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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 CO₂than picocyanobacteria, accounting for roughly half of the volumetric CO₂fixation by the microbial phytoplankton, or a third of the total primary production. Cell‐specific PO₄assimilation 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 PO₄than small eukaryotes, indicating different strategies to cope with PO₄limitation. Our results underline an imbalance in the CO₂: PO₄uptake 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⁻¹h⁻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 PO₄was added during experimental incubations, indicating that feeding rate by P‐Euk is regulated by PO₄availability. This may be in response to the higher cost associated with assimilating PO₄by phagocytosis compared to osmotrophy.