Consistent cell-specific carbon fixation rates by small eukaryotic phytoplankton in contrasting nutrient-limited conditions

Small phytoplankton, consisting of pico and nano size fractions, are diverse in size and taxonomy. Yet, the differences in their productivity and taxonomic diversity are poorly described. Here, we measured the cell-specific carbon fixation rates of picocyanobacteria Synechococcus, picoeukaryote, and nanoeukaryote populations, while unveiling their taxonomic composition in oligotrophic subtropical and high-nutrient low-chlorophyll subantarctic waters. We coupled 24 h in situ radiolabeled ¹⁴C incubations to flow cytometry sorting and DNA metabarcoding from the same incubated samples, offering a direct account of the community associated with the carbon fixation rates measured. In both water masses, nanoeukaryotes had the highest cell-specific carbon fixation rate, followed by picoeukaryotes and Synechococcus (2.24 ± 29.99, 2.18 ± 2.08, and 0.78 ± 0.55 fgC cell⁻¹ h⁻¹, respectively). The cell-specific carbon fixation rates and growth rates of Synechococcus were threefold higher in subtropical compared to subantarctic waters, while the rates of picoeukaryotes and nanoeukaryotes had no significant difference between the biogeochemically-contrasting water masses. Sorted picoeukaryote populations were dominated by Mamiellophyceae, Pelagophyceae, Prymnesiophyceae, and Chrysophyceae, while nanoeukaryote populations were dominated by Dinophyceae and Prymnesiophyceae. Despite significant differences in their taxonomic composition, the sorted picoeukaryote populations in subantarctic waters and nanoeukaryote populations in subtropical and subantarctic waters were dominated by taxa reported in the literature as able to engage in phago-mixotrophic strategies (Prymnesiophyceae, Chrysophyceae, and Dinophyceae), suggesting that such trophic strategy might be applied by discrete small photosynthetic eukaryote populations to alleviate macronutrient and iron stress.