Evidence of oxygenic phototrophy in ancient phosphatic stromatolites from the Paleoproterozoic Vindhyan and Aravalli Supergroups,India

Fossil microbiotas are rare in the early rock record, limiting the type of ecological information extractable from ancient microbialites. In the absence of body fossils, emphasis may instead be given to microbially derived features, such as microbialite growth patterns, microbial mat morphologies, and the presence of fossilized gas bubbles in lithified mats. The metabolic affinity of micro‐organisms associated with phosphatization may reveal important clues to the nature and accretion of apatite‐rich microbialites. Stromatolites from the 1.6 Ga Chitrakoot Formation (Semri Group, Vindhyan Supergroup) in central India contain abundant fossilized bubbles interspersed within fine‐grained in situ‐precipitated apatite mats with average δ¹³C_org indicative of carbon fixation by the Calvin cycle. In addition, the mats hold a synsedimentary fossil biota characteristic of cyanobacterial and rhodophyte morphotypes. Phosphatic oncoid cone‐like stromatolites from the Paleoproterozoic Aravalli Supergroup (Jhamarkotra Formation) comprise abundant mineralized bubbles enmeshed within tufted filamentous mat fabrics. Construction of these tufts is considered to be the result of filamentous bacteria gliding within microbial mats, and as fossilized bubbles within pristine mat laminae can be used as a proxy for oxygenic phototrophy, this provides a strong indication for cyanobacterial activity in the Aravalli mounds. We suggest that the activity of oxygenic phototrophs may have been significant for the formation of apatite in both Vindhyan and Aravalli stromatolites, mainly by concentrating phosphate and creating steep diurnal redox gradients within mat pore spaces, promoting apatite precipitation. The presence in the Indian stromatolites of alternating apatite‐carbonate lamina may result from local variations in pH and oxygen levels caused by photosynthesis–respiration in the mats. Altogether, this study presents new insights into the ecology of ancient phosphatic stromatolites and warrants further exploration into the role of oxygen‐producing biotas in the formation of Paleoproterozoic shallow‐basin phosphorites.     

Modern marine stromatolites in the Exuma Cays,Bahamas: Uncommonly common

Summary Modern stromatolites in open marine environments, unknown until recently, are common throughout the Exuma Cays, Bahamas. They occur in three distinct settings: subtidal tidal passes, subtidal sandy embayments and intertidal beaches. These stromatolites have a relief of up to 2.5 m and occur in water depths ranging from intertidal to 10 m. Surfaces near the sediment-water interface are typically colonized by cyanobacterial mats, whereas high relief surfaces are commonly colonized by algal turf and other macroalgae such asBatophora, Acetabularia, andSargassum. The internal structure of the stromatolites is characterized by millimeter-scale lamination defined by differential lithification of agglutinated sediment. In thin section, the lithified laminae appear as micritic horizons with distinct microstructures: they consist of thin micritic crusts (20–40 μm thick) overlying layers of micritized sediment grains (200–1000 μm thick); the micritized grains are cemented at point-contacts and are trucated along a surface of intense microboring. The Exuma stromatolites are built by cyanobacterial-dominated communities. These laminated prokaryotic structures grade to unlayered thrombolites built by eukaryotic algae. The variety of sites, settings and shapes of stromatolites in the Exuma Cays present excellent opportunities for future studies of stromatolite morphogenesis.     

