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 c haracteristic 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 relativ ely high C-13 content could have arisen biologically from recently discover ed autotrophic pathways. We focused on the green non-sulphur bacterium Chlo roflexus aurantiacus that uses the 3-hydroxypropionate pathway for inorgani c carbon fixation and is geologically significant as it forms modern mat co mmunities 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 isotopicall y heavy (-8.9 parts per thousand to -18.5 parts per thousand). Organic matt er in mats constructed by Chloroflexus in conjunction with cyanobacteria ha d 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 cy anobacteria (-33.9 parts per thousand to -36.3 parts per thousand). This su ggests that, in mats formed by both cyanobacteria and Chloroflexus, autotro phy must have a greater effect on Chloroflexus carbon metabolism than the p hotoheterotrophic consumption of cyanobacterial photosynthate. Chloroflexus cell components were also selectively preserved. Hence, Chloroflexus autot rophy and selective preservation of its products constitute one purely biol ogical mechanism by which isotopically heavy organic carbon could have been introduced into important Precambrian geological features.