Carbon isotope fractionation by extant cyanobacteria and physiological analogs of their ancestors

Oxygenic photosynthesis arose once, in the ancestors of modern single-celled cyanobacteria and fundamentally changed the course of geological and biological evolution on Earth. Although oxygenic photosynthesis is thought be responsible for the Paleoproterozoic accumulation of oxygen ~2.3-2.4 billion years ago, the oldest microfossils widely agreed to be cyanobacterial appear ~1.9 billion years ago. The mid Proterozoic oceans remained largely anoxic, with iron-rich and sulfide-rich regions separated in space and time. It therefore remains unclear if primary production was the domain of oxygenic cyanobacteria or if environmental conditions favored other phototrophic metabolisms. Carbon isotope ratios in preserved sedimentary organic matter provide a uniquely continuous record through this time and, in principle, a way to identify the dominant carbon fixation pathway in the marine environment. I have developed Monte Carlo simulations of the Proterozoic carbon isotope record to place new constraints on the range of different metabolic fractionations associated with marine primary production during this time.

Modern cyanobacteria employ a sophisticated biochemical and biophysical carbon concentrating mechanism to increase the efficiency of carbon fixation under low CO2, complicating the relationship between CO2 concentrations and carbon isotope fractionation. I investigate carbon isotope fractionation in ancestral cyanobacteria by growing physiological analogs lacking carbon concentrating mechanisms at relevant Proterozoic pH and CO2 levels.