THE ROLE OF DIATOMS IN STROMATOLITE GROWTH: TWO EXAMPLES FROM MODERN FRESHWATER SETTINGS1

Some modern laminated and calcified stromatolitic structures are partially or completely formed by eukaryotes. Diatom populations in freshwater environments with elevated ionic concentrations contribute to calcite precipitation, and the formation of distinctive mineral-rich stromatolitic laminae. Two types of stromatolite-forming diatom populations were observed. In the first example, in stromatolies growing on a quarry ledge near Laegerdorf, North Germany, calcite crystals with biogenic imprints form around polysaccharide stalks of the diatom Gomphonema olivaceum var. calcarea (Cleve) Cleve-Euler. These individually precipitated crystals eventunally become cemented together in layers, forming rigid, laminated stromatolitic deposits which drape over the quarry ledge. In the second example, in stromatolites forming in a shallow stream near Cuatro Ciénegas, Coahuila, Mexico, diatomaceous laminae also form by the accumulation of carbonate particles in a matrix of diatoms and their extracellular polysaccharide products. These laminae become thick enough to drape over individual stromatolite heads. The diatoms responsible for these deposits are Amphora aff. A. Katii Selva, Nitzschia denticula Grun., and six other species. At Cuatro Ciénegas, in addition to the diatomaceous laminae, carbonate-rich cyanobacterial layers, dominated by two cyanobacterial species with different fabrics and porosities, are also present and contribute substantially to the growth of the stromatolites. In both the Laegerdorf and Cuatro Ciénegas examples, entire stromatolites or thick laminations on stromatolites are built by a small number of diatom species which produce copious amounts of extracellular stalk, gel, and sheath material, a property they share with cyanobacterial stromatolite builders.     

Quantifying CaCO3 Microprecipitates Within Developing Surface Mats of Marine Stromatolites Using GIS and Digital Image Analysis

The unique geochemical coupling of organic molecules and mineral CaCO₃ provides a fluorescence signature detectable using conventional confocal scanning laser microscopy (CSLM). The surface microbial mats of open-water marine stromatolites (Bahamas) exist in a continuum of states ranging from a Type 1 (i.e., nonlithifying) to Type 2 (i.e., lithified micritic laminae present) to Type 3 (i.e., fused grain layer). An approach was developed here, that utilizes geographical information systems (GIS) and digital image analysis, coupled with CSLM to estimate concentrations of calcium carbonate precipitates in developing marine stromatolites. We propose that the area occupied by particles within each image can be used to estimate concentrations of precipitates. Fluorescent polymeric microbeads and bacteria were used to calibrate the approach. We used this approach to demonstrate that CaCO₃ precipitates in lithifying layers were quantifiable and significantly different (p < 0.0001) from those in nonlithifying layers. The approach provided a useful tool for the unambiguous assessment of relative changes in microbial precipitates occurring over small ( μ m to mm) spatial scales, and that characterize the formation of lithified layers (micritic laminae) in open-water marine stromatolites.     

THE ROLE OF DIATOMS IN STROMATOLITE GROWTH: TWO EXAMPLES FORM MODERN FRESHWATER SETTINGS1

Some modern laminated find calcified stromatolitic structures are partially or completely formed by eukaryotes. Diatom populations in freshwater environments with elevated ionic concentrations contribute to calcite precipitation, and the formation of distinctive mineral-rich stromatolitic laminae. Two types of stromatolite-forming diatom populations were observed. In the first example, in stromatolites growing on a quarry ledge near Laegerdorf, North Germany, calcite crystals with biogenic imprints form around polysaccharide stalks of the diatom Gomphonema olivaceum var. calcarea (Cleve) Cleve-Euler. These individually precipitated crystals eventually become cemented together in layers, forming rigid, laminated stromatolitic deposits which drape over the quarry ledge. In the second example, in stromatolites forming in a shallow stream near Cuatro Ciénegas, Coahuila, Mexico, diatomaceous laminae also form by the accumulation of carbonate particles in a matrix of diatoms and their extracellular polysaccharide products. These laminae become thick enough to drape over individual stromatolite heads. The diatoms responsible for these deposits are Amphora aff. A. katii Selva, Nitzschia denticula Grun., and six other species. At Cuatro Ciénegas, in addition to the diatomaceous laminae, carbonate-rich cyanobacterial layers, dominated by two cyanobacterial species with different fabrics and porosities, are also present and contribute substantially to the growth of the stromatolites. In both the Laegerdorf and Cuatro Ciénegas examples, entire stromatolites or thick laminations on stromatolites are built by a small number of diatom species which produce copious amounts of extracellular stalk, gel, and sheath material, a propertuy they share with cyanobacterial stromatolite builders.     

