Small pigmented eukaryotes play a major role in carbon cycling in the P-depleted western subtropical North Atlantic, which may be supported by mixotrophy

We found that in the phosphate (PO₄)-depleted western subtropical North Atlantic Ocean, small-sized pigmented eukaryotes (P-Euk; < 5 μm) play a central role in the carbon (C) cycling. Although P-Euk were only ~ 5% of the microbial phytoplankton cell abundance, they represented at least two thirds of the microbial phytoplankton C biomass and fixed more CO₂ than picocyanobacteria, accounting for roughly half of the volumetric CO₂ fixation by the microbial phytoplankton, or a third of the total primary production. Cell-specific PO₄ assimilation rates of P-Euk and nonpigmented eukaryotes (NP-Euk; < 5 μm) were generally higher than of picocyanobacteria. However, when normalized to biovolumes, picocyanobacteria assimilated roughly four times more PO₄ than small eukaryotes, indicating different strategies to cope with PO₄ limitation. Our results underline an imbalance in the CO₂ : PO₄ uptake rate ratios, which may be explained by phagotrophic predation providing mixotrophic protists with their largest source of PO₄. 18S rDNA amplicon sequence analyses suggested that P-Euk was dominated by members of green algae and dinoflagellates, the latter group commonly mixotrophic, whereas marine alveolates were the dominant NP-Euk. Bacterivory by P-Euk (0.9 ± 0.3 bacteria P-Euk⁻¹ h⁻¹) was comparable to values previously measured in the central North Atlantic, indicating that small mixotrophic eukaryotes likely exhibit similar predatory pressure on bacteria. Interestingly, bacterivory rates were reduced when PO₄ was added during experimental incubations, indicating that feeding rate by P-Euk is regulated by PO₄ availability. This may be in response to the higher cost associated with assimilating PO₄ by phagocytosis compared to osmotrophy.