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Microbialites: Organosedimentary Deposits of Benthic Microbial Communities

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  • Australian National University & University of Queensland

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Microbialites are organosedimentary deposits formed from interaction between benthic microbial communities (BMCs) and detrital or chemical sediments. Processes involved in the formation of calcareous microbialites include trapping and binding of detrital sediment (forming microbial boundstones), inorganic calcification (forming microbial tufa), and biologically influenced calcification (forming microbial framestones). Microbialites contrast with other biological sediments in that they are generally not composed of skeletal remains. -from Authors
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... Biofilms are made up of secreted extracellular polymeric substances (EPS) that serve as protection for the cells, provide the exchange of metabolites, and work as a buffer against environmental factors, such as sudden changes in salinity, mechanical stresses, radiation and many others (Decho, 1999). By the trapping and binding of sedimentary particles and in situ mineral precipitation, the biofilms may construct organosedimentary structures called stromatolites (Hofmann, 1969;Walter, 1976;Burne and Moore, 1987;Riding, 2011;Noffke and Awramik, 2013). These are the result of biologically induced precipitation of minerals, especially carbonate, in a laminated form, whose process of mineral precipitation and bacterial growth is repeated until the structure grows vertically (Burne and Moore, 1987;Riding, 2011;Noffke and Awramik, 2013). ...
... By the trapping and binding of sedimentary particles and in situ mineral precipitation, the biofilms may construct organosedimentary structures called stromatolites (Hofmann, 1969;Walter, 1976;Burne and Moore, 1987;Riding, 2011;Noffke and Awramik, 2013). These are the result of biologically induced precipitation of minerals, especially carbonate, in a laminated form, whose process of mineral precipitation and bacterial growth is repeated until the structure grows vertically (Burne and Moore, 1987;Riding, 2011;Noffke and Awramik, 2013). These structures can follow different morphologies such as domical, stratiform, flat layered, columnar, branched, conical, among others, generally reflecting the environmental parameters in which the structure grew up (Hoffman, 1976;Walter, 1976;Grotzinger, 1989;Awramik, 1992;Hofmann, 2000;Zhang et al., 2021). ...
... But not all microbial mats develop a stromatolite; it depends on the microbiota composition, the dominant metabolisms and the surrounding geochemical environment (Havemann and Foster, 2008;Foster and Green, 2011). Stromatolites, in general, are formed mainly by photosynthesizing microorganisms, especially filamentous cyanobacteria (Awramik and Margulis, 1974;Burne and Moore, 1987;Riding, 2011;Noffke and Awramik, 2013), but its microbial diversity may be greater, depending on the environmental parameters. Foster and Green (2011), in an experiment with cultivation-independent molecular techniques in several modern stromatolites, attested to a greater microbial diversity in marine and hypersaline stromatolites. ...
Article
Full-text available
Deciphering the evolution of ecological interactions among the metabolic types during the early diversification of life on Earth is crucial for our understanding of the ancient biosphere. The stromatolites from the genus Conophyton cylindricus represent a datum for the Proterozoic (Meso to Neoproterozoic) on Earth. Their typical conical shape has been considered a result of a competition between microorganisms for space, light and nutrients. Well-preserved records of this genus from the “Paleontological Site of Cabeludo”, Vazante Group, São Francisco Craton (Southern Brazil) present in situ fossilized biofilms, containing preserved carbonaceous matter. Petrographic and geochemical analyses revealed an alternation between mineral laminae (light grey laminae) and fossilized biofilms (dark grey laminae). The dark grey laminae comprise three different biofilms recording a stratified microstructure of microbial communities. These three biofilms composing the dark grey laminae tend to be organized in a specific pattern that repeats through the stromatolite vertical section. Iron and manganese are distributed differently along the dark and light grey laminae; X-ray absorption and luminescence data showed possible different areas with authigenic iron and iron provided from diagenetic infiltration. Cryptocrystalline apatite in the lowermost biofilms in each dark grey laminae may suggest past metabolic activity of sulfide-oxidizing bacteria. These findings suggest that the microorganisms reached a complex metabolic diversification in order to maintain an equilibrium situation between the three different biofilms along the vertical section of the structures, thus benefiting the whole microbial community. This means that the stromatolites from the Conophyton genus may have formed as a result of a greater complexity of interactions between microorganisms, and not only from competition between photosynthesizers.
