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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 δ 13 C 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., (© 2018 John Wiley & Sons Ltd.)

Complete budgets for carbon and oxygen have been constructed for cyanobacterial mats dominated by Microcoleus chthonoplastes from the evaporating ponds of a salt works located in Guerrero Negro, Baja California Sur, Mexico. Included in the budget are measured rates of O2 production, sulfate reduction, and elemental exchange across the mat/brine interface, day and night, at various temperatures and times of the year. We infer from this data the various sinks for O2, as well as the sources of carbon for primary production. To summarize, although seasonal variability exists, a major percentage of the O2 produced during the day did not diffuse out of the mat but was used within the mat to oxidize both organic carbon and the sulfide produced by sulfate reduction. At night, most of the O2 that diffused into the mat was used to oxidize sulfide, with O2 respiration of minor importance. During the day, the internal mat processes of sulfate reduction and O2 respiration generated as much or more inorganic carbon (DIC) for primary production as diffusion into the mat. Also, oxygenic photosynthesis was the most important process of carbon fixation, although anoxygenic photosynthesis may have been important at low light levels during some times of the year. At night, the DIC lost from the mat was mostly from sulfate reduction. Elemental fluxes across the mat/brine interface indicated that carbon with an oxidation state of greater than zero was taken up by the mat during the day and liberated from the mat at night. Overall, carbon with an average oxidation state of near zero accumulated in the mat. Both carbon fixation and carbon oxidation rates varied with temperature by a similar amount. These mats are thus closely coupled systems where rapid rates of photosynthesis both require and fuel rapid rates of heterotrophic carbon oxidation.

Nitrogen fixation is a critical part of the global nitrogen cycle, replacing biologically available reduced nitrogen lost by denitrification. The redox-sensitive trace metals Fe and Mo are key components of the primary nitrogenase enzyme used by cyanobacteria (and other prokaryotes) to fix atmospheric N2 into bioessential compounds. Progressive oxygenation of the Earth's atmosphere has forced changes in the redox state of the oceans through geologic time, from anoxic Fe-enriched waters in the Archean to partially sulfidic deep waters by the mid-Proterozoic. This development of ocean redox chemistry during the Precambrian led to fluctuations in Fe and Mo availability that could have significantly impacted the ability of prokaryotes to fix nitrogen. It has been suggested that metal limitation of nitrogen fixation and nitrate assimilation, along with increased rates of denitrification, could have resulted in globally reduced rates of primary production and nitrogen-starved oceans through much of the Proterozoic. To test the first part of this hypothesis, we grew N2-fixing cyanobacteria in cultures with metal concentrations reflecting an anoxic Archean ocean (high Fe, low Mo), a sulfidic Proterozoic ocean (low Fe, moderate Mo), and an oxic Phanerozoic ocean (low Fe, high Mo). We measured low rates of cellular N2 fixation under [Fe] and [Mo] estimated for the Archean ocean. With decreased [Fe] and higher [Mo] representing sulfidic Proterozoic conditions, N2 fixation, growth, and biomass C:N were similar to those observed with metal concentrations of the fully oxygenated oceans that likely developed in the Phanerozoic. Our results raise the possibility that an initial rise in atmospheric oxygen could actually have enhanced nitrogen fixation rates to near modern marine levels, providing that phosphate was available and rising O2 levels did not markedly inhibit nitrogenase activity. [ABSTRACT FROM AUTHOR]

Gypsum crusts containing multicolored stratified microbial populations grow in the evaporation ponds of a commercial saltern in Eilat, Israel. These crusts contain two prominent cyanobacterial layers, a bright purple layer of anoxygenic phototrophs, and a lower black layer with active sulphate reduction. We explored the diel dynamics of oxygen and sulphide within the crust using specially constructed microelectrodes, and further explored the crust biogeochemistry by measuring rates of sulphate reduction, stable sulphur isotope composition, and oxygen exchange rates across the crust–brine interface. We explored crusts from ponds with two different salinities, and found that the crust in the highest salinity was the less active. Overall, these crusts exhibited much lower rates of oxygen production than typical organic-rich microbial mats. However, this was mainly due to much lower cell densities within the crusts. Surprisingly, on a per cell-volume basis, rates of photosynthesis were similar to organic-rich microbial mats. Due to relatively low rates of oxygen production and deep photic zones extending from 1.5 to 3 cm depth, a large percentage of the oxygen produced during the day accumulated into the crusts. Indeed, only between 16% to 34% of the O2 produced in the crust escaped, and the remainder was internally recycled, used mainly in O2 respiration. We view these crusts as potential homologs to ancient salt-encrusted microbial ecosystems, and we compared them to the 3.45 billion-year-old quartz barite deposits from North Pole, Australia, which originally precipitated gypsum. [ABSTRACT FROM AUTHOR]