Optimization of sulphur production in a biotechnological sulphide-removing reactor

A new biotechnological process for sulphide removal is proposed. The principle of this process is that sulphide is converted into elemental sulphur, which can be removed by sedimentation. In this article, investigations on the optimization of the sulphur production are reported. It seems that less than 10% sulphate is produced at low oxygen concentration, when the sulphide concentration in the reactor exceeds 10 mg/L. At sulphide concentrations higher than 20 mg/L only 5% of the incoming sulphide is converted to sulphate even at high oxygen concentrations. An immobilized biomass on recticulated polyurethane produced more sulphate than a free cell suspension at the same oxygen and sulphide concentration.     

Comparative Characterization of the Microbial Diversities of an Artificial Microbialite Model and a Natural Stromatolite

Microbialites are organosedimentary structures that result from the trapping, binding, and lithification of sediments by microbial mat communities. In this study we developed a model artificial microbialite system derived from natural stromatolites, a type of microbialite, collected from Exuma Sound, Bahamas. We demonstrated that the morphology of the artificial microbialite was consistent with that of the natural system in that there was a multilayer community with a pronounced biofilm on the surface, a concentrated layer of filamentous cyanobacteria in the top 5 mm, and a lithified layer of fused oolitic sand grains in the subsurface. The fused grain layer was comprised predominantly of the calcium carbonate polymorph aragonite, which corresponded to the composition of the Bahamian stromatolites. The microbial diversity of the artificial microbialites and that of natural stromatolites were also compared using automated ribosomal intergenic spacer analysis (ARISA) and 16S rRNA gene sequencing. The ARISA profiling indicated that the Shannon indices of the two communities were comparable and that the overall diversity was not significantly lower in the artificial microbialite model. Bacterial clone libraries generated from each of the three artificial microbialite layers and natural stromatolites indicated that the cyanobacterial and crust layers most closely resembled the ecotypes detected in the natural stromatolites and were dominated by Proteobacteria and Cyanobacteria. We propose that such model artificial microbialites can serve as experimental analogues for natural stromatolites.     

Grain trapping by filamentous cyanobacterial and algal mats: implications for stromatolite microfabrics through time

Archean and Proterozoic stromatolites are sparry or fine‐grained and finely laminated; coarse‐grained stromatolites, such as many found in modern marine systems, do not appear until quite late in the fossil record. The cause of this textural change and its relevance to understanding the evolutionary history of stromatolites is unclear. Cyanobacteria are typically considered the dominant stromatolite builders through time, but studies demonstrating the trapping and binding abilities of cyanobacterial mats are limited. With this in mind, we conducted experiments to test the grain trapping and binding capabilities of filamentous cyanobacterial mats and trapping in larger filamentous algal mats in order to better understand grain size trends in stromatolites. Mats were cut into squares, inclined in saltwater tanks at angles from 0 to 75° (approximating the angle of lamina in typical stromatolites), and grains of various sizes (fine sand, coarse sand, and fine pebbles) were delivered to their surface. Trapping of grains by the cyanobacterial mats depended strongly on (i) how far filaments protruded from the sediment surface, (ii) grain size, and (iii) the mat's incline angle. The cyanobacterial mats were much more effective at trapping fine grains beyond the abiotic slide angle than larger grains. In addition, the cyanobacterial mats actively bound grains of all sizes over time. In contrast, the much larger algal mats trapped medium and coarse grains at all angles. Our experiments suggest that (i) the presence of detrital grains beyond the abiotic slide angle can be considered a biosignature in ancient stromatolites where biogenicity is in question, and, (ii) where coarse grains are present within stromatolite laminae at angles beyond the abiotic angle of slide (e.g., most modern marine stromatolites), typical cyanobacterial‐type mats are probably not solely responsible for the construction, giving insight into the evolution of stromatolite microfabrics through time.     

