Abstract
Summary Oceanic photosynthetic picoeukaryotes ( 40% of total primary production at low latitudes such as the North‐Eastern tropical Atlantic. In the world ocean, warmed by climate changes, the expected gradual shift towards smaller primary producers could render the role of photosynthetic picoeukaryotes even more important than they are today. Little is still known, however, about how the taxonomic composition of this highly diverse group affects primary production at the basin scale. Here, we combined flow cytometric cell sorting, NaH 14 CO 3 radiotracer incubations and class‐specific fluorescence in situ hybridization (FISH) probes to determine cell‐ and biomass‐specific inorganic carbