How did anoxic conditions affect nitrogen fixing Cyanobacteria on early Earth?

The Earth’s atmosphere was without free oxygen until the Great Oxygenation Event, thought to have been driven by oxygenic photosynthesis. The expansion of early Cyanobacteria was proposed to be restricted by the lack of bioavailable nitrogen. The effects of an anoxic Archean atmosphere on the growth of a the nitrogen fixing Cyanobacterium Nostoc sp. PCC7524 was compared to control cultures grown under present day atmospheric levels (PAL) of O2 and CO2. Additionally, we assessed how the early Archean atmosphere affected the gas diffusion barrier, consisting of heterocyte glycolipids, of the heterocyte and the ability of early Cyanobacteria to fix N2. While no significant changes were observed for growth rates under N-depleted conditions in the experimental and control atmospheres, upregulation of the C- and N2-fixation associated genes, were observed under Archean conditions relative to PAL. This correlated with increased levels of the C-fixing Rubisco protein and O2 production. The glycogen and protein content of the Archean endpoint culture material showed raised levels of these long-term storage compounds compared to those grown under PAL conditions. No significant changes in the heterocyte glycolipid content or composition was observed. This data suggests that diazotrophic Cyanobacteria were able to fix nitrogen and carbon more efficiently under the anoxic conditions of the Archean, thereby releasing more biologically available carbon and nitrogen into the immediate environment than under PAL conditions. The fact that no significant changes in the heterocyte glycolipid content occurred suggests they are suitable biomarkers for cyanobacterial N2-fixation in geological records.