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 CO2 pressure (pCO2) representative of the early Paleoproterozoic. We find 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 and sustain oxygenation of the planetary surface.
Highlights
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
With ≥75% of the Paleoproterozoic planet covered by oceans[17], and assuming negligible burial of terrestrial organic matter, oceanic planktonic unicellular cyanobacteria ought to contribute to the global sequestration of organic carbon
To grow cyanobacteria under the ultrahigh levels of CO2 estimated for Paleoproterozoic, we used a single-cell marine cyanobacterium as a model because of its planktonic growth and resilience to high pCO2
Summary
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. In the Paleoproterozoic, formation of such ballasted cell aggregates could have resulted in efficient organic carbon export from the surface to deep ocean by unicellular planktonic cyanobacteria. To grow cyanobacteria under the ultrahigh levels of CO2 estimated for Paleoproterozoic, we used a single-cell marine cyanobacterium as a model because of its planktonic growth and resilience to high pCO2.
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