Turbulence simultaneously stimulates small- andlarge-scale CO2 sequestration by chain-formingdiatoms in the sea

Johanna Bergkvist, Isabell Klawonn, Martin J. Whitehouse, Gaute Lavik, Volker Brüchert & Helle Ploug

Chain-forming diatoms are key CO2-fixing organisms in the ocean. Under turbulent condi-tions they form fast-sinking aggregates that are exported from the upper sunlit ocean to theocean interior. A decade-old paradigm states that primary production in chain-forming dia-toms is stimulated by turbulence. Yet, direct measurements of cell-specific primary pro-duction in individualfield populations of chain-forming diatoms are poorly documented. Herewe measured cell-specific carbon, nitrate and ammonium assimilation in twofield popula-tions of chain-forming diatoms (SkeletonemaandChaetoceros) at low-nutrient concentrationsunder still conditions and turbulent shear using secondary ion mass spectrometry combinedwith stable isotopic tracers and compared our data with those predicted by mass transfertheory. Turbulent shear significantly increases cell-specific C assimilation compared to stillconditions in the cells/chains that also form fast-sinking, aggregates rich in carbon andammonium. Thus, turbulence simultaneously stimulates small-scale biological CO2 assim-ilation and large-scale biogeochemical C and N cycles in the ocean.