dissolve quickly, their hard shells resist dissolution for a long time.D.Empty snail or clam shells that are strong enough not to dissolve may stay in their original state for a long time.5.Why does the author mention "aragonite" in the passage (paragraph 2)?A.To emphasize that some fossils remain unaltered for millions of years.B.To contrast fossil formation in organisms with soft tissue and in organisms with hard shells.C.To explain that some marine organisms must undergo chemical changes in order to fossilize.D.To explain why fossil shells are more likely to survive than are fossil skeletons.6.The word "enhance" in the passage (paragraph 3) is closest in meaning toA.control.B.limit.bine.D.increase.7.Which of the following best explains the process of permineralization mentioned in paragraph 3?A.Water containing calcium carbonate circulates through a shell and deposits sediment.B.Liquid containing chemicals hardens an already existing fossil structure.C.Water passes through sediment surrounding a fossil and removes its chemical content.D.A chemical substance enters a fossil and changes its shape.8.The word "precise" in the passage (paragraph 4) is closest in meaning toplex.B.quick.C.exact.D.reliable.9.Paragraph 5 suggests which of the following about the carbonization process?A.It is completed soon after an organism dies.B.It does not occur in hard-shell organisms.C.It sometimes allows soft-tissued organisms to bepreserved with all their parts.D.It is a more precise process of preservation than is replacement.10.The word "prospect" in the passage (paragraph 6) is closest in meaning topletion.B.variety.C.possibility.D.speed.11.According to paragraph 7, how do environments containing oxygen affect fossil preservation?A.They increase the probability that soft-tissued organisms will become fossils.B.They lead to more bacteria production.C.They slow the rate at which clay and silt are deposited.D.They reduce the chance that animal remains will be preserved.12.According to paragraph 7， all of the following assist in fossil preservation EXCEPTA.the presence of calcite in an organism's skeleton.B.the presence of large open areas along an ocean floor.C.the deposition of a fossil in sticky substances such as sap or tar.D.the rapid burial of an organism under layers of silt.13. Look at the four squares [■] that indicate where the following sentence can be added to the passage. Where would the sentence best fit? Click on a square [■] to insert the sentence in the passage. But the evidence of past organic life is not limited to petrifaction. ■【A】Another type of fossilization, known as carbonization, occurs when soft tissues are preserved as thinfilms of carbon. ■【B】Leaves and tissue of soft-bodied organisms such as jellyfish or worms may accumulate, become buried and compressed, and lose their vola tile constituents. ■【C】The carbon often remains behind as a blackened silhouette.■【D】14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. The remains of ancient life are amazingly well preserved in the form of fossils.A.Environmental characteristics like those present on ocean floors increase the likelihood that plant and animal fossils will occur.B.Fossils are more likely to be preserved in shale deposits than in deposits of clay and silt.C.The shells of organisms can be preserved by processes of chemical precipitation or mineral exchange.D.Freezing enables the soft parts of organisms to survive longer than the hard parts.paratively few fossils are found in the terrestrial deposits of streams and lakes.F.Thin films of carbon may remain as an indication of soft tissue or actual tissue may be preserved if exposure to bacteria is limited.托福阅读答案1.agency代理，中介，作用，所以答案是force，选C。
Soil Microbial Diversity Soil microbial diversity is a critical component of healthy soil ecosystems, playing a crucial role in nutrient cycling, soil structure, and overall ecosystem functioning. The diversity of soil microorganisms, including bacteria, fungi, archaea, and other microbes, is essential for maintaining soil health and productivity. However, various factors, such as land use changes, agricultural practices, and climate change, can have a significant impact on soil microbial diversity, leading to potential negative consequences for soil health and ecosystem functioning. One of the primary reasons why soil microbial diversity is essential is its role in nutrient cycling. Soil microorganisms are responsible for decomposing organic matter and recycling nutrients, making them available forplant uptake. This process is crucial for maintaining soil fertility and productivity, as it ensures that essential nutrients are continuously availablefor plant growth. Additionally, soil microbial diversity also contributes to the stability of soil aggregates, which is essential for maintaining soil structureand preventing erosion. Furthermore, soil microbial diversity plays a crucialrole in supporting plant health and resilience. Certain soil microorganisms form symbiotic relationships with plants, such as mycorrhizal fungi, which help improve nutrient uptake and enhance plant tolerance to environmental stress. Additionally, soil microbes can also suppress plant pathogens and diseases, contributing to overall plant health and productivity. Therefore, a