... Microbialites are defined by Burne and Moore (1987) as ''organosedimentary deposits that have accreted as a result of a benthic microbial community trapping and binding detrital sediment and/or forming the locus of mineral precipitation.'' These structures have diverse morphogenesis resulting from environmental influence, biologic and ecologic controls, and processes and rates of lithification (Burne and Moore 1987). ...
... Microbialites are defined by Burne and Moore (1987) as ''organosedimentary deposits that have accreted as a result of a benthic microbial community trapping and binding detrital sediment and/or forming the locus of mineral precipitation.'' These structures have diverse morphogenesis resulting from environmental influence, biologic and ecologic controls, and processes and rates of lithification (Burne and Moore 1987). Recent microbialites have gained importance after the discovery of giant petroleum reservoirs in the Brazilian pre-salt section (Barra Velha Formation, Santos Basin) as they are considered as among the possible analogs to understanding the processes involved in the formation of limestone facies of potential microbial origin (Estrella et al. 2009). ...
... So, they displaced the matrix because they were deposited on the bottom of the lagoon, where microbial mats flourished. Then, they got trapped into its structure by the sticky EPS, as seen in Burne and Moore (1987). As the layers of the microbial mats grew, mucous-rich cyanobacteria covered the grains, preserving them. ...
Article
The presence of microbialites in the hypersaline lagoons of Rio de Janeiro is especially important in the study of recent analogs of carbonate rocks with microbial origins, mostly after the discovery of giant petroleum reservoirs in the Brazilian pre-salt section and their similarities with stromatolites from Lagoa Salgada (Rio de Janeiro State). Many studies have been conducted to analyze the biology, geochemistry, mineralogy, and geomicrobiology of these microbialites. This paper, however, focuses on the petrography, sedimentology, and geochemistry of recent and superficial microbial mats from Lagoa Vermelha to understand the interaction of carbonate and siliciclastic grains with an organic matrix and discuss their similarities and differences with pre-salt rocks. A sedimentologic description was performed to understand the sediment dynamics in microbial mats. A petrographic description involved the characterization of components and textures in microscale. Furthermore, geochemical analyses were performed using scanning electron microscopy and X-ray diffraction for detailed mineralogical characterization. This multitechnique study showed the lamellar and cracked texture of the matrix being displaced by biologically induced carbonate growth and siliciclastic grains. In addition, chemical analysis showed the concentration of magnesium and silica in the matrix, with the absence of Mg-clay minerals. Even though the studied microbial mats present relevant similarities with some pre-salt facies, a microbially dominated genesis for the pre-salt limestones cannot be supported by the studied data.
... Organo-mineralisation processes cause precipitation of minerals within microbial mats (Dupraz et al., 2009;Eymard et al., 2020;Konhauser, 2007), which usually consist of a variety of cyanobacterial communities (Aref et al., 2020;Cardoso et al., 2019;Jorgensen et al., 1983;van Gemerden, 1993;Zhao et al., 2020). The early diagenetic precipitation of minerals in the mats leads to the mat lithification (Arp et al., 2001;Awramik and Sprinkle, 1999;Riding, 2011;Sarkar et al., 2020), and results in the formation of organo-sedimentary deposits, also known as microbialites (Burne and Moore, 1987). Microbial fingerprints in carbonates (Choudhuri et al., 2020;Della Porta, 2015;Dupraz et al., 2009;Köhler et al., 2013;Sánchez-Román et al., 2014;Visscher et al., 1998), 4 siliceous deposits (Baele et al., 2008;Handley et al., 2008;Konhauser et al., 2001;Konhauser and Jones, 2011), phosphorites (Cosmidis et al., 2013;Salama et al., 2015) and ironstones (Burkhalter, 1995;Dahanayake and Krumbein, 1986;Lazǎr et al., 2013;Lin, 2016;Lin et al., 2019;Salama et al., 2013) have been extensively reported before. ...
