Photosynthetic primary production in the Mesoproterozoic

The Mesoproterozoic atmosphere had more CO₂ and less O₂ than at present. While the upper ocean was oxygenated, the deeper ocean was euxinic or ferruginous. Primary production was performed by Chlorobia, Cyanobacteria, Proteobacteria, and Archaeplastida. Cyanobacteria and Archaeplastida presumably performed oxygenic photosynthesis above the chemocline, while photosynthesis below the chemocline involved anoxygenic photolithotrophic oxidation of S²⁻ or Fe²⁺ by Proteobacteria and S²⁻ by Chlorobia and Cyanobacteria. Photolithotrophic growth of extant Proteobacteria with Rubisco, and Chlorobia with the reverse tricarboxylic acid cycle, involves diffusive CO₂ entry; this likely also occurred under high Mesoproterozoic CO₂. By contrast, Cyanobacteria may already have possessed CO₂-concentrating mechanisms comparable to extant lineages. Here, we evaluate how differences in primary carboxylation pathways and inorganic carbon acquisition among these groups influence stable carbon isotope fractionation and assess how isotopic signatures in modern representatives inform interpretations of Mesoproterozoic carbon isotope records. Although culture-based fractionation patterns broadly overlap with Mesoproterozoic sedimentary δ¹³C values, these records are strongly biased towards continental margin settings, and direct evidence for open-ocean planktonic primary producers is lacking. The evidence reviewed highlights that physiological constraints on carbon acquisition and metabolism provide a framework for interpreting Mesoproterozoic carbon isotope signals with implications for primary production and the biological pump.