High pCO ₂-induced exopolysaccharide-rich ballasted aggregates of planktonic cyanobacteria could explain Paleoproterozoic carbon burial

Abstract

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 that had to be linked to oceanic cyanobacteria. Here to elucidate that link, we experiment with unicellular planktonic cyanobacteria acclimated to high partial CO ₂ pressure ( pCO ₂) representative of the early Paleoproterozoic. We find that high pCO ₂ 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 and sustain oxygenation of the planetary surface.

Abstract

A Paleoproterozoic carbon isotope anomaly is likely linked to burial of oceanic cyanobacteria, but it is not clear how burial occurred. Here, the authors find that, under Paleoproterozoic pCO ₂ conditions, planktonic cyanobacteria increase exopolysaccharide production and mineralization, leading to aggregation and faster sinking.