Article
This study investigates the authigenically formed Late Cretaceous and Paleogene marine ironstones in Western Siberia (Russia) based on petrographic and spectroscopic investigations. In the Upper Cretaceous Kuznetsovo, Ipatovo, Slavgorod, Gan'kino and Paleogene Lyulinvor Formations of the West Siberian ironstone basin, there are three main ore horizons, known as the Narym, Kolpashevo, Bakchar layers. The marine succession predominantly comprises ooidal or peloidal ironstones, glauconitolites, glauconitic rocks, sandstones, siltstones, claystones and gritstones. The ooidal ironstone exhibits predominantly abiogenic precipitates with subordinate microbial signatures, including micro-oncoids, relicts of lipids, carbohydrates and microfilaments. Iron-rich ooids, peloids and micro-oncoids formed primarily by adsorption of iron and occasionally by microbial iron-oxidising and sulphate-reduction actions. The abiogenic formation of ooid and peloid depends on physico-chemical conditions of environment during ion adsorption. While berthierine-goethite in ooid cortex formed in an oxic or dysoxic environment, siderite laminae therein represent an anoxic condition. Microbial mediation influenced the precipitation of berthierine and goethite, pyrite, siderite, greigite, pyrrhotite, wurtzite, barite, As-Ni-Co-Fe sulphide, and probably monazite. Siderite, pyrite, and to a lesser extent greigite and pyrrhotite formed exclusively within organic remains. Goethite with a high content of phosphate within micro-oncoids probably formed with the mediation of microbial activities. Bacterial microfilaments and Raman peaks of lipids and carbohydrates strongly support a bacterial origin of micro-oncoids. Abundant intraclasts at the base of the Ipatovo Formation that marks the Coniacian-Santonian boundary, and those within the Lyulinvor Formation that marks the Palaeocene-Eocene boundary, correspond to active tectonics, which provided an enhanced supply of metal-rich nutrients. Distribution of trace elements in the main iron-rich minerals supports a hydrothermal source of iron and other metals for iron-rich ooids and peloids in Western Siberia. The study of the Meso-Cenozoic marine ironstones provides insights into the biotic or abiotic processes involved in case of Precambrian banded iron formations (BIFs).
... Microbialites are organosedimentary structures formed due to microbial-mediated sedimentation or accretion of minerals (Burne and Moore, 1987). The massive development of microbialites is dated to the Proterozoic, when epicontinental alkaline geochemical settings were widely represented on Earth (Zavarzin, 1993;Stüeken et al., 2015). ...
Article
Modern microbialites formed in soda-saline and soda lakes are of interest as model systems for studying geobiological interactions in the Precambrian, when such geochemical settings were widespread. This work describes the structure and mineral composition of microbialites from the soda-saline Laguna de Los Cisnes (Isla Grande, Tierra del Fuego, Chile), and also characterizes the biodiversity of microorganisms involved in their formation. The microbialites consist mainly of carbonate minerals, of which monohydro-calcite is of particular interest. It was shown that the formation of microbialites occurs under alkaline conditions in the presence of taxonomically and functionally diverse microorganisms and in direct contact with exopolysaccharides produced by the microbial community.
... The growth of stromatolites is influenced by biological and paleo-environmental factors, such as morphological algal mat, microenvironment, microbial community, and ecosystem (Burne and Moore 1987;Dupraz et al. 2009;Golubic 1976). In the lacustrine environments, the changes of d 18 O are commonly attributed to temperature or precipitation/ evaporation ratio (Leng and Marshall 2004). ...
... Less abundant siliciclastic rocksare also found within the dominantly carbonate sequence (Fragoso et al. 2008). The carbonates belong to the Nova América stratigraphic unit (Bonfim et al. 1985) and exhibit microbial laminites (Burne and Moore, 1987;Riding, 1991;Dupraz et al., 2009) with tepee structures. This unit is interpreted to have been deposited in a supratidal environment and is overlain by intraclasts/peloidal calcarenites with planar and trough cross-stratification, which are interpreted as deposits of intertidal calcareous sandflats. ...
