DOLOMITE PRECIPITATION
Our experiments demonstrate that it is possible to produce partially ordered,nonstoichiometric dolomite in the laboratory under low-temperature,anoxic,hypersaline conditions in the presence of sulfate-reducing bacteria.During the bacterial growth experiments using formate,the pH of the medium and sulfide and bicarbonate concentrations increased,and the formate and sulfate concentrations decreased,according to the following reaction:4CHO 2–+ SO 42–+ H +→4 HCO 3
–+ HS –(at pH ≈7.5). These changes show the influence of the bacteria in the geochemical alteration of their closed en-vironment and,undoubtedly,in the mediation of the dolomite precipitation.
Nucleation and growth of the dolomite dumbbells occurred in an organic matrix,and sulfate-reducing bacteria were found to be associated with the mineral dumbbell surfaces; both facts indicate that microbial mediation might be an active process. The organic mucous encapsulating the bacteria may pro-
Figure 2.Scanning electron microscope photo-micrographs of sulfate-reducing bacteria strain L Vform6 associated with precipitation of dolo-mite with dumbbell morphology.A:Possible initial stage of crystal growth at polar end of bacterial cell.B:Crystal growth appears to begin with close spatial relationship to bac-terial cells,mainly inside larger aggregates of bacteria.C:Growth of d olomite d umbbells showing d rastic mass increase within first 2–3weeks after initiation.Size of bacteria (4–8μm in length) can be used as comparative scale.D:Later stage of mineral precipitation after ~4–6weeks,when d umbbell shape is transformed
into cauliflower-like structure.
Figure 3.Scanning electron microscope photomicrograph showing close-up of dolomite dumbbell with attached D.hydrogenovorans bac-teria (a) and desiccated organic filaments.Dolomite is rather well crys-tallized,
as demonstrated by clearly defined crystal faces.Figure 4.Dolomite sample from upper few centimeters of Lagoa Vermelha sed iment.Scanning electron microscope photomicrograph shows dumbbell and cauliflower shapes similar to those observed in pure cul-tures of sulfate-reducing bacteria strain LVform6,which was isolated from Lagoa Vermelha sediment.
mote diffusion gradients,whereby ions can diffuse through it and enable the development of the physicochemical conditions promoting dolomite precipi-tation. In the dolomite formation process,the concentration of the magnesium ions is likely to be very important. By consuming sulfate ions,which are con-sidered to inhibit dolomite formation (Baker and Kastner,1981),sulfate-reducing bacteria remove a kinetic inhibitor and thereby promote dolomite formation. Moreover,as sulfate-reducing bacteria take up sulfate together with H +or Na +(Warthmann and Cypionka 1990),the strong ion pair Mg 2+-SO 4
2–has to be dissociated. Under diffusion-controlled conditions in bacterial colonies,a high concentration of free available Mg 2+would tend to combine with the bicarbonate ions released by the cells; the result would be dolomite.
Kinetic inhibition is generally thought to control dolomite precipita-tion under Earth surface conditions. Based on the results of our culture experiments,we propose that the presence of specific sulfate-reducing bac-teria can mediate dolomite precipitation and overcome the kinetic barriers.The metabolic activity results in an elevated pH and removal of the kinetic inhibitor (sulfate ions),and leads to a concentration of magnesium and bi-carbonate ions and the nucleation of the microcrystals.
CRYSTAL MORPHOLOGY
The end product of our culture experiments is a nonstoichometric dolo-mite with a dumbbell morphology. Examining the development of the dumb-bell structures (Fig.2,A–C),we conclude that the shape changes with time as precipitation continues. With longer reaction times,the individual dumbbells continue to grow,exhibiting a botryoidal-like habit and merging together to form agglomerates with semispherical or cauliflower shapes (Fig.2D). These shapes are very similar to the dumbbell morphology observed for the modern dolomite found in the Lagoa V ermelha sediment (Fig. 4).
