Late acquisition of the rTCA carbon fixation pathway by Chlorobi

Compound-specific carbon isotope measurements of the 1.64-billion-year-old Barney Creek Formation show an ecosystem dominated by cyanobacteria and heterotrophic bacteria. Isotope data for carotenoids suggest that the assembly of the reverse tricarboxylic acid cycle in Chlorobi occurred later. The reverse tricarboxylic acid (rTCA) cycle is touted as a primordial mode of carbon fixation due to its autocatalytic propensity and oxygen intolerance. Despite this inferred antiquity, however, the earliest rock record affords scant supporting evidence. In fact, based on the chimeric inheritance of rTCA cycle steps within the Chlorobiaceae, even the use of the chemical fossil record of this group is now subject to question. While the 1.64-billion-year-old Barney Creek Formation 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. We demonstrate chlorobactane is C-13-depleted when compared to contemporary equivalents, showing only slight C-13-enrichment over co-existing cyanobacterial carotenoids. The absence of this diagnostic isotopic fingerprint, in turn, confirms phylogenomic hypotheses that call for the late assembly of the rTCA cycle and, thus, the delayed acquisition of autotrophy within the Chlorobiaceae. 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 shift towards energy-efficient photoautotrophy within the Chlorobiaceae.

Automated summary

Compound-specific carbon isotope measurements indicate that the Barney Creek Formation, 1.64 billion years ago, was dominated by cyanobacteria and heterotrophic bacteria. The formation of the reverse tricarboxylic acid cycle in Chlorobi appears to have occurred at a later stage. However, the earliest rock record does not provide strong evidence in support of this. Through specific carbon isotope measurements, we have identified a cyanobacterially dominated ecosystem in the Barney Creek Formation, confirming phylogenomic hypotheses about the late assembly of the rTCA cycle within the Chlorobiaceae and the delayed acquisition of autotrophy.