Article
We applied field structural data and isotope geochemical (δ13C, δ18O, and 87Sr/86Sr) analyses to understand the relationship among calcite veins, fault damage zones, and carbonate host rocks in a thrust fault damage zone in the Achado quarry, Irecê Basin in the São Francisco Craton, NE Brazil. Our results reveal three hydrologic packages with different rheological behaviors in a stratified carbonate succession. The upper package includes the Achado fault damage zone that is characterized by interlayered dolomitized grainstones and mudstones. These rocks display high positive δ13C values (10‰ to 13‰), negative δ18O values (mean ‐6.34‰), and radiogenic (87Sr/86Sr) isotope values (0.70885 to 0.71519). A second package marked by a cataclastic brittle shear zone lateral parallel to dolograinstones bedding. These rocks show low to positive δ13C values (‐3.41‰ to +8.85‰), more positive δ18O values (mean ‐3.73‰), and radiogenic (87Sr/86Sr) isotope values (0.71039 to 0.71373). The lower package is characterized by well‐preserved pristine limestone succession that shows δ13C values ranging between ‐0.46‰ to +3.17‰, mean δ18O = ‐5.41‰, and less radiogenic 87Sr/86Sr values (0.70762 to 0.70818). In contrast to the upper and intermediate packages, rocks from the lower one exhibit very low permeability and behaved as a seal for fluid migration. Fluid flow occurred several times during basin evolution, e.g., along syn‐rift fault damage zones, bedding‐parallel carbonate breccia, thrust faults, cataclastic shear zones, synorogenic conjugate shear fractures or joints, and opening mode‐I fracture‐fill calcite veins. These fractures allowed pervasive fluid flow in the porous intermediate cataclastic shear zone where fluids flowed and formed veins, as diffuse fluid flow in randomly oriented fracture swarms, or channelized fluid flow in aligned fracture corridors. They record significant centimeter‐scale to km‐scale hydrologic behavior within carbonate layers. Most carbonates that are associated with veins, fault damage zones, and hydraulic breccia were formed by fluids of the same origin with low δ13C (‐6.0 to ‐2.0‰) and δ18O (‐6.0 to ‐8.5‰) values, and more radiogenic 87Sr/86Sr values compared to the carbonate host rocks.
... This might be attributed to their strict depositional conditions: 1) frequent currents which help to mobilize grains, 2) high electrolyte concentration and diversity, which can enhance the attachment of grains on organics, especially EPSs, 3) relatively low CaCO 3 saturation states which are not too high to induce direct microbial calcification, and 4) grain-rich environments which can supply sufficient grains (Suarez-Gonzalez et al., 2019). Thus, trapping and binding are generally considered to only dominate the microbialite formation in agitated shallow-marine settings (Burne and Moore, 1987;Chow and George, 2004;Suarez-Gonzalez et al., 2019). ...
Article
Travertine-depositing hot springs can generate various carbonate minerals with or without the participation of microorganisms. They thus serve as good natural laboratories to study abiotic and biotic factors controlling the precipitation of different carbonate minerals and CaCO3 polymorphism. Through tens of years' investigations on travertines, considerable advances about carbonate mineral precipitation have been made. Here, we summarized general hydrological features of travertine systems, and driving forces, microenvironments, and mechanisms of the precipitation of different carbonate minerals in travertine-depositing hot springs by integrating present advances and conducting hydrochemical simulations. Travertine-depositing hot springs can be divided into near-neutral pH hot springs and hyperalkaline hot springs and contain four types of microenvironments: hypogean solid-water interface, epigean solid-water interface, air-water interface, and subaerial exposure surface. Both abiotic (passive CO2 degassing, atmospheric CO2 uptake, evaporation, and fluid mixing) and biotic (metabolism, organism-related crystal nucleation, and trapping and binding) processes may drive carbonate formation. The specific driving forces, however, depend on bulk water hydrochemistry and microenvironments. Calcite and aragonite are the most common minerals in travertines and the calcite-aragonite polymorphism might be under decisive influences from bulk water chemistry, especially [Mg2+]/[Ca2+], in some conditions. However, calcium carbonate precipitation in hot springs is controlled by solute transport in microenvironments, and any local change induced by hydrodynamics, microorganisms, and extracellular polymeric substances may significantly modify CaCO3 precipitation and polymorphism. Such integrated control from bulk water composition and microenvironments also affects the formation of other carbonate minerals, but their exact roles remain unclear. Overall, despite the fruitful recent findings, further investigations, especially those focusing on microenvironments, are still imperative to better understand carbonate precipitation in hot springs. These microenvironment-scale studies might also provide insights on carbonate precipitation in other environments.
... Maciej Bąbel 1 , Danuta Olszewska-Nejbert 1 , Barbara Kremer 2 , Andrii Bogucki 3 , Andrii Yatsyshyn 3 & Maciej Śliwiński 4 Gypsum microbialites (GM) can be defined as organosedimentary deposits which have accreted as a result of a benthic microbial community trapping and binding detrital gypsum sediment and/or forming the locus of gypsum precipitation (Burne & Moore 1987). Two main depositional mechanisms forming GM are thus considered and form the basis of distinguishing between two groups of GM: (i) trapping and binding (Group A) and (ii) inorganic gypsum precipitation or precipitation mediated or influenced by biological processes (Group B). ...
