Autotrophy of green non-sulphur bacteria in hot spring microbial mats: biological explanations for isotopically heavy organic carbon in the geological record

Inferences about the evidence of life recorded in organic compounds within the Earth's ancient rocks have depended on C-13 contents low enough to be characteristic of biological debris produced by the well-known CO2 fixation pathway, the Calvin cycle. "Atypically' high values have been attributed to isotopic alteration of sedimentary organic carbon by thermal metamorphism. We examined the possibility that organic carbon characterized by a relatively high C-13 content could have arisen biologically from recently discovered autotrophic pathways. We focused on the green non-sulphur bacterium Chloroflexus aurantiacus that uses the 3-hydroxypropionate pathway for inorganic carbon fixation and is geologically significant as it forms modern mat communities analogous to stromatolites. Organic matter in mats constructed by Chloroflexus spp. alone had relatively high C-13 contents (-14.9 parts per thousand) and lipids diagnostic of Chloroflexus that were also isotopically heavy (-8.9 parts per thousand to -18.5 parts per thousand). Organic matter in mats constructed by Chloroflexus in conjunction with cyanobacteria had a more typical Calvin cycle signature (-23.5 parts per thousand). However, lipids diagnostic of Chloroflexus were isotopically enriched (-15.1 parts per thousand to -24.1 parts per thousand) relative to lipids typical of cyanobacteria (-33.9 parts per thousand to -36.3 parts per thousand). This suggests that, in mats formed by both cyanobacteria and Chloroflexus, autotrophy must have a greater effect on Chloroflexus carbon metabolism than the photoheterotrophic consumption of cyanobacterial photosynthate. Chloroflexus cell components were also selectively preserved. Hence, Chloroflexus autotrophy and selective preservation of its products constitute one purely biological mechanism by which isotopically heavy organic carbon could have been introduced into important Precambrian geological features.