Stromatolitic knobs in Storr's Lake (San Salvador,Bahamas): a model system for formation and alteration of laminae

The initial lamination in young, metabolically active Scytonema knobs developing in Storr's Lake (Bahamas) results from the iterative succession of two different stages of microbial growth at the top of this microbialite. Stage 1 is dominated by vertically oriented cyanobacterial filaments and is characterized by a high porosity of the fabric. Stage 2 shows a higher microbial density with the filaments oriented horizontally and with higher carbonate content. The more developed, dense microbial community associated with Stage 2 of the Scytonema knobs rapidly degrades extracellular organic matter (EOM) and coupled to this, precipitates carbonate. The initial nucleation forms high‐Mg calcite nanospheroids that progressively replace the EOM. No precipitation is observed within the thick sheath of the Scytonema filaments, possibly because of strong cross‐linking of calcium and EOM (forming EOM‐Ca‐EOM complexes), which renders Ca unavailable for carbonate nucleation (inhibition process). Eventually, organominerals precipitate and form an initial lamina through physicochemical and microbial processes, including high rates of photosynthetic activity that lead to ¹³C‐enriched DIC available for initial nucleation. As this lamina moves downward by the iterative production of new laminae at the top of the microbialite, increased heterotrophic activity further alters the initial mineral product at depth. Although some rare relic preservation of ‘Stage 1–Stage 2’ laminae in subfossil knobs exists, the very fine primary lamination is considerably altered and almost completely lost when the knobs develop into larger and more complex morphologies due to the increased accommodation space and related physicochemical and/or biological alteration. Despite considerable differences in microstructure, the emerging ecological model of community succession leading to laminae formation described here for the Scytonema knobs can be applied to the formation of coarse‐grained, open marine stromatolites. Therefore, both fine‐ and coarse‐grained extant stromatolites can be used as model systems to understand the formation of microbialites in the fossil record.     

Bacterial oxidation of sulphide under denitrifying conditions

Anoxic H2S oxidation under denitrifying conditions produced sulphur and sulphate in almost equal proportions by an isolated Thiobacillus denitrificans. Under nitrate reducing conditions the rate of sulphide oxidation was approximately 0.9 g sulphide/g biomass h. Nitrate was reduced to nitrite and accumulated during sulphide oxidation. Above 100 mg nitrite/l, the sulphide oxidation rate declined and at 500 mg/l it was totally arrested. The optimum pH for the anoxic sulphide oxidation was around 7.5. Concentrations of sulphate 1500 mg/l and acetate 400 mg/l had no effect on anoxic sulphide oxidation.     

Discovery of large conical stromatolites in Lake Untersee, Antarctica

Lake Untersee is one of the largest (11.4 km(2)) and deepest (>160 m) freshwater lakes in East Antarctica. Located at 71°S the lake has a perennial ice cover, a water column that, with the exception of a small anoxic basin in the southwest of the lake, is well mixed, supersaturated with dissolved oxygen, alkaline (pH 10.4) and exceedingly clear. The floor of the lake is covered with photosynthetic microbial mats to depths of at least 100 m. These mats are primarily composed of filamentous cyanophytes and form two distinct macroscopic structures, one of which--cm-scale cuspate pinnacles dominated by Leptolyngbya spp.--is common in Antarctica, but the second--laminated, conical stromatolites that rise up to 0.5 m above the lake floor, dominated by Phormidium spp.--has not previously been reported in any modern environment. The laminae that form the conical stromatolites are 0.2-0.8 mm in thickness consisting of fine clays and organic material; carbon dating implies that laminations may occur on near decadal timescales. The uniformly steep sides (59.6 ± 2.5°) and the regular laminar structure of the cones suggest that they may provide a modern analog for growth of some of the oldest well-described Archean stromatolites. Mechanisms underlying the formation of these stromatolites are as yet unclear, but their growth is distinct from that of the cuspate pinnacles. The sympatric occurrence of pinnacles and cones related to microbial communities with distinct cyanobacterial compositions suggest that specific microbial behaviors underpin the morphological differences in the structures.     

