Analysis of C-isotope signatures of iron- and sulfur-metabolizing anoxygenic phototrophic bacteria from modern lakes to understand biogeochemical cycling of carbon, iron and sulfur in ancient environments.

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Anoxygenic photoautotrophic bacteria utilizing sulfur (S) and iron (Fe) as electron source to fix carbon dioxide into biomass are considered one of the earliest life forms on earth and are studied to investigate the origins of mineral-microbe-hydrosphere interactions. The turnover of carbon (C) linked to the S and Fe cycle by microbial processes, therefore, lies at the heart of emergence of life and C-Fe-S studies. The C-isotope signatures of individual strains of anoxygenic phototrophs due to their carbon fixation pathways are only partially studied. Importantly, in mixed communities of microorganisms, such as occur in natural systems, multiple C-fixation pathways are used. It is not known whether the final C-isotope composition measured in situ is determined by a dominant species, or whether the resulting signature is an integration of all. Likewise, it is unknown how diagenesis due to microbial carbon turnover influences the final isotopic composition measured in the water column, sediment and finally, the rock record. A combination of laboratory and in situ experiments at model lakes La Cruz (Spain) and Cadagno (Switzerland) are proposed to determine the preservation potential of anoxygenic phototroph C-isotope signatures 1) due to C-fixation in the absence and presence of Fe and S, as well as C-limitation 2) as isolates and in microbial communities, and 3) after diagenesis due to microbial carbon turnover linked to the Fe and S cycles. The findings will be compared to current studies simulating diagenesis due to temperature and pressure influence, as well as rock record investigations. The geological carbon record harbors the key to persistent questions about the evolution of the atmosphere and hydrosphere together with life on earth. Analysis of microbial C-isotope fractionation signatures may decode this carbon chronicle and lend insight to how organisms shape the geosphere, as well as potential markers of life both on earth and other planets.