The earth’s carbon record is a potential chronicle of biogeosphere evolution. Understanding how the earth and the life on it have influenced one another will help estimate how the biogeosphere will react to new natural and anthropogenic pressures. Anoxygenic phototrophic bacteria are studied as one of the earliest organisms due to their phylogenic antiquity and ability to metabolize iron and sulfur using light energy. Iron and sulfur were abundant on early earth, and their relative oxidation states and isotopic signatures in the rock record are routinely utilized as markers for the rise of oxygen. The carbon chronicle and anoxygenic phototroph evolution can be studied by anlayzing C-isotope signatures. Isotope fractionations due to bacterial carbon fixation and C-isotope signatures in ancient sedimentary rock have been studied. Yet, in order to understand how anoxygenic phototrophs evolved in their environment, the C-isotope pathways between bacterial C-fixation and organic matter burial must be elucidated. The role of organic matter in the geosphere is difficult to access isotopically as it results from a complex mixture of source organisms, biosynthetic pathways, and diagenetic transformations. Furthermore, mixed communities of microorganisms use multiple C-fixation pathways. It is unknown 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. This study will determine anoxygenic phototroph C-isotope signatures under various conditions by linking lab experiments with pure strains isolated from iron-rich Lake La Cruz and sulfur-rich Lake Cadagno to in situ experiments in the lakes. Microbial cycling experiments will determine how C-isotope signature of biomass produced via bacterial C-fixation is altered by turnover and sediment burial. In particular, it aims to elucidate the preservation potential of bacterial C-isotope pathways.