Bacterially mediated precipitation in marine stromatolites

Stromatolites are laminated, lithified (CaCO₃) sedimentary deposits formed by precipitation and/or sediment accretion by cyanobacterial–bacterial mat communities. Stromatolites have been associated with these communities as far back as the Precambrian era some 2+ billion years ago. The means by which microbial communities mediate the precipitation processes have remained unclear, and are the subject of considerable debate and speculation. Two alternative explanations for microbially mediated precipitation include: (i) cyanobacterial photosynthesis increases pH in a system supersaturated in respect of CaCO₃, resulting in CaCO₃ precipitation and then laminated lithification, and (ii) decomposition of cyanobacterial extracellular organic matter (e.g. sheaths, mucilage and organic acids) by microheterotrophs leads to release of organic-bound Ca²⁺ ions and CaCO₃ precipitation. We evaluated these explanations by examining metabolically active, lithifying stromatolitic mat communities from Highborne Cay, Bahamas, using microautoradiography. Microautoradiographic detection of ¹⁴CO₂ fixation and ³H organic matter ( d -glucose and an amino acid mixture) utilization by photosynthetically active cyanobacteria and microheterotrophs, combined with community-level uptake experiments, indicate that bacteria, rather than cyanobacteria are the dominant sites of CaCO₃ deposition. In the oligotrophic waters in which stromatolites exist, microheterotrophs are reliant on the photosynthetic community as a main source of organic matter. Therefore, autotrophic production indirectly controls microbially mediated precipitation and stromatolite formation in these shallow marine environments.     

Biological sulphide oxidation in a fed-batch reactor

This study shows that, in a sulphide-oxidizing bioreactor with a mixed culture of Thiobacilli, the formation of sulphur and sulphate as end-products from the oxidation of sulphide can be controiledinstantaneously and reversibiy by the amount of oxygen supplied. It was found that at sulphide loading rates of up to 2.33 mmol7/L . h, both products can be formed already at oxygen concentrations below 0.1 mg/L. Because the microorganisms tend to form sulphate rather than forming sulphur, the oxygen concentration is not appropriate to optimize the sulphur production. Within less than 2 h, the system can be switched reversibly from sulphur to sulphate formation by adjusting the oxygen flow. This is below the minimum doubling time (2.85 h) of, e.g., Thiobacillus neapolitanus and Thiobacillus 0,(18) which indicates that one metabolic type of organism can probably perform both reactions. Under highly oxygen-limited circumstances, that is, at an oxygen/sulphide consumption ratio below 0.7 mol . h(-1) mol . h(-1) thiosulphate is abundantly formed. Because the chemical sulphide oxidation results mainly in the formation of thiosulphate, it is concluded that, under these circumstances, the biological oxidation capacity of the system is lower than the chemical oxidation capacity. The oxidation rate of the chemical sulphide oxidation can be described by a first-order process (k =-0.87 h(-1)).(c) 1995 John Wiley & Sons, Inc.     

LIPOPHILIC PIGMENTS FROM CYANOBACTERIAL (BLUE-GREEN ALGAL) AND DIATOM MATS IN HAMELIN POOL,SHARK BAY,WESTERN AUSTRALIA1

Lipophilic pigments were examined in microbial mat communities dominated by cyanobacteria in the intertidal zone and by diatoms in the subtidal and sublittoral zones of Hamelin Pool, Shark Bay, Western Australia. These microbial mats have evolutionary significance because of their similarity to lithified stromatolites from the Proterozoic and Early Paleozoic eras. Fucoxanthin, diatoxanthin, diadinoxanthin, β-carotene, and chlorophylls a and c characterized the diatom mats, whereas cyanobacterial mats contained myxoxanthophyll zeaxanthin, echinenone, β-carotene, chlorophyll a and, in some cases, sheath pigment. The presence of bacteriochlorophyll a with in the mats suggest a close association of photosynthetic bacteria with diatoms and cyanobacteria. The high carotenoids: chlorophyll a ratios (0.84–2.44 wt/wt) in the diatom mats suggest that carotenoids served a photoprotective function in this high light environment. By contrast, cyanobacterial sheath pigment may have largely supplanted the photoprotective role of carotenoids in the intertidal mats.     

