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Project Progress

Studies of microbially-shaped rocks supported the use of mm-scale clumps as an early morphological biosignature of oxygenic photosynthesis, offering independent evidence that some stromatolites were shaped in the presence of oxygen 0.2-0.4 Ga before the rise of atmospheric oxygen. Experiments tracking the cycling of carbon in photosynthetic biofilms related the shapes of pinnacles and conical stromatolites to the flow of nutrients. These insights were used to develop quantitative tests for the presence of former biofilms in the laminae of conical stromatolites and were summarized in a review of recent process-oriented models of stromatolite morphogenesis. Isotopic imprints of microbial processes in sediments were explored by studies of environmental factors and enzymatic processes that control the magnitude of microbial sulfur fractionation during microbial sulfate reduction. Culturing studies of sulfate reducers underscored the complexity of factors that control the magnitude of microbial sulfur isotope fractionation by demonstrating higher fractionations in cultures limited by iron and nitrogen, as well as in mutants of sulfate reducing bacteria with impaired enzymes in the electron transport chain. These studies have implications for the environmental distribution of sulfate reducing bacteria and for the interpretations of large S-isotope fractionations during ocean anoxic events. The search for fossils of early modern eukaryotes in carbonate strata deposited between the two Neoproterozoic global glaciations uncovered the fossils of the earliest putative foraminiferans, revealed different agglutinated eukaryotes at different localities of the Rasthof Formation, Namibia and described the earliest macroscopic, multicellular organic-walled fossils that are similar to some modern red algae. We have started using organic remnants of Neoproterozoic eukaryotes to explore the cycling of carbon during the deposition of isotopically extreme carbonates between the Sturtian and the Marinoan glaciation.