Conference Paper
Full-text available
Gypsum microbialites (GM) can be defined as organosedimentary deposits which have accreted as a result of a benthic microbial community trapping and binding detrital gypsum sediment and/or forming the locus of gypsum precipitation. Two main depositional mechanisms forming GM are thus considered and form the basis of distinguishing between two groups of GM: (i) trapping and binding (Group A) and (ii) inorganic gypsum precipitation or precipitation mediated or influenced by biological processes (Group B). In the Badenian (Middle Miocene) evaporites of the Carpathian Foredeep (best exposed in Ukraine and in Poland), GM are very common and variable. Group B is represented by gypsum encrusted (or gypsified) microbial mats containing fenestral structures and showing wavy, crinkled laminations (sometimes only ghosts thereof) reflecting the morphology of the living mats prior to gypsification during periods of increased evaporation. The less common Group A is represented by crudely laminated, flat-topped domes, most commonly composed of coarse-grained gypsum. These domes occur in channels filled with clastic laminated gypsum and as intercalations within grass-like selenites (i. e. rows of large, bottom-grown gypsum crystals) (Fig. 1e). Some domes show asymmetric or elongated shapes interpreted as having been modeled by waves or currents. GM are commonly associated with selenite crystals, forming specific selenite-gypsum microbialite facies, typified by large domal structures and considered to be both microbial and chemical in origin. The gypsum microbialites of the Badenian accreted on semi-emerged shoals (Group B) and in shallow channels and on downwind shores of saline pans (Groups A and B) of a very shallow evaporite basin.
Article
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Microbialites and peloids are commonly associated throughout the geologic record. Proterozoic carbonate megafacies are composed predominantly of micritic and peloidal limestones often interbedded with stromatolitic textures. The association is also common throughout carbonate ramps and platforms during the Phanerozoic. Recent investigations reveal that Hamelin Pool, located in Shark Bay, Western Australia, is a microbial carbonate factory that provides a modern analog for the microbialite-micritic sediment facies associations that are so prevalent in the geologic record. Hamelin Pool contains the largest known living marine stromatolite system in the world. Although best known for the constructive microbial processes that lead to formation of these stromatolites, our comprehensive mapping has revealed that erosion and degradation of weakly lithified microbial mats in Hamelin Pool leads to the extensive production and accumulation of sand-sized micritic grains. Over 40 km2 of upper intertidal shoreline in the pool contain unlithified to weakly lithified microbial pustular sheet mats, which erode to release irregular peloidal grains. In addition, over 20 km2 of gelatinous microbial mats, with thin brittle layers of micrite, colonize subtidal pavements. When these gelatinous mats erode, the micritic layers break down to form platey, micritic intraclasts with irregular boundaries. Together, the irregular micritic grains from pustular sheet mats and gelatinous pavement mats make up nearly 26% of the total sediment in the pool, plausibly producing ~ 24,000 metric tons of microbial sediment per year. As such, Hamelin Pool can be seen as a microbial carbonate factory, with construction by lithifying microbial mats forming microbialites, and erosion and degradation of weakly lithified microbial mats resulting in extensive production of sand-sized micritic sediments. Insight from these modern examples may have direct applicability for recognition of sedimentary deposits of microbial origin in the geologic record.