Biotechnological process for sulphide removal with sulphur reclamation

A new biotechnological process for sulphide removal is proposed. The process is based on the oxidation of sulphide into elemental sulphur, which can be removed by sedimentation. In this study it was found that elemental sulphur and sulphate are the main oxidation products of the biological sulphide oxidation. The settling characteristics become worse as the sulphide concentration increases, due to polysulphide formation. The start-up phase of this biological system is very short; Only four days are needed to reduce the sulphide concentration of 100 to 2 mg/l at a HRT (Residence time) of 22 minutes. Also some environmental factors were evaluated. The optimal pH is situated in the pH-range 8.0–8.5. Significantly lower conversion rates are found at pH = 6.5 to 7.5 and pH = 9.0, while at pH = 9.5 the sulphide oxidation capacity of the system detoriates. The process temperature was 20°C, although the optimal temperature is situated in the range 25–35°C. No substrate inhibition of sulphide was found at sulphide concentrations up to 100 mg/l.     

Biotic and Abiotic Imprints on Mg-Rich Stromatolites: Lessons from Lake Salda,SW Turkey

Abstract

Modern hydrated Mg rich stromatolites are actively growing along the shallow shorelines of Lake Salda (SW Turkey). An integrated approach involving isotopic, mineralogical, microscopic, and organic/geochemical techniques along with culture-independent molecular methods were applied to various lake samples to assess the role of microbial processes on stromatolite formation. This study further explores the biosignature preservation potential of fossil stromatolites by comparing with textures, lipid profiles and isotopic composition of the modern stromatolites. Similar lipid profile and δ¹³C isotope values in active and fossil stromatolites argue that CO₂ cycling delicately balanced between photosynthetic and heterotrophic (aerobic) activity as in the active ones may have regulated stromatolite formation in the lake. A decrease in the exopolymeric substances (EPS) profile of the mat and concurrent hydromagnesite precipitation imply a critical role for EPS in the formation of stromatolite. Consistently, a discrete, discontinuous lamination and clotted micropeloidal textures with cyanobacterial remnants in the fossil stromatolites likely refer to partial degradation of EPS, creating local nucleation sites and allowing precipitation of hydrated Mg minerals and provide a link to the active microbial mat in the modern stromatolites. Our results for the first time provide strong evidence for close coupling of cyanobacterial photosynthesis and aerobic heterotrophic respiration on hydromagnesite textures involved in the stromatolite formation of Lake Salda. The creation of photosynthesis induced high-pH conditions combined with a change in the amount and properties of the EPS and the repetition of these processes over time seems to be a possible pathway for stromatolite growth in the lake. Understanding these microbial symbioses and their mineralized records may provide new insights on the formation mechanism of Mg-rich carbonates not only for terrestrial geological records but also for planetary bodies like Mars, where hydrated Mg-carbonate deposits have been identified in possible paleolake deposits at Jezero crater, the landing site of the NASA Mars 2020 rover.     

Cyanobacterial construction of hot spring siliceous stromatolites in Yellowstone National Park