diverse soil microbial community is essential for supporting healthy and resilient plant communities, which is particularly important in agricultural and natural ecosystems. Inaddition to supporting plant health, soil microbial diversity also contributes to overall ecosystem functioning. Soil microorganisms play a crucial role inregulating greenhouse gas emissions, such as carbon dioxide and methane, through processes like carbon sequestration and methane oxidation. Moreover, soilmicrobial diversity also contributes to the degradation of pollutants and contaminants, helping to maintain soil quality and environmental health. Therefore, preserving soil microbial diversity is essential for mitigating climate change and protecting overall environmental quality. However, soil microbial diversity is increasingly threatened by various human activities and environmental changes. Forexample, intensive agricultural practices, such as monocropping and excessive use of chemical fertilizers and pesticides, can lead to a loss of soil microbial diversity. These practices can disrupt the natural balance of soil microorganisms, leading to a decrease in beneficial microbes and an increase in harmful pathogens. Similarly, land use changes, such as deforestation and urbanization, can also have a significant impact on soil microbial diversity, leading to the loss of important microbial communities and functions. Climate change is another significant threat to soil microbial diversity. Changes in temperature and precipitation patterns can directly impact the composition and activity of soil microorganisms, potentially leading to shifts in microbial communities and functions. Additionally, extreme weather events, such as droughts and floods, can also disrupt soil microbial diversity, leading to potential negative consequences for soil health and ecosystem functioning. Therefore, addressing climate change and implementing sustainable land management practices are essential for preserving soil microbial diversity. In conclusion, soil microbial diversity is a critical component of healthy soil ecosystems, playing a crucial role in nutrient cycling, soil structure, and overall ecosystem functioning. Preserving soil microbial diversity is essential for maintaining soil health and productivity, supporting plant health and resilience, and protecting overall environmental quality. However, various factors, such as land use changes, agricultural practices, and climate change, pose significant threats to soil microbial diversity. Therefore, it is essential to implement sustainable land management practices and address climate change to preserve soil microbial diversity and ensure the long-term health and productivity of soil ecosystems.。
Microbial Community Structure Microbial community structure refers to the composition and organization of microorganisms within a particular environment. This structure is a keydeterminant of the functions and dynamics of microbial communities, impacting various ecological processes such as nutrient cycling, carbon sequestration, and disease resistance. Understanding microbial community structure is essential for numerous fields, including environmental science, agriculture, medicine, and biotechnology. In this discussion, we will explore the significance of microbial community structure, the factors influencing it, and the methods used to study and manipulate it. The significance of microbial community structure lies in its pivotal role in maintaining ecosystem stability and functioning. Microorganismsare ubiquitous and diverse, existing in various habitats ranging from soil and water to the human body. The interactions and relationships among different microbial species within a community shape its structure and determine its overall impact on the environment. For example, in soil ecosystems, microbial communities play a crucial role in nutrient cycling, decomposition of organic matter, and soil fertility. In the human gut, the composition of microbial communities has been linked to host health, metabolism, and immune function. Therefore, studying microbial community structure is essential for understanding and harnessing the potential of microbial communities in diverse applications. Several factors influence microbial community structure, including environmental conditions, resource availability, and microbial interactions. Environmental factors such as pH, temperature, and oxygen levels can significantly impact the composition and diversity of microbial communities. For instance, acidic soils may harbordifferent microbial species compared to alkaline soils. Additionally, the availability of resources such as carbon, nitrogen, and energy sources can shape the competitive interactions among microorganisms, influencing community structure. Microbial interactions, including competition, predation, and mutualism, also play a crucial role in shaping community structure by affecting the relative abundance of different microbial taxa. Studying microbial community structure involves the use of various techniques and approaches, including high-throughput sequencing, metagenomics, and bioinformatics. High-throughput sequencing technologies, such asnext-generation sequencing, allow researchers to