Modern Microbial Mats Research Articles

Bacterial fossils and microbial dolomite in Triassic stromatolites

Triassic stromatolitic dolomite from Italy preserves mineralized bacterial remains, one of the first unequivocal identifications of such structures in the geological record. They consist of empty spheroids ∼1.0 µm diameter resembling coccoid bacteria, and smaller, 150–400 nm, objects interpreted as dwarf bacterial forms, occurring within and between syn-sedimentary dolomite crystals. Moreover, gently folded sheets, 100–200 nm thick and several micrometers long, form a sub-polygonal network reminiscent of EPS (extracellular polymeric substance). Their granular-textured surfaces suggest bacterial degradation of original organic matter. These features confirm a biological origin for the stromatolites, as in modern microbial mats, and the preserved original geochemical signatures indicate early precipitation of Mg-carbonates induced through microbial sulfate-reducing metabolic activities.

Changes in carbon cycling ascertained by stable isotopic analyses in a hypersaline microbial mat

Modern microbial mats have been used as analogs for early life because of the longevity of microbial life on Earth. Mats collected from hypersaline salterns in Baja California were maintained for over a year and a half under both normal (85 ppt salinity, 50 mM SO42−) and reduced salinity (35 ppt salinity; near modern seawater) and sulfate concentrations (20 mM, ≤1 mM SO42−) to assess carbon processing under sulfate conditions more similar to the Archean oceans. As sulfate was removed from the mats by diffusion out into the overlying water, methane concentrations within the mats increased. Highest methane concentrations occurred in mats with reduced salinity and little sulfate. The δ13C values of bulk particulate organic matter in all of the mats averaged −11‰, similar to what had been observed previously for these Microcoleus mats. In mats with sulfate, pore water concentrations of dissolved inorganic carbon (DIC) δ13C values averaged about −3‰. However, in the mats with ≤1 mM sulfate concentrations, the DIC δ13C values increased substantially with depth, from about −1‰ in the overlying water to +10‰ by 20 mm depth. Rates of methanogenesis, calculated from pore water dissolved methane concentration profiles, were too low to account for the total increase in DIC δ13C values. These positive isotopic signatures, however, are also consistent with the occurrence of acetogenesis, as are the higher acetate concentrations in the low‐sulfate mats. Acetogens may be poised to become successful competitors for substrates in these mats, given the right environmental conditions.

Methane production by microbial mats under low sulphate concentrations

ABSTRACTCyanobacterial mats collected in hypersaline salterns were incubated in a greenhouse under low sulphate concentrations ([]) and examined for their primary productivity and emissions of methane and other major carbon species. Atmospheric greenhouse warming by gases such as carbon dioxide and methane must have been greater during the Archean than today in order to account for a record of moderate to warm palaeoclimates, despite a less luminous early sun. It has been suggested that decreased levels of oxygen and sulphate in Archean oceans could have significantly stimulated microbial methanogenesis relative to present marine rates, with a resultant increase in the relative importance of methane in maintaining the early greenhouse. We maintained modern microbial mats, models of ancient coastal marine communities, in artificial brine mixtures containing both modern [] (c. 70 mm) and ‘Archean’[] (<0.2 mm). At low [], primary production in the mats was essentially unaffected, while rates of sulphate reduction decreased by a factor of three, and methane fluxes increased by up to 10‐fold. However, remineralization by methanogenesis still amounted to less than 0.4% of the total carbon released by the mats. The relatively low efficiency of conversion of photosynthate to methane is suggested to reflect the particular geometry and chemical microenvironment of hypersaline cyanobacterial mats. Therefore, such mats were probably relatively weak net sources of methane throughout their 3.5 Ga history, even during periods of low environmental levels oxygen and sulphate.

Geomicrobiology of carbonate–silicate microbialites from Hawaiian basaltic sea caves

Abstract Modern microbial mats and microbialites are described from basaltic sea caves on the island of Kauai, HI. The mats grow on the ceilings and walls in the photic zone of several open caves where fresh water seeps out of the rock. Scanning (SEM) and transmission electron microscopy (TEM) showed that the active mats are dominated by filamentous and nonfilamentous cyanobacteria in the surface layers and heterotrophic bacteria in deeper layers. Energy dispersive X-ray analysis revealed that copious amounts of extracellular polymeric substances (EPS) are rich in Mg, Si, O, and Ca, likely concentrated from solution. Petrographic microscopy and electron microprobe analysis of the mineralized microbialites showed textures reminiscent of stromatolitic laminations, consisting mainly of alternating calcium carbonate (calcite and aragonite) and magnesium-rich silicate (kerolite). Thin coatings rich in magnesite, hydromagnesite and monohydrocalcite surround the microbialites on the rock surfaces and are likely inorganic in origin. Within the mats, minerals tend to form and concentrate within, or around, dense matrices of EPS. Microenvironments with geochemical conditions favorable for mineral crystallization likely develop in the mats as a result of the mucilaginous extracellular material and the development of bacterial microcolonies. In addition, copious amounts of extracellular polymers bind ions from solution and provide nucleation sites for mineral crystallization and growth. This combination of biological and inorganic processes can explain the occurrence of the secondary minerals in these caves, as well as the stromatolitic textures of the microbialites.

