Sedimentary precipitation of carbonate induced by microbes
Microbialites in Lake Salda rocksEmerged microbialite formation at Lake Van, East AnatoliaClassification of microbialites (redrawn and simplified from Schmid, 1996[1]).Stromatolites – laminated microbialites (Precambrian silicified stromatolite, Strelley Pool Chert, (Pilbara Craton),W. Australia)
Microbialite is abenthic sedimentary deposit made ofcarbonate mud (particle diameter less than 5μm) that is formed with the mediation ofmicrobes. The constituent carbonate mud is a type ofautomicrite (orauthigenic carbonate mud); therefore, it precipitatesin situ instead of being transported and deposited. Being formed in situ, a microbialite can be seen as a type ofboundstone wherereef builders are microbes, and precipitation of carbonate is biotically induced instead of formingtests, shells or skeletons.
Microbialites can also be defined asmicrobial mats withlithification capacity.[2]Bacteria can precipitate carbonate both in shallow and in deep water (except forCyanobacteria) and so microbialites can form regardless of the sunlight.[3]
Microbialites are the foundation of manylacustrine ecosystems, such as the biosystem of theGreat Salt Lake with its millions of migratory birds[4] or, serving in theAlchichica Lake as nurseries for axolotl (Ambystoma taylori) and a variety of fish.
Microbialites were very important to the formation ofPrecambrian andPhanerozoic limestones in many different environments, marine and not. Thebest age[clarify] forstromatolites was from 2800Ma to 1000 Ma when stromatolites were the main constituents ofcarbonate platforms[3]
Microbialites can have three different fabrics:[3]
Stromatolitic: microbialite layered, laminated or agglutinated to form astromatolite.
Thrombolitic: microbialite with a clotted peloidal fabric if observed with a petrographic microscope. The density ofpeloids is variable. At the scale of the hand sample, the rock shows a dendritic fabric, and can be namedthrombolite.
Leiolitic: a microbialite with no layering nor clotted peloidal fabric. It is only made of a denseautomicrite.
Microbialites played an important role in the evolution of the Earth's atmosphere, since they were ancestralniches where the first microbial metabolisms capable of releasing oxygen arose. Microbialites saturated coastal systems and later the primitive atmosphere with oxygen, changing it from a reduced state to an oxidized state.[5] The fossil microbialites (also called stromatolites) of thePrecambrian andPhanerozoic are one of the first evidences of communal life. The oldest microbialites are dated at 3.5 billion years.[6][needs update] Fossil evidence suggests that microbialite-producing organisms were a very abundant life form from the early Archaean to the late Proterozoic, until their communities decreased due to the predation offoraminifera and other eukaryotic microorganisms.[7] Microbialites again became common briefly after thePermian-Triassic extinction event.[8][9][10]
The formation of microbialites is complex and is a continuous process ofprecipitation anddissolution, where different microbial metabolisms are coupled and a high saturation index (SI) of ions in water is present.[11]
Microbialites have two possible genesis mechanisms:
1) Mineral precipitation: is the main formation process of microbialites and it can be due to inorganic precipitation or to the passive influence of microbial metabolisms. There can also be precipitation due to saturation of the microenvironment whenextracellular polymeric substances (EPS) are rapidly degraded, increasing ion saturation.[12]
2) Trapping and binding: when the microbial community includes mineral particles of the environment that adhere to theextracellular polymeric substances (EPS). This process is very popular, since it was described in modern microbialites ofShark Bay (Australia) andBahamas, but it has been shown to be very uncommon throughout the 3500 million year long geological history of microbialites.[13]
Microbialites are built up with layers of some organic component and of some mineral.[14] The organic component is an elaborate microbial mat where different communities of microorganisms interact according to different metabolisms and create a micro-niche where oxygenic and anoxygenic phototrophic organisms coexist:nitrogen fixers,sulfur reducers,methaneotrophs,methanogens,iron oxidizers, and an infinity ofheterotrophic decomposers.[15] The mineral component is composed of carbonates, generallycalcium carbonate or magnesium carbonates such ashydromagnesite, although there may also besintered silicones, that is,silicates; and include mineral forms of sulfur, iron (pyrite) or phosphorus.[12] Carbonate is usually a type of autogenic automicrite, therefore it precipitates in situ. Microbialites can be viewed as a type of biogenic sedimentary rock where the reef builders are microbes and carbonate precipitation is induced. Microorganisms can precipitate carbonate in both shallow and deep waters
A broad number of studies have analyzed the diversity of microorganisms living at the surface of microbialites.[16][17] Very often, this diversity is very high and includes bacteria, archaea and eukaryotes. While the phylogenetic diversity of these microbial communities is pretty well assessed using molecular biology, the identity of the organisms contributing to carbonate formation remains uncertain. Interestingly, some microorganisms seem to be present in microbialites forming in several different lakes, defining a core microbiome.[18][16] Microbes that precipitate carbonate to build microbialites are mostlyprokaryotes, which include bacteria andarchaea. The best known carbonate-producing bacteria areCyanobacteria andSulfate-reducing bacteria.[19] Additional bacteria may play a prominent role, such as bacteria performing anoxygenic photosynthesis[20] is. Archaea are oftenextremophiles and thus live in remote environments where other organisms cannot live, such aswhite smokers at the bottom of the oceans.
There is great interest in studying fossil microbialites in the field of paleontology since they provide relevant data on paleoclimate and function as bioclimatic indicators.[22] There is also an interest in studying them in the field of astrobiology, as they are one of the first forms of life, one would expect to find evidence of these structures on other planets.[23] The study of modern microbialites can provide relevant information and serve as environmental indicators for the management and conservation of protected natural areas.[24] Due to their ability to form minerals and precipitate detrital material, biotechnological and bioremediation applications have been suggested in aquatic systems for carbon dioxide sequestration, since microbialites can function as carbon sinks.[25]
^Chagas, Anderson A.P.; Webb, Gregory E.; Burne, Robert V.; Southam, Gordon (November 2016). "Modern lacustrine microbialites: Towards a synthesis of aqueous and carbonate geochemistry and mineralogy".Earth-Science Reviews.162:338–363.Bibcode:2016ESRv..162..338C.doi:10.1016/j.earscirev.2016.09.012.ISSN0012-8252.