analyze the genetic material of entire microbial communities, providing insights into their composition and diversity. Metagenomics, which involves the direct sequencing of DNA from environmental samples, enables the study of the functional potential of microbial communities. Bioinformatic tools and computational analyses are essential for processing and interpreting large-scale microbial community data, allowing researchers to identify key microbial taxa and their functional roles within a community. Manipulating microbial community structure holds great potential for applications in agriculture, bioremediation, and human health. In agriculture, the use of microbial inoculants and biofertilizers aims to enhance soil microbial communities, promoting plant growth and nutrient uptake. Bioremediation strategies leverage the metabolic capabilities of microbial communities to degrade pollutants and contaminants in the environment. In human health, efforts to modulate the gut microbiota through probiotics and fecal microbiota transplantation highlight the potential for manipulating microbial community structure to improve host health and treat diseases. In conclusion, microbial community structure is a complex and dynamic aspect of microbial ecology with far-reaching implications for ecosystem functioning, human health, and biotechnological applications. Understanding the factors influencing microbial community structure and the methods used to study and manipulate it is essential for harnessing the potential of microbial communities in diverse contexts. Continued research in this field will contribute to advancements in environmental sustainability, agriculture, medicine, and biotechnology, ultimately benefiting society as a whole.。
化石的形成过程简短200字作文英文回答：Fossils are formed through a process called fossilization. This process begins when an organism diesand its remains are buried in sediment, such as mud or sand. Over time, the sediment gradually hardens and turns into rock, which helps to preserve the remains.As the remains are buried deeper, they may undergo a process called permineralization. This occurs when minerals in the surrounding sediment seep into the pores of the organism's tissues, replacing the original organic material. This helps to strengthen and preserve the remains.Another way fossils can form is through the process of replacement. In this process, the original organic material is completely dissolved and replaced by minerals, such as silica or calcium carbonate. This creates a replica of the organism's structure in the rock.The final step in fossil formation is exposure. Over millions of years, the layers of sediment containing the fossil may be eroded away, exposing the fossil on the surface. Fossils can also be uncovered through human activities, such as excavation or mining.中文回答：化石是通过一种叫做化石化的过程形成的。
海洋微型生物碳泵——从微型生物生态过程到碳循环机制效应焦念志;郑强;李彦玲;骆庭伟;张瑶;张锐;汤凯;陈峰;曾永辉;张永雨;赵艳琳【期刊名称】《厦门大学学报（自然科学版）》【年(卷),期】2011(050)002【摘要】The ubiquitous picoplankton are major components of food webs and thus they play key roles in biogeochemical cycle and energy flow in the marine ecosystem. Thereinto,aerobic anoxygenic phototrophic bacteria (AAPB) is an important functional group with capability of harvesting light energy,and has a particular role in the ocean's carbon cycling. By application of new methods and further innovation of related techniques, and through extensive field investigations in the Chinese coastal wasters and the three Oceans, great data sets have been set up and systemic results have been obtained on microbial processes in the ocean' s carbon and energy flows. The following are the major achievements. 1) We established a new protocol as "time series observation based infrared epifluorescence microscopy" (TIREM) which enabled us to enumerate AAPB accurately; 2) And consequently revealed the wide distribution and unexpected abundance of AAPB in coastal waters (more abundant than in oligotrophic oceanic water which was thought to be the major niche of AAPB) ; 3) Based on the above data sets, a novel model is proposed , where bacteriochlorophyll a induced anoxygenic phototrophy and aproteiorhodopsin based proton pump are included; 4) Furthermore for carbon sequestration below the surface ocean, a conceptual model of microbial carbon pump ( MCP) is proposed, which is in contrast to the well known sinking flux-based biological pump. Overall, these achievements have filled several research banks in China, and lead in the international field of marine microbial ecology.%微型生物是海洋生态系统中"看不见的主角",在资源环境以及全球变化中扮演着举足轻重的角色.本研究通过方法创新和大量现场调查,从一类特殊微型生物类群--好氧不产氧光合异养细菌(从PB)入手,展开了微型生物生态过程与机制的系统研究,修正了国际同行AAPB计数方法的误差,获得了全球海洋AAPB的分布规律,解释了以往现场实测结果的分歧,澄清了以往理论上的偏颇认识;建立了包括不产氧光能利用途径的上层海洋碳循环模型,并通过大量现场实测揭示:细菌光能利用关系到海区碳循环的"源""汇"格局;在这些研究基础上探讨了新的海洋碳循环机制,提出了"微型生物碳泵"理论框架,为全面认识海洋储碳机制、促进学科交叉、研发海洋碳汇奠定了基础.【总页数】15页(P387-401)【作者】焦念志;郑强;李彦玲;骆庭伟;张瑶;张锐;汤凯;陈峰;曾永辉;张永雨;赵艳琳【作者单位】厦门大学近海海洋环境科学国家重点实验室,福建,厦门,361005;厦门大学近海海洋环境科学国家重点实验室,福建,厦门,361005;厦门大学近海海洋环境科学国家重点实验室,福建,厦门,361005;厦门大学近海海洋环境科学国家重点实验室,福建,厦门,361005;厦门大学近海海洋环境科学国家重点实验室,福建,厦门,361005;厦门大学近海海洋环境科学国家重点实验室,福建,厦门,361005;厦门大学近海海洋环境科学国家重点实验室,福建,厦门,361005;厦门大学近海海洋环境科学国家重点实验室,福建,厦门,361005;厦门大学近海海洋环境科学国家重点实验室,福建,厦门,361005;厦门大学近海海洋环境科学国家重点实验室,福建,厦门,361005;厦门大学近海海洋环境科学国家重点实验室,福建,厦门,361005【正文语种】中文【中图分类】Q148;P76【相关文献】1.海洋微型和微微型浮游生物的区域分布与影响因素 [J], 钟瑜;黄良民2.灵活的微型离心泵——适用于化工、医药产品生产领域的微型磁力离心泵 [J],3.海洋微型和超微型浮游生物 [J], 宁修仁4.微型生物碳泵研究进展 [J], 蔡阮鸿;郑强;陈晓炜;徐大鹏;王煜;骆庭伟;张锐5.海洋浮游生物碳循环过程(生物泵)简介 [J], 杨振雄;柠语(制图)因版权原因，仅展示原文概要，查看原文内容请购买。