The dumbbell morphology has been previously observed in carbonate minerals produced in laboratory experiments and found in the rock record.Buczynski and Chafetz (1991) showed that aerobic marine bacteria could in-duce the precipitation of calcium carbonate that can have dumbbell morpholo-gies,similar to the observation of Sagemann et al. (1999) under microbial sul-fate reduction. The production of different crystal morphologies,including dumbbells,is apparently related to the percentage Mg in the calcite crystal lat-tice (Devery and Ehlmann,1981; Fernández-Díaz et al.,1996). In the rock record,the occurrence of characteristic holes centered in the dolomite rhombo-hedral crystals,often with a dumbbell shape,has been reported (e.g.,Fig.15 of Feldmann and McKenzie,1997; Fig.5,k–l,of Clari and Martire,2000). These molds may represent the former presence of metastable dumbbell-shaped nuclei around which additional stoichiometric dolomite rhombohedra precipi-tated. Thus,the crystal shape,whether the original or a relict mold,may be a useful tool to trace microbial processes in the geologic record.
CONCLUSIONS
In our culture experiments,we have demonstrated that modern species of sulfate-reducing bacteria are capable of mediating dolomite formation in a synthetic anoxic hypersaline medium under well-defined conditions. The production of microbial-mediated dolomite by a pure strain of sulfate-reducing bacteria verifies the microbial dolomite model developed by V asconcelos and McKenzie (1997).
A similarity in crystal shape and crystallography of natural and experi-mentally produced dolomite is strong evidence for the involvement of these microorganisms in the precipitation process. Typical dumbbell-shaped dolomite,as formed in cultures of the sulfate-reducing strain LVform6,appears to be uniquely mediated by microbes. Future investigations will attempt to elucidate whether the mineral shape and composition are actively influenced by the bacteria or by the extracellular material. We propose that bacterial sulfate reduction can induce significant amounts of dolomite pre-cipitation,implying a major microbial contribution to the carbonate sedi-mentary budget. During Earth’s early history,anaerobic bacteria may have been particularly important for dolomite production because anoxic condi-tions were vastly more prevalent than today.
ACKNOWLEDGMENTS
We thank Ernst Wehrli and Martin Müller (Servicelabor für Elektronen-mikroskopie of the Department of Biology,ETH-Zürich) for their assistance with scanning electron microscope imaging and Gilles Morvan,Centre Nacional de la Recherche Scientifique,Strasbourg,for his talented operation of the transmission electron microscope. We acknowledge Paul A. Wilson and Eugine A. Shinn for their helpful reviews. This work is partially supported by the Swiss National Science Foundation Grant 21-49612.96.
REFERENCES CITED
Baker,P.A.,and Kastner,M.,1981,Constraints on the formation of sedimentary
dolomite:Science,v. 213,p. 214–216.
Buczynski,C.,and Chafetz,H.S.,1991,Habit of bacterially induced precipitates of
calcium carbonate and the influence of medium viscosity on mineralogy:Jour-nal of Sedimentary Petrology,v. 61,p. 226–233.
Burns,S.T.,McKenzie,J.A.,and V asconcelos,C.,2000,Dolomite formation and bio-geochemical cycles in the Phanerozoic:Sedimentology,v. 47,supplement 1,p.49–61.
Castanier,S.,Métayer-Levrel,G.L.,and Perthuisot,J.-P .,1999,Ca-carbonates pre-cipitation and limestone genesis—The microbiogeologist point of view:Sedi-mentary Geology,v. 126,p. 9–23.
Clari,P .A.,and Martire,L.,2000,Cold seep carbonates in the Tertiary of northwest
Italy:Evidence of bacterial degradation of methane,in Riding,R.E.,and Awramik,S.M.,eds.,Microbial sediments:Berlin,Springer,p. 261–269.
Devery,D.M.,and Ehlmann,A.J.,1981,Morphological changes in a series of syn-thetic Mg-calcites:American Mineralogist,v. 66,p. 592–595.
Ehrlich,H.L.,1998,Geomicrobiology:Its significance for geology:Earth-Science
Reviews,v. 45,p. 45–60.