Living stromatolites growing in a hot spring in Yellowstone National Park are composed of silica-encrusted cyanobacterial mats. Two cyanobacterial mat types grow on the stromatolite surfaces and are preserved as two distinct lithofacies. One mat is present when the stromatolites are submerged or at the water-atmosphere interface and the other when stromatolites protrude from the hot spring. The lithofacies created by the encrustation of submerged mats constitutes the bulk of the stromatolites, is comprised of silica-encrusted filaments, and is distinctly laminated. To better understand the cyanobacterial membership and community structure differences between the mats, we collected mat samples from each type. Molecular methods revealed that submerged mat cyanobacteria were predominantly one novel phylotype while the exposed mats were predominantly heterocystous phylotypes (Chlorogloeopsis HTF and Fischerella). The cyanobacterium dominating the submerged mat type does not belong in any of the subphylum groups of cyanobacteria recognized by the Ribosomal Database Project and has also been found in association with travertine stromatolites in a Southwest Japan hot spring. Cyanobacterial membership profiles indicate that the heterocystous phylotypes are 'rare biosphere' members of the submerged mats. The heterocystous phylotypes likely emerge when the water level of the hot spring drops. Environmental pressures tied to water level such as sulfide exposure and possibly oxygen tension may inhibit the heterocystous types in submerged mats. These living stromatolites are finely laminated and therefore, in texture, may better represent similarly laminated ancient forms compared with more coarsely laminated living marine examples.     

Halophilic algal-bacterial and cyanobacterial communities and their role in carbonate precipitation

This work studies the diversity of cyanobacterial and algal-bacterial communities of saline water bodies in the Crimean Peninsula and Altai Region. Plant-bacterial communities are described for the first time. The dependence of the production and destruction on the season and salinity of the water body is shown. The development of planktonic cyanobacteria is related to the presence of zooplankton, the development of which is controlled by hydrogen sulfide. The high hydrogen sulfide tolerance of benthic cyanobacteria secures the integrity of cyanobacterial communities. Observations in nature and laboratory modeling show that the formation of mineral layers is restricted to conditions of supersaturation with mineral components. Carbonate precipitation can take place in cyanobacterial communities under conditions of mixing sea water enriched with Ca and Mg with continental water enriched with sodium carbonate. Cyanobacteria are able to form and transform various Ca-Mg-carbonates. Dolomite formation is a derived process that occurs in cyanobacterial mats in the presence of sulfate-reducing bacteria. Carbonatization of cyanobacterial cells is considered using the example of the unicellular halophilic-alkaliphilic cyanobacterium Euhalothece sp. The accomplished study is of certain interest for interpretation of geological and paleontological data in the context of the supposed analogy between cyanobacterial mats and ancient stromatolites.     

中国叠层石研究的历史和现状

中国前寒武纪地层分布广泛。在20世纪70-80年代,中国学者对晚前寒武纪的叠层石进行了系统的研究,描述了类型众多的叠层石属种,并把叠层石组合应用于地层的划分和对比。近年来,为了揭示叠层石的形态发生,中国学者正在探讨硅质叠层石的生物组构模式和叠层石微层理的成因,以及叠层石中微生物生长、运动和造席过程。     

A comparison of carbon/energy and complex nitrogen sources for bacterial sulphate-reduction: potential applications to bioprecipitation of toxic metals as sulphides

Detailed nutrient requirements were determined to maximise efficacy of a sulphate-reducing bacterial mixed culture for biotechnological removal of sulphate, acidity and toxic metals from waste waters. In batch culture, lactate produced the greatest biomass, while ethanol was more effective in stimulating sulphide production and acetate was less effective. The presence of additional bicarbonate and H₂ only marginally stimulated sulphide production. The sulphide output per unit of biomass was greatest using ethanol as substrate. In continuous culture, ethanol and lactate were used directly as efficient substrates for sulphate reduction while acetate yielded only slow growth. Glucose was utilised following fermentation to organic acids and therefore had a deleterious effect on pH. Ethanol was selected as the most efficient substrate due to cost and efficient yield of sulphide. On ethanol, the presence of additional carbon sources had no effect on growth or sulphate reduction in batch culture but the presence of complex nitrogen sources (yeast extract or cornsteep) stimulated both. Cornsteep showed the strongest effect and was also preferred on cost grounds. In continuous culture, cornsteep significantly improved the yield of sulphate reduced per unit of ethanol consumed. These results suggest that the most efficient nutrient regime for bioremediation using sulphate-reducing bacteria required both ethanol as carbon source and cornsteep as a complex nitrogen source.