微生物岩与微生物碳酸盐在当前地质记录中的进展摘要：本文综述了微生物岩和微生物碳酸盐当前研究的进展和问题。
微生物岩和微生物碳酸盐，形成于微生物的增长形式钙化以及其所绑定的碎屑沉积过程,最近成为最受欢迎的地质话题之一。
他们的发展变化贯穿于整个地质历史中,由于其复杂的结构和形成过程，使得他们具有重要理论研究意义以及经济意义。
微生物岩是栖生物微生物的代谢产物,而微生物碳酸盐可以分为两种:稳定的微生物碳酸盐(即碳酸盐岩微生物岩,如叠层石和和凝块石)和动员微生物碳酸盐(例如,微生物碳酸盐岩颗粒,如胶状物以及微生物肿块)。
不同质地、结构和形态的微生物岩和微生物碳酸盐阻碍了系统的描述和分类。
不仅仅如此，复杂的钙化途径以及成岩修改之后进一步掩盖了一些微生物岩和微生物碳酸盐。
在发现“微生物岩”改变传统生物起源观念之前，现代研究发现了丰富的海绵骨针，这些“微生物岩”影响了微生物碳酸盐礁进化论的过程。
在恐龙灭绝事件发生后，微生物岩微生物碳酸盐的发展并不兴旺。
一些其他证据表明,他们不仅被后生动物所影响,还受其他地质因素所影响。
由于他们依赖水深,清晰度,营养,微生物岩光线等因素，因此他们的增长、发展和消亡也与海平面的变化密切相关。
细致的研究微生物岩和微生物碳酸盐，对于帮助理解地质历史上重大地质事件的原因和影响以及生活和环境的共同进化有重要意义。
关键词：微生物岩微生物碳酸盐礁石钙化1 引言微生物岩和微生物碳酸盐是由微生物的增长形式钙化以及其所捕获或者绑定的碎屑沉积过程所形成的，他们的发展贯穿了整个地质时期:其发展历程最早可以追溯到34.5亿年前，然而，最新一批的微生物盐和微生物碳形成于地质环境的不同变换过程中，微生物岩和微生物碳酸盐，在一些较为关键的历史时期很盛行，例如中央新元,寒武纪，晚奥陶系,石炭纪,三叠纪早期等等。
一些重大地质事件、古气候和古海洋条件与微生物岩和微生物碳酸盐的繁荣和衰退是密切相关的，因此，对于这些微生物的研究的过程，离不开对于沉积物、古海洋,气候,和古代生态条件的研究学习,由于其复杂的结构和生物的起源，微生物岩和微生物碳酸盐也能作为水库的矿物质和碳氢化合物,并且具备一定的经济意义.微生物岩和微生物碳酸盐最近成为地质领域的热门话题。
Microbial fossilization in carbonate sediments:a result of the bacterial surface involvement in dolomite precipitation YVONNE VAN LITH1,ROLF WARTHMANN,CRISOGONO VASCONCELOSand JUDITH A.MCKENZIELaboratory of Geomicrobiology,Geological Institute,Swiss Federal Institute of Technology(ETH), Sonneggstrasse5,CH-8092Zu¨rich,SwitzerlandABSTRACTRecent dolomitic sediment samples from Lagoa Vermelha,Brazil,wereexamined microscopically to study the process of bacterial fossilization incarbonate sediments.Bacteria-like bodies were intimately associated withcarbonate mineral surfaces,and coatings on the former demonstrate thecalcification of single bacterial cells.The bacterial fossilization process inLagoa Vermelha sediments was simulated in the laboratory by cultivation ofmixed and pure cultures of sulphate-reducing bacteria,which were isolatedfrom the Lagoa Vermelha sediments.These cultures produced carbonateminerals that were studied to provide insight into the initiation of thefossilization process.In mixed culture experiments,bacterial colonies becamecalcified,whereas in pure culture experiments,single bacterial cells wereassociated with dolomite surfaces.Dolomite nucleated exclusively in bacterialcolonies,intimately associated with extracellular organic matter and bacterialcells.Electrophoretic mobility measurements of the bacterial cells inelectrolyte solutions demonstrated the specific adsorption of Ca2+and Mg2+onto the cell surfaces,indicating the role of the bacterial surface in carbonatenucleation and bacterial fossilization.The affinity of the cells for Mg2+wasrelated to the capability of the strains to mediate dolomite formation.Combined with sulphate uptake,which dissociates the[MgSO4]0ion pairand increases the Mg2+availability,the concentration of Mg2+ions in themicroenvironment around the cells,where the conditions are favourable fordolomite precipitation,may be the key to overcome the kinetic barrier todolomite formation.These results demonstrate that bacterial fossilization is aconsequence of the cell surface involvement in carbonate precipitation,implying that fossilized bacterial bodies can be used as a tool to recognizemicrobially mediated carbonates.Keywords Bacterial fossils,carbonates,cell surface,dolomite,fossilization,zetapotential.INTRODUCTIONThe relationship between microbial activity and carbonate precipitation has long been recognized in modern and ancient sedimentary environ-ments,and in laboratory studies(Nadson,1928;Neher&Rohrer,1958;Krumbein,1979;Morita, 1980;Chafetz&Buczynski,1992;Vasconcelos et al.,1995;Rivadeneyra et al.,1997).Bacteria are common and ubiquitous constituents in modern carbonate sediments and are involved in their formation through metabolic processes, such as photosynthesis,ammonification and sulphate reduction,that increase alkalinity and bicarbonate concentration and induce carbonate precipitation under both oxic and anoxic condi-tions(Buczynski&Chafetz,1991;Vasconcelos&1Present address:Department of Geochemistry,Facultyof Earth Sciences,University of Utrecht,Budapest-laan4,3508TA Utrecht,The Netherlands(E-mail:y.vanlith@geo.uu.nl)Sedimentology(2003)50,237–245Ó2003International Association of Sedimentologists237McKenzie,1997;Ehrlich,1998;Visscher et al., 1998;Castanier et al.,1999;Sagemann et al., 1999;Wright,1999;Warthmann et al.,2000). Encrustment and fossilization of microbes can be studied in recent sediments and may provide information on the processes responsible for mineral formation.Recent laboratory experiments have shown that microbial sulphate reduction can be respon-sible for the formation of carbonates with differ-ent Mg/Ca ratios(Sagemann et al.,1999; Warthmann et al.,2000).Bacterial sulphate reduction may overcome the kinetic barrier to dolomite formation by increasing pH and car-bonate alkalinity(Vasconcelos et al.,1995; Vasconcelos&McKenzie,1997;Castanier et al., 1999;Wright,1999;Warthmann et al.,2000)and by removing sulphate,an inhibitor to dolomite formation(Baker&Kastner,1981).In addition, sulphate occurs in sea water as a magnesium sulphate ion pair,and removal of sulphate ions by bacteria may increase