Award: OCE-1124272

Currently, there is only limited information on the identity and activity of the microorganisms carrying out CO2-fixation in the natural environment, despite the fact that these organisms form the basis of their respective ecosystems. To this end, we carried out an interdisciplinary, international research program aiming to improve our understanding of autotrophic carbon fixation and its chemical and isotopic signature along environmental gradients in a natural hydrothermal system. To perform these studies, we chose a shallow-water vent system off Paleochori Bay (Milos Island, Greece) as a natural laboratory. In May 2012, we conducted a 12-day long expedition to the shallow water vent sites in Paleochori Bay to sample vent fluids and sediments using SCUBA diving. The scientific party included the three PIs (Foustoukos, Sievert, Vetriani), international collaborators (Le Bris [UPMC, Banyuls-sur-Mer, France], Bühring [Bremen University, Germany]) and members of the respective labs. This expedition was highly successful and resulted in the collection of sediment, fluid and volatile samples from different areas of venting (subaerial and submarine). In addition, we successfully deployed in situ injection core units that are used to identify the microorganisms actively performing autotrophic carbon fixation. The spatial and temporal geochemical dynamics of a shallow-water vent site in Paleochori Bay has been described (Yücel et al., 2013, Chemical Geology 356:11-20). This includes a 6-day long, high-resolution time series, which is the first of its kind for such an environment. This data set provides the framework for assessing the microbial communities inhabiting the sediments along the studied environmental gradient. To this end, we have extracted nucleic acids from the various frozen samples and performed a survey based on the gene coding for the 16S ribosomal RNA specific for Bacteria and Archaea to assess the general diversity. These data are being analyzed in the context of the geochemcial parameters to obtain further insights into the controls of the distribution and abundance of specific microbial groups along the gradient. Already, we can see that both temperature and distance from the center of the vent exert a strong influence. Clear trends are apparent, with Epsilonproteobacteria being the dominant groups in the more strongly hydrothermally influenced sediments and Gammaproteobacteria being more dominant in the periphery. These two groups are known to utilize different pathways for autotrophic carbon fixation, i.e., the reductive TCA cycle vs. the Calvin cycle, which result in distinct stable carbon isotopic composition of the produced biomass. To test the hypothesis that this would be reflected in the stable carbon isotopic composition of the organic carbon along the transect, we analyzed the concentration and the stable carbon isotopic composition of the total organic carbon (TOC) in the sediments. Surprisingly and in contrast to the microbial community analyses, no obvious trend emerged from these data, with TOC being consistently low and the isotopic composition staying constant around -18 permille, in line with the use of the rTCA cycle for autotrophic carbon fixation. Analyzing the stable carbon isotopic composition of specific lipid biomarkers (in collaboration with Dr. Bühring) as well as mRNA analyses of indicative functional genes (in collaboration with Co-PI Vetriani) will provide further insights to explain this pattern. The data generated in this study will allow us to constrain the relationship between autotrophic carbon fixation and the resulting isotopic signatures of biomass and specific biomarker molecules, such as lipids, in a natural system, which does not only have implications for assessing the importance of carbon fixation in extant ecosystems, but will also provide a tool to improve the interpretation of isotopic values in the geological record. This project provided opportunitie...