Eugster,H.P .,1970,Chemistry and origin of the brines of Lake Magadi,Kenya,in
Morgan,B.A.,ed.,Fiftieth anniversary symposia,Mineralogy and geochem-istry of non-marine evaporites:Mineralogical Society of America Special Paper 3,p. 215–235.
Feldmann,M.,and McKenzie,J.A.,1997,Messinian stromatolite-thrombolite asso-ciations,Santa Pola,SE Spain:An analogue for the Palaeozoic?:Sedimentol-ogy,v. 44,p. 893–914.
Fernández-Díaz,L.,Putnis,A.,Prieto,M.,and Putnis,C.-V .,1996,The role of mag-nesium in the crystallization of calcite and aragonite in a porous medium:Jour-nal of Sedimentary Research,v. 66,p. 482–491.
Krumbein,W.E.,1979,Photolithotrophic and chemoorganotrophic activity of bac-teria and algae as related to beachrock formation and degradation (Gulf of Aqaba,Sinai):Geomicrobiology Journal,v. 1,p. 139–203.
Land,L.S.,1998,Failure to precipitate dolomite at 25 °C from dilute solution despite
1000-fold oversaturation after 32 years:Aquatic Geochemistry,v. 4,p. 361–368.Lumsden,D.N.,1979,Discrepancy between thin-section and X-ray estimates of
dolomite in limestone:Journal of Sedimentary Petrology,v. 49,p. 429–435.McKenzie,J.A.,1991,The dolomite problem:An outstanding controversy,in Müller,
D.W.,et al.,eds.,Controversies in modern geology:Evolution of geological theories in sedimentology,Earth history and tectonics:London,Academic Press,p. 37–54.
Morita,R.Y .,1980,Calcite precipitation by marine bacteria:Geomicrobiology Jour-nal,v. 1,p.63–82.
Rivadeneyra,M.A.,Delgado,G.,Ramos-Cormenzana,A.,and Delgado,R.,1997,
Precipitation of carbonates by Deleya halophila in liquid media:Pedological im-plications in saline soils:Arid Soil Research and Rehabilitation,v. 11,p.35–47.Sagemann,J.,Bale,S.J.,Briggs,D.E.G.,and Parkes,R.J.,1999,Controls on the for-mation of authigenic minerals in association with decaying organic matter:An ex-perimental approach:Geochimica et Cosmochimica Acta,v. 63,p.1083–1095.V asconcelos,C.,and McKenzie,J.A.,1997,Microbial mediation of modern dolomite
precipitation and diagenesis under anoxic conditions (Lagoa V ermelha,Rio de Janeiro,Brazil):Journal of Sedimentary Research,v. 67,p. 378–390.
Vasconcelos,C.,McKenzie,J.A.,Bernasconi,S.,Grujic,D.,and Tien,A.J.,1995,
Microbial mediation as a possible mechanism for natural dolomite formation at low temperatures:Nature,v. 377,p. 220–222.
Warthmann,R.,and Cypionka,H.,1990,Sulfate transport in Desulfobulbus propionicus
and Desulfococcus multivorans :Archives of Microbiology,v. 154,p.144–149.Wright,D.T.,1999,The role of sulphate-reducing bacteria and cyanobacteria in dolo-mite formation in distal ephemeral lakes of the Coorong region,South Australia:Sedimentary Geology,v. 126,p. 147–157.
Yates,K.K.,and Robbins,L.L.,1998,Production of carbonate sediments by a uni-cellular green alga:American Mineralogist,v. 83,p. 1503–1509.
Zhilina,T.N.,Zavarzin,G.A.,Rainey,F.A.,Pikuta,E.N.,Osipov,G.A.,and Kostrikina,
N.A.,1997,Desulfonatronovibrio hydrogenovorans gen. nov.,sp. nov.,an alkaliphilic,sulfate-reducing bacterium:International Journal of Systematic Bacteriology,v. 47,p. 144–149. Manuscript received May 25,2000
Revised manuscript received September 5,2000Manuscript accepted September 14,2000