the availability of magnesium ions for dolomite precipitation (Vasconcelos&McKenzie,1997).Understanding carbonate formation by sulphate-reducing bac-teria under anoxic conditions may furnish information on Precambrian carbonate sedimen-tation,as well as on diagenetic processes that occur in the marine subsurface.The recognition of microbial carbonates in the geological record is often controversial.Scanning electron microscopy(SEM)of natural or labora-tory-produced microbial carbonate has a high potential to reveal the association of bacteria with carbonate minerals(Chafetz&Folk,1984;Folk, 1994;Rivadeneyra et al.,1998,2000;Casanova et al.,1999)but does not prove their involvement in mineral formation.In this study,bacterial fossilization processes in recent dolomitic sedi-ment from Lagoa Vermelha,Brazil were investi-gated.In addition,carbonate precipitates formed in the laboratory by mixed and pure cultures of sulphate-reducing bacteria isolated from Lagoa Vermelha sediments were studied with SEM and compared with the modern environment.To investigate the role of the cell surface of sul-phate-reducing bacteria in dolomite nucleation and bacterial fossilization,electrophoretic mobil-ity measurements were carried out to determine the affinity of the cells for magnesium and calcium ions.Insight into the mechanism of microbial fossilization in carbonate sediments may provide clues to recognize microbial carbon-ate formation and trace it throughout the geolo-gical record.METHODSediment cores were collected from Lagoa Verme-lha,a coastal lagoon located about100km east of Rio de Janeiro city,Brazil.A microbial dolomite model wasfirst described in Lagoa Vermelha (Vasconcelos&McKenzie,1997),and sulphate-reducing bacterial cultures from the lagoon have been shown to mediate dolomite precipitation in the laboratory(Warthmann et al.,2000).The upper1m of Lagoa Vermelha sediment(up to 98%total carbonate)is characterized by alternat-ing carbonate and organic carbon-rich layers.Low Mg-calcite,aragonite,high Mg-calcite(7–35 mol.%MgCO3)and Ca-dolomite(42–48mol.% MgCO3)are the carbonate mineral phases present in the lagoon sediment(Ho¨hn et al.,1986;Vas-concelos,1994;Vasconcelos&McKenzie,1997). Dolomitic sediment samples from2cm depth in the sediment cores were prepared for SEM studies.A sample of Lagoa Vermelha dolomitic sediment was diluted in a saline phosphate buffer and used to inoculate a Postgate medium(Postgate,1984). The resulting mixed culture of sulphate-reducing bacteria was grown at30°C under anoxic condi-tions using lactate as a carbon source and sulphate as the sole electron acceptor.Pure bacterial cul-tures were obtained from the sediment using Lagoa Vermelha medium(Warthmann et al., 2000)for deep agar dilution series.The isolated pure cultures Desulfovibrio sp.strains LVform6 and LVform1(EMBL accession no.AJ548465and AJ544687respectively)and a reference strain Desulfonatronovibrio hydrogenovorans(Zhilina et al.,1997)were grown in Lagoa Vermelha medium on formate,and incubated without sha-king at30°C.Parallel sterile control experiments without bacteria were run simultaneously for all experiments.X-ray diffraction spectra of precipi-tates were obtained using a Scintag diffractometer to identify the minerals present.In order to visualize microbial cells in the samples,the precipitate was treated with DAPI,a highly speci-ficfluorescent stain for DNA.The DNA–DAPI complexfluoresces bright blue when excited with light at a wavelength of365nm.Samples were studied using a Zeiss Axioskop-2phase-contrast microscope with an appropriatefilter.Scanning electron microscope studies were conducted using a Hitachi S-700field emission SEM.Dolomitic sediment samples from Lagoa Vermelha were chemicallyfixed in2Æ5%glutar-aldehyde in0Æ2M Na-cacodylate buffer for 90min at4°C.Cells grown on glass plates and cell mineral pellets from centrifuged cultures238Y.van Lith et al.Ó2003International Association of Sedimentologists,Sedimentology,50,237–245werefixed with2Æ5%glutaraldehyde within their culture medium.Fixed samples were subse-quently washed with distilled water,30%eth-anol,dehydrated in acetone and critical point dried in liquid CO2.The samples were coated with8nm platinum before SEM imaging.Con-trols of culture medium without bacteria and minerals were treated as samples and prepared identically for SEM imaging,functioning as back-ground controls.In addition,a methodological comparison was made for some of the samples by cryofixation in10%methanol,freeze substitution, dehydration in acetone and critical point drying.A comparison of twofixation techniques permit-ted a better interpretation of the images because fixation artifacts could be excluded.Sulphate-reducing bacterial strains LVform1, LVform6,D.hydrogenovorans and Desulfovibrio profundus(Bale et al.,1997)were grown in a marine mineral medium with formate or lactate as a substrate and sulphate as an electron acceptor. Cells from these pure cultures were collected by centrifugation,washed in phosphate-buffered saline and distilled water and resuspended in a small volume of distilled water.The bacterial suspensions were diluted in different electrolytes until the optical density measured at578nm was about0Æ05OD,corresponding to about2500–3000 counts on a Malvern