Significant CO₂ fixation by small prymnesiophytes in the subtropical and tropical northeast Atlantic Ocean

Global estimates indicate the oceans are responsible for approximately half of the carbon dioxide fixed on Earth. Organisms ≤5 μm in size dominate open ocean phytoplankton communities in terms of abundance and CO₂ fixation, with the cyanobacterial genera Prochlorococcus and Synechococcus numerically the most abundant and more extensively studied compared with small eukaryotes. However, the contribution of specific taxonomic groups to marine CO₂ fixation is still poorly known. In this study, we show that among the phytoplankton, small eukaryotes contribute significantly to CO₂ fixation (44%) because of their larger cell volume and thereby higher cell-specific CO₂ fixation rates. Within the eukaryotes, two groups, herein called Euk-A and Euk-B, were distinguished based on their flow cytometric signature. Euk-A, the most abundant group, contained cells 1.80.±1 μm in size while Euk-B was the least abundant but cells were larger (2.8±0.2 μm). The Euk-B group comprising prymnesiophytes (73±13%) belonging largely to lineages with no close cultured counterparts accounted for up to 38% of the total primary production in the subtropical and tropical northeast Atlantic Ocean, suggesting a key role of this group in oceanic CO₂ fixation.