Isotopic evidence of photoheterotrophy in Palaeoproterozoic Chlorobi

Abstract The reductive tricarboxylic acid (rTCA) cycle is touted as a primordial mode of carbon fixation due to its autocatalytic propensity and oxygen intolerance 1,2 . Despite this inferred antiquity, however, the earliest rock record affords scant supporting evidence. In fact, based on the chimeric inheritance of rTCA cycle genes within the Chlorobiaceae 3 , even the utility of the chemical fossil record is now subject to question. While the 1.64-billion-year-old Barney Creek Formation (BCF) contains chemical fossils of the earliest known putative Chlorobiaceae-derived carotenoids, interferences from the accompanying hydrocarbon matrix have hitherto precluded the carbon isotope measurements necessary to establish the physiology of the organisms that produced them. Overcoming this obstacle, here we report a suite of compound-specific carbon isotope measurements identifying a cyanobacterially dominated ecosystem featuring heterotrophic bacteria. Crucially, we demonstrate chlorobactane is 13C-depleted when compared to contemporary equivalents, showing only slight 13C-enrichment over co-existing cyanobacterial carotenoids. These observations demonstrate the late acquisition of autotrophy and the rTCA cycle as predicted via phylogenomic- and molecular-clock-grounded approaches3. We suggest that progressive oxygenation of the Earth System caused an increase in the marine sulfate inventory thereby providing the selective pressure to fuel the Neoproterozoic niche shift toward energy-efficient photoautotrophy within the Chlorobiaceae 4 .