Zetamaster(Malvern Instru-ments,UK).The electrophoretic mobility of sus-pended bacteria was measured by dynamic light scattering on the Zetamaster.All samples were measured in duplicate.A Latex standard(GMP, Switzerland)was applied for calibration.In order to determine the isoelectric point,the electroph-oretic mobility of the strains was measured at pH3Æ5–10Æ5in solutions with ionic strengths from 10)2M to10)3M KNO3(Hunter,1981).Elec-trophoretic mobility of bacterial cells is an indi-rect measurement of the cell’s surface charge.To study the cation adsorption to bacterial surfaces, the electrophoretic mobility of bacterial suspen-sions was measured as a function of aqueous H+, K+,Na+,Mg2+,Ca2+and SO42–concentrations with ionic strengths from10)4to10)1M.RESULTSSEM study of Lagoa Vermelha sediment Bacteria were found to be closely associated with the carbonate mineral phase in sediment samples taken from2cm depth in Lagoa Vermelha(Fig.1A–C).Some of these bacteria-like bodies had irregular rather than smooth surfaces,indicating that the bacteria are coated by a carbonate precipitate(Fig.1B and C).Bac-teria with both smooth and irregular surfaces were observed,indicating that the latter wasnot Fig.1.Scanning electron micrographs of a dolomitic Lagoa Vermelha sediment sample from2cm depth.(A)Bacteria(arrow)are closely associated with thesediment particles.The size of the bacteria is similar to that of the isolated pure culture LVform6.(B)Oval-shaped bacterium(arrow)coated with afine-grained carbonate precipitate.(C)A long,curved,rod-shaped bacterium(arrow)with afine-grained carbonate preci-pitate on its surface.The bacterium appears to rest on a layer of dolomite crystals(black arrow)with well-developed orthorhombic shapes.Microbial fossilization in carbonate sediments239Ó2003International Association of Sedimentologists,Sedimentology,50,237–245an artifact of fixation and sample coating for SEM analysis.Mixed cultures simulating the sedimentary environmentA visible amount of carbonate formed after about 3weeks in mixed cultures of sulphate-reducing bacteria from Lagoa Vermelha.SEM revealed bacterial colonies consisting of single-type sul-phate-reducing bacteria held together by extracel-lular organic matter.Within bacterial colonies,some bacteria were coated with a fine-grained carbonate precipitate (Fig.2),demonstrating the process by which colonies become mineralized.Dolomite formation in pure bacterial culturesAfter 3weeks’incubation of LVform6and D.hydrogenovorans ,a visible amount of dolomite formed in the reaction vials.The apparent preci-pitation rate was 500mg L )1month )1.Dumb-bell-shaped bodies consisting of partially ordered,non-stoichiometric dolomite formed in cultures of LVform6and D.hydrogenovorans (Fig.3A and B respectively;Warthmann et al .,2000).Bacteria were sometimes attached to the dumbbell surfaces,which formed in dense col-onies of D.hydrogenovorans culture (Fig.3B).Dolomite nucleated exclusively in bacterial colonies of some 10000cells aggregated inan organic matrix composed of extracellular polymeric material (Fig.3B).Phase-contrast micr-oscopy of a DAPI-stained sample also revealed that the dumbbells were embedded in extracellu-lar material surrounding the biomass (Fig.4).Electrophoretic mobility measurementsStrain D.hydrogenovorans had an isoelectric point at pH 5with an electrophoretic mobility of )4Æ6m 2V )1s )1.Strain LVform6had its iso-electric point at pH 3with an electrophoretic mobility of )2Æ5m 2V )1s )1.Electrophoretic mobility measurements of D.hydrogenovorans showed two trends:(1)electrolytes with mono-valent cations (KNO 3,Na 2SO 4,NaCl)demonstra-ted a decreasing electrophoretic mobility with increasing ionic strength;and (2)electrolytes with bivalent cations (CaCl 2and MgCl 2)demon-strated an increasing electrophoretic mobility with increasing ionic strength,and the charge inverted at high ionic strength (Fig.5).The electrophoretic mobility of D.hydrogenovorans ,measured in different electrolytes at 10)4M,varied from )6to )3m 2V )1s )1(Fig.5).The sulphate-reducing bacterial strains ana-lysed for their electrophoretic mobility all had different affinities for Mg 2+,Ca 2+,Na +and H +(Fig.6).Relative to the average electrophoretic mobility measured in the most dilute solution,the electrophoretic mobility of bacteria in 0Æ1M MgCl 2increased most for strain LVform6,fol-lowed by D.profundus , D.hydrogenovorans and strain LVform1(Fig.6).The electrophoretic mobility of bacteria in 0Æ1M CaCl 2increased most for D.hydrogenovorans ,followed by D.profun-dus ,strain LVform6and strain LVform1(Fig.6).DISCUSSIONBacterial fossilizationMicrobes are the most abundant and widespread organisms in sediments and like to adhere to inorganic surfaces (Riding &Awramik,2000).Therefore,it is no surprise that living bacteria are commonly observed in close spatial relationship with mineral particles (Folk,1994;Castanier et al .,1999).Fossilized and entombed bacteria,as observed in Lagoa Vermelha sediments (Fig.1),however,may indicate bacterial involvement in the carbonate precipitation process.Being well aware of potential artifacts,such as fixation,etching and coating artifacts (Kirkland et al.,Fig.2.Scanning electron micrographs of a mixed cul-ture of sulphate-reducing bacteria.Within bacterial colonies,individual bacterial cells are coated with an unidentified fine-grained mineral precipitate.An example of one coated bacterial cell is shown in the centre of the picture.240Y.van Lith et al.