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Carbonate deposits and sedimentary records from lake in southern Patagonia provide an excellent contribution in the regional environmental register. Laguna Timone maar crater is situated in the Pali Aike Volcanic Field, a Quaternary volcano-tectonic complex in southern Patagonia and represent one of hundreds of “pools” of brines developed after explosive volcanic eruptions in a periglacial environment. The lake constitutes an endorheic hydrological system where processes leading to carbonate precipitation under extreme physicochemical conditions and biological influences can be explored. Laguna Timone is recharged by groundwater and sporadic episodes of precipitation (ca. 200 mm per year). The strong winds regimes are responsible for high evaporation rates in this area. Carbonate precipitation was studied in microbialites fragments of tufa deposits and carbonate crust located in the edge of lake. The mineralogy of both is composed by calcite … The clay fraction of sediment that underlying carbonates was characterized. HRTEM analysis shows that authigenic smectites has influence on calcite crystals precipitation. The positive δ¹⁸O values (2.28‰) in the crust are associated to evaporation processes. In contrast, the δ¹⁸O values (-6.52‰) in the microbialites shows meteoric and/or groundwater influences. The δ¹³C (-0.43 ‰ to 2.50 ‰) values indicate physico-chemical and biochemical processes controlling the precipitation. The carbonate precipitation involves the interrelations of hydrogeological properties, climate and biological influences. Laguna Timone provides a natural laboratory for understanding of mineral precipitation processes that register continuous climatic and environmental archives
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Extensive laminated mats of algae are forming on the protected intertidal and supratidal flats of a highly saline lagoon, the Khor al Bazam, Abu Dhabi, southwest Persian Gulf. At the east end of the lagoon, the largest algal flat parallels the coast for 42 kilometers, and to the west another smaller one parallels the coast for nine kilometers. These flats, part of the seaward edge of a prograding coastal flat, have an average width of approximately two kilometers and a thickness of at least thirty centimeters. In some areas they extend landward in the subsurface for more than two kilometers, beneath a thin cover of evaporites and wind blown and storm washover sediments. Smaller flats occur in the shelter of islands, headlands, and swash bars. The larger algal flats are divided on the basis of surface morphology, into four geographical belts. From the high-water mark seaward these are: (1) Flat zone--firm, smooth algal mat, with no topographic relief, overlying quartz-rich carbonate sand and evaporites; (2) Crinkle zone--leathery algal skin forming a blistered surface over gypsum and carbonate mush; (3) Polygonal zone--algal mat separated into desiccation polygons a few centimeters to two meters in diameter, which cover laminated algal peat; carbonate sand and mud fills the cracks between the polygons. (4) Cinder zone--a warty black algal surface, the color and size of the raised bumps resembling a weathered volcanic cinder layer. These bumps, shaped like small pustules two to three centimeters in diameter, cap an unlaminated algal and sediment peat. The algal growth and structures appear to be determined by the frequency and duration of subaerial exposure and the salinity of the tidal waters; they are only related to wave energy when limited by wave and tidal scour at the edge of the Cinder zone and along ebb channels.
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Three textural features seem especially useful in classifying those carbonate rocks that retain their depositional texture (1) Presence or absence of carbonate mud, which differentiates muddy carbonate from grainstone; (2) abundance of grains, which allows muddy carbonates to be subdivided into mudstone, wackestone, and packstone; and (3) presence of signs of binding during deposition, which characterizes boundstone. The distinction between grain-support and mud-support differentiates packstone from wackestone—packstone is full of its particular mixture of grains, wackestone is not. Rocks retaining too little of their depositional texture to be classified are set aside as crystalline carbonates.
Chapter
Cohesive benthic microbial communities (BMC) are common in Australian salt lakes. They are dominated by photosynthetic prokaryotes and, less frequently, eukaryotic microalgae. Structural integrity is derived primarily from filamentous prokaryotes or stalked diatoms. The organic matter produced by in situ (or external) photosynthetic CO2 fixation is then decomposed by a series of microbial processes culminating in sulfate reduction and methanogenesis, from which the final carbon degradation products are CO2 and CH4. Degradative processes are most active just below the photic zone but are also interspersed throughout it. BMC are characteristically laminated, reflecting the spatial organization of the various physiological groups present and their interaction with each other and with the physicochemical environment. The totality is a dynamic equilibrium exhibiting steep gradients and striking diurnal fluctuations. Community members must adopt suitable metabolic and behavioural strategies to deal with these and other, less regular, fluctuations in parameters such as salinity and desiccation. Still unresolved are questions concerning the following topics: the quantitative importance of BMC primary production in relation to that of phytoplankton and aquatic macrophytes, the environmental factors promoting formation of BMC and their ability to compete with other primary producers, the quantitative significance of anoxygenic photoautotrophy in BMC, the effect of meiofauna on internal elemental cycling and exchange with the overlying water, and the significance of photoheterotrophy at high salinities.
Chapter
Stromatolites built by the coccoid cyanophyte Entophysalis major in Shark Bay, W. Australia undergo seasonal lithification by carbonate precipitation within the polysaccharide envelopes of the organism. Mineral incorporation obliterates the biological structures. The lithified stromatolite surface is then colonized by destructive, carbonate penetrating microbial endoliths. Stromatolite growth resumes when Entophysalis recolonizes the surface. Entophysalis stromatolites serve as a direct interpretational model for domal stromatolites built by Eoentophysatis (a silicified microfossil) in Precambrian strata. Eoentophysalis stromatolites occurred worldwide over a period of 1 Ga in the Precambrian (1.9–0.9 Ga). Entophysalis may be its direct descendant.