Wrinkle structures: Microbially mediated sedimentary structures common in subtidal siliciclastic settings at the Proterozoic-Phanerozoic transition

Research Article| November 01, 1997 Wrinkle structures: Microbially mediated sedimentary structures common in subtidal siliciclastic settings at the Proterozoic-Phanerozoic transition James W. Hagadorn; James W. Hagadorn 1Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740 Search for other works by this author on: GSW Google Scholar David J. Bottjer David J. Bottjer 1Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740 Search for other works by this author on: GSW Google Scholar Geology (1997) 25 (11): 1047–1050. https://doi.org/10.1130/0091-7613(1997)025<1047:WSMMSS>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James W. Hagadorn, David J. Bottjer; Wrinkle structures: Microbially mediated sedimentary structures common in subtidal siliciclastic settings at the Proterozoic-Phanerozoic transition. Geology 1997;; 25 (11): 1047–1050. doi: https://doi.org/10.1130/0091-7613(1997)025<1047:WSMMSS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Sedimentary structures produced by microbial activity are well known from ancient carbonate facies, but little is known about equivalent microbial structures in ancient siliciclastic facies. Wrinkle marks and Kinneyia ripples are closely related sedimentary structures that are commonly found in ancient siliciclastics, and they may represent microbial activity. To evaluate this possibility, these structures were studied in Vendian–Cambrian strata of North America and compared to structures formed by modern microbial mat communities in Redfish Bay, Texas. Striking similarities in sedimentologic, petrographic, and morphologic characteristics of modern and ancient occurrences suggest that the enigmatic but widespread ancient structures could have been formed by microbial processes. The relative abundance of these structures in siliciclastic facies of this interval further suggests that prior to the onset of relatively intense bioturbation in the Ordovician, microbial mats could have played a significant role in the evolution and diversification of early life in a broad variety of sea-floor environments. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A model for diurnal patterns of carbon fixation in a Precambrian microbial mat based on a modern analog

Microbial mat communities are one of the first and most prevalent biological communities known from the Precambrian fossil record. These fossil mat communities are found as laminated sedimentary rock structures called stromatolites. Using a modern microbial mat as an analog for Precambrian stromatolites, a study of carbon fixation during a diurnal cycle under ambient conditions was undertaken. The rate of carbon fixation depends primarily on the availability of light (consistent with photosynthetic carbon fixation) and inorganic carbon, and not nitrogen or phosphorus. Atmospheric PCO2 is thought to have decreased from 10 bars at 4 Ga (10(9) years before present) to approximately 10(-4) bars today, implying a change in the availability of inorganic carbon for carbon fixation. Experimental manipulation of levels of inorganic carbon to levels that may have been available to Precambrian mat communities resulted in increased levels of carbon fixation during daylight hours. Combining these data with models of daylength during the Precambrian, models are derived for diurnal patterns of photosynthetic carbon fixation in a Precambrian microbial mat community. The models suggest that, even in the face of shorter daylengths during the Precambrian, total daily carbon fixation has been declining over geological time, with most of the decrease having occurred during the Precambrian.

Model of carbon fixation in microbial mats from 3,500 Myr ago to the present.

Biological carbon fixation is an important part of global carbon cycling and ecology. Fixation that took place 3,500 million years ago is recorded in the laminated sedimentary rock structures known as stromatolites, which are fossilized remains of microbial mat communities. Stromatolites are the most abundant type of fossil found in the Proterozoic (2,500 to 590 Myr ago), but they then declined, possibly because of predation and competition. Using modern microbial mats as analogues for ancient stromatolites, we show that the rate of carbon fixation is higher at the greater levels of atmospheric CO2 that were probably present in the past. We suggest that carbon fixation in microbial mats was not carbon-limited during the early Precambrian, but became carbon-limited as the supply of inorganic carbon decreased. Carbon limitation led to a lower rate of carbon fixation, especially towards the end of the Precambrian. Thus, another reason for the decline of the stromatolites could have been a decrease in available CO2.