Ó2003International Association of Sedimentologists,Sedimentology ,50,237–2451999)that may occur during sample preparation for SEM studies,two fixation methods were tested,and a control study was undertaken without bacteria.Chemical fixation and cryofixa-tion led to identical images and the background signal was negligible,indicating that fixation and coating artifacts could be ruled out.Bacteria were intimately associated with car-bonate mineral surfaces in both Lagoa Vermelha sediment samples and bacterial culture experi-ments.The bacteria-like bodies were observed attached to the mineral surface of Lagoa Vermelha dolomitic sediment (Fig.1C)or partly entombed (Fig.1B),in contrast to the integrated bacteria observed by Castanier et al .(1999)that emerged perpendicular from crystal planes and eventually left no traces with ongoing precipitation.With etching,Castanier et al .(1999)observed rounded structures becoming visible,similar to rounded surface structures on other carbonate mineral surfaces,which have commonly been interpreted as calcified entombed bacterial bodies (Casanova et al .,1999;Castanier et al .,1999;Rivadeneyra et al .,2000).In previous SEM investigations of etched Lagoa Vermelha sediment,entombed bac-teria were observed (Vasconcelos et al .,1995;Vasconcelos &McKenzie,1997),demonstrating that,with etching,more integrated bacteria and bacteria protruding from crystals may become visible.Another reason for not observing deep entombed bacteria may be the low preservation potential of encrusted bacteria.Encrusted bac-teria most probably die and the cell lyses,possibly leaving only a cast with the particular shape behind.The present study of the bacterial mixed culture demonstrates calcification of bacterial clusters,which resembles the well-documented ‘biolith’formation process of Rivadeneyra et al .(1998).Nucleation and growth of dolomite dumbbells occurred exclusively in an organic matrix in laboratory pure culture experiments,and bacteria (LVform6and D.hydrogenovorans )were found to be associated with the dolomite dumbbell surface.The organic matrix encapsulating the bacteria may promote diffusion gradients,whereby ions can diffuse through and enable the development of the physicochemical conditions promoting dolomite precipitation.Under diffusion-controlled condi-tions in bacterial colonies,a high concentration of freely available or adsorbed Mg 2+would tend to combine with the bicarbonate ions released by the cells;the result would be the nucleation of dolo-mite microcrystals in close spatial relation with the cells (Warthmann et al .,2000).Entombed bacteria or bacteria emerging from the crystal planes were not observed in these pure cultures.The close spatial relationship of bacteria and dolomite crystals,however,suggests that bacteria could be entombed by increased dolo-mite precipitation rates,similar to fossilization processes observed in the sediment.DolomiteFig.3.(A)Sulphate-reducing bacteria of strain LVform6(arrows)are closely related to the dolomite dumbbells that precipitated in this culture.(B)A dolomite dumbbell,typically embedded in a matrix of sulphate-reducing bacteria D.hydrogenovorans (arrow)and extracellular organic material (eom).XRD analysis identified the mineral dumbbells formed in both sulphate-reducing bacterial cultures as nearly stoichiometric and well-ordered dolomite,as also indicated by the well-defined crystal texture visible on the outer surface of the dumbbell.Microbial fossilization in carbonate sediments 241Ó2003International Association of Sedimentologists,Sedimentology ,50,237–245formed in the bacterial pure culture experiments at a rate of 500mg L )1month )1.This rate was obtained in experiments beginning withlow bacterial density and 40mM formate and 10mM sulphate.In Lagoa Vermelha,where the bacterial density is higher and the sulphate concentration is greater at 50mM,the carbonate crystallization rate is faster.Differences in car-bonate precipitation rate may also explain the different fossilization of mixed and pure bacterial cultures.The mixed cultures may simulate bac-terial fossilization in the natural environment better because of higher carbonate precipitation rates than the pure cultures.Involvement of the cell surfaceThe isoelectric point of strain D.hydrogenovo-rans and LVform6was determined at low pH,respectively 0Æ5to 2Æ5pH units lower than the isoelectric point reported for other sulphate-reducing bacterial strains (Ulanovskii et al .,1980).The indication that the isoelectric point was at low pH for these strains demonstrates that the bacterial surface consists of proteins,lipo-polysaccharides and peptides with carboxyl,hydroxyl,amino and phosphate groups,slightly dominated by the acidic groups.This implies that,at a neutral pH (pH >pH isoelectric point),the bacterial surface is negatively charged.The different trends in electrophoretic mobility,meas-ured in monovalent and bivalent electrolytes with increasing ionic strength,indicate that monova-lent cations interacted with the cell surface differently than bivalent cations.Previous