PNNL

Abstract

The decoupling of CO2 fixation by cyanobacterial oxygenic photosynthesis (OP) from respiration of the synthesized biomass led to oxygenation of Earth’s surface ~2.3 gigaannum ago (Ga). Because OP discriminates against the 13C isotope, burial of biomass on a global scale can be deduced from 13C enrichment in inorganic carbonates, which formed during and after the Great Oxygenation Event (GOE). Whilst biomass burial in shallow water stromatolites is well documented, the biomass burial of oceanic cyanobacteria remains an open question. With scarcity of geological records, the contribution of planktonic cyanobacteria to burial of organic carbon in deep-sea sediments has been considered negligible owing to the slow sinking speed of their small cells. Here we present a mechanism for efficient export of organic carbon from the surface to the bottom layers of a deep ocean by unicellular planktonic cyanobacteria acclimated to high partial CO2 pressure (pCO2) representative of early Paleoproterozoic. We found that high pCO2 boosted cyanobacterial photosynthesis and generation of extracellular polysaccharides (EPS). The negatively charged EPS aggregated cells, adsorbed Ca and Mg ions, and facilitated mineral precipitation and formation of ballasted particles. Hence, in the Paleoproterozoic high-CO2 environment unicellular planktonic cyanobacteria could form similar large ballasted particles, which drove efficient export of organic matter from surface to deep ocean zones. The flux of self-assembled cyanobacterial particles would decouple the oxygenic photosynthesis from oxidative respiration at the ocean-scale, sustain oxygenation of the planetary surface, and decrease the relative abundance of the light carbon isotope (12C) in the oceanic carbonate pool.