Community living long before man: Fossil and living microbial mats and early life

Microbial mats are layered communities of bacteria that form cohesive structures, some of which are preserved in sedimentary rocks as stromatolites. Certain rocks, approximately three and a half thousand million years old and representing the oldest known fossils, are interpreted to derive from microbial mats and to contain fossils of microorganisms. Modern microbial mats (such as the one described here from Matanzas, Cuba) and their fossil counterparts are of great interest in the interpretation of early life on Earth. Since examination of microbial mats and stromatolites increases our understanding of long-term stability and change, within the global environment, such structures should be protected wherever possible as natural science preserves. Furthermore, since they have existed virtually from the time of life's origin, microbial mats have developed exemplary mechanisms of local community persistence and may even play roles in the larger global environment that we do not understand.

Heliotropism in Modern Stromatolites

Modern microbial mats and stromatolites exhibiting a preferred orientation toward specular sunlight were found at two sites. In Hamelin Pool of Shark Bay, Western Australia, subtidal decimeter-sized columns and intertidal centimeter-sized tufts were found pointing north. In thermal spring effluents and pools of Yellowstone National Park, mats were found with columnar and conical centimeter-sized structures inclined to the south. These examples of heliotropism in modern stromatolites are each built by a different community of photosynthetic microbes under markedly different environmental conditions. These new observations support the proposal that stromatolites can orient themselves toward the sun.

Early Biological Evolution in Relation to Mineral and Energy Resources: Igcp Project 157

Project 157 of the International Geological Correlation Program is concerned with the timing of major events in biological history (such as the advent of bacterial sulfate reduction and the development of oxygen-releasing photosynthesis) and how these events may have been related to the formation of mineral and fossil fuel deposits. Both direct and indirect forms of microbial involvement are being investigated. Examples of direct involvement would be contributions of sulfate-reducing bacteria to the formation of certain stratiform sulfide mineral deposits and massive accumulation of microbial remains to form bog head coals and oil shales. An example of indirect involvement would be photosynthetic oxygenation of the Proterozoic atmosphere providing conditions amenable to the formation of red bed copper deposits. Two aspects of Project 157 are directly applicable to the search for petroleum reserves. One is the subproject thut deals with pre-Devonian crude oils; a major conclusion of our work in this area is that many Proterozoic sedimentary sequences have oil and gas potential and should not be neglected by explorationists. The other is the subproject that deals with organic diagenesis of modern microbial mat communities; among other things, work in this area should lead to the recognition of new biological tracer compounds that can be used for oil-to-source correlations.

Stromatolite morphogenesis—progress and problems

Stromatolites are laminated, lithified, sedimentary growth structures that accrete away from a point or limited surface of attachment. They are commonly, but not necessarily, of microbial origin and calcareous composition. Although familiar to geologists, they remain enigmatic as to origin and uncertain as to their full potential for historical geology.We summarize here the results of collective inquiry and discussion concerning central problems of stromatolite morphogenesis. We focus on relations between microstructure, laminar details, and gross morphology of ancient, probably biogenic, stromatolites and on the microbial composition and laminar characteristics of analogous modern microbial mats and sedimentary structures produced by them.The basic microstructure-determining unit of the modern stromatolite analog of biogenic origin is the mat-building community of organisms and particularly its dominant species, operating within a particular ecological setting. Biological factors dominate at the laminar and sublaminar level. Gross morphology probably reflects a balance between ecological processes and microbial activity.Nomenclatural clarity and stratigraphic utility turn on a proper understanding of morphogenesis. Although stromatolites will continue to function as stratigraphic and paleoecologic indicators, the reasons behind and limits to their applicability will remain obscure while morphogenetic processes remain uncertain. Finally, we outline some problems whose resolution could reduce that uncertainty.

On the experimental silicification of microorganisms. III. Implications of the preservation of the green prokaryotic alga prochloron and other coccoids for interpretation of the microbial fossil record

Abstract Evidence for Precambrian fossil eukaryotic microorganisms has been based on: (1) the presence of internal ‘spots’ which have been variously interpreted to be remains of nuclei or pyrenoids of photosynthetic plastids or other organelles; (2) tetrahedral tetrad arrangement of cells; (3) trilete scars interpreted to be indicative of meiotic division: (4) large cell diameters; and (5) putative mitotic cell divisions. These features have been reported in fossils preserved in Precambrian cherts. We have studied modern microbial mats, thought to be analogues of Precambrian fossil communities, and found they may be silicified by laboratory procedures. In microbial mats from Baja California we have found many ‘spot cells’ that we could identify as remains of cyanophytes. We have silicified the newly discovered large prokaryotic coccoid green alga Prochloron and have found that it, like many cyanophytes previously silicified, preserves its structure and maintains its initial dimensions. In laboratory-silicified prokaryotic organisms we have found that all of the above criteria, supposedly characteristic of eukaryotes, can be observed. We conclude that there is no compelling morphological evidence for fossil eukaryotic microbes from Precambrian cherts.