research on the electrophoretic mobil-ity of bacterial cells has shown that cations affected the electrostatics at the negatively charged bacterial surfaces,but anions did not (Simoni et al .,2000).The negative charge of bacterial surfaces is counterbalanced by positive charges in the diffuse ion cloud in the electrolyte.This causes a decrease in electrophoretic mobility with increasing ion strength,as observed in electrolytes comprising monovalent cations (Fig.6).Bivalent cations bind specifically to phosphate and carboxyl surface groups and reduce the net charge within the electrokinetic shear plane.This implies that fewer counterions accumulate in the diffusive layer (Hunter,1981).Specific binding of bivalent cations explains the initial decrease in electrophoretic mobility with increasing ionic strength and the eventual inver-sion of charge (Fig.6).Bivalent cations are known to stabilize the outer membrane of Gram-negative bacteria by reducing the charge repulsion between highly anionic lipopolysaccharide mol-ecules (Coughlin et al .,1983).TheincreasedFig.4.Dolomite dumbbells embedded in bacterial clusters.This light micrograph shows a bacterial aggregate of 105sulphate-reducing bacteria in blue fluorescence by the DNA-binding DAPI dye.The purple to bright blue colour represents the concentration of cells.The dolomite minerals also show some fluores-cence,but at a different wavelength (bright pink),not representing the DNA–DAPI complex.Dolomite dumbbells in this culture were formed exclusively in bacterialaggregates.Fig.5.Electrophoretic mobility (l E)of D.hydrogeno-vorans measured in electrolyte CaCl 2,MgCl 2,KNO 3,NaCl and Na 2SO 4with ionic strengths ranging from 10)4to 10)1M.242Y.van Lith et al.Ó2003International Association of Sedimentologists,Sedimentology ,50,237–245electrophoretic mobility with increased electro-lyte concentration indicates that both Mg2+and Ca2+ions were specifically adsorbed to the bacterial surface of strain LVform6,LVform1, D.hydrogenovorans and D.profundus.This is consistent with previous research showing that Mg2+and Ca2+are the main ions bound to the outer membrane of Gram-negative bacteria (Coughlin et al.,1983).The electrophoretic mobility at the point of zero charge was similar to the average electrophoretic mobility measured in most dilute electrolytes.A comparison demonstrates that D.hydrogenovo-rans has the largest negative permanent surface charge,followed by D.profundus,strain LVform6 and LVform1.All analysed sulphate reducers specifically adsorbed Mg2+and Ca2+cations;Mg2+ions were adsorbed most strongly by strain LVform6and D.hydrogenovorans,strains that were also found to mediate dolomite formation. Sulphate-reducing bacteria take up sulphate without Mg2+(Warthmann&Cypionka,1990), thereby dissociating the[MgSO4]0ion pair and making Mg2+available to adsorb to the cell surface.Mg2+and Ca2+are concentrated at the cell surface,where bicarbonate is also produced. This leads to a supersaturation of the microenvi-ronment around the cell promoting dolomite nucleation.Ca2+ions,adsorbed onto bacterial surfaces,can induce the precipitation offine-grained calcium carbonate by the complexation of counterions(Schultze-Lam et al.,1996;Fortin et al.,1997;Douglas&Beveridge,1998).In the same way,it is proposed here thatspecificbinding of Mg2+and Ca2+ions on the bacterial surface may catalyse dolomite formation by the complexation of carbonate counterions. CONCLUSIONSThis study shows that an intimate association of sulphate-reducing bacteria and carbonate min-erals exists in both Lagoa Vermelha sediments and bacterial culture experiments.It is proposed that carbonate precipitation was mediated by the bacteria in both cases.In the Lagoa Vermelha sediments,the occurrence of encrusted,calcified bacterial bodies provides evidence of the sul-phate-reducing bacterial mediation of carbonate formation,as also demonstrated in laboratory culture experiments.In bacterial mixed cultures, bacterial clusters were found to be mineralized as a whole,whereas in pure cultures,the bacterial cells were associated only with the surfaces of carbonate minerals,and could be entombed with continuing carbonate precipitation.Fossilized bacteria are an indicator of microbial carbonates in the sedimentary rock record.Ca2+and Mg2+were found specifically to adsorb onto the negatively charged surface of halotoler-ant,alkaliphilic sulphate-reducing bacteria.In addition to the sulphate-reducing activity,the bacterial cell surface of sulphate-reducing bacteria is involved in the mediation of dolomite precipi-tation through the capability of concentrating Mg2+around the cells where dolomite nucleation occurs.Bacterial fossilization is thus a result of the microbial mediation of carbonate precipitation. ACKNOWLEDGEMENTSWe thank Ernst Wehrli and Martin Mu¨ller(Ser-vice laboratory for electron microscopy of the Department of Biology,ETH-Zu¨rich)for their assistance with SEM imaging.Paolo Landini (EAWAG Duebendorf)is kindly acknowledged for his help with the analysis on the Zetamaster. We also thank H.S.Chafetz and an anonymous reviewer for their constructive comments.This work is partially supported by the Swiss National Science Foundation grant no.21-49612.96. 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