Ballasted aggregates of planktonic cyanobacteria could sustain carbon burial in Paleoproterozoic

The contribution of planktonic cyanobacteria to burial of organic carbon in deep-sea sediments before the emergence of eukaryotic predators ~1.5 Ga has been considered negligible owing to the slow sinking speed of their small cells. However, global, highly positive excursion in carbon isotope values of inorganic carbonates ~2.22-2.06 Ga implies massive organic matter burial. With ≥75% of the Paleoproterozoic planet covered by oceans, the massive burial of organic matter had to be linked to oceanic cyanobacteria. To elucidate this link, we experimented with unicellular planktonic cyanobacteria acclimated to high partial CO₂ pressure (pCO₂) representative of the early Paleoproterozoic. We found that high pCO₂ boosts generation of acidic extracellular polysaccharides (EPS) that adsorb Ca and Mg cations, support mineralization, and aggregate cells to form ballasted particles. The down flux of such self-assembled cyanobacterial aggregates would decouple the oxygenic photosynthesis from oxidative respiration at the ocean scale, drive export of organic matter from surface to deep ocean, sustain oxygenation of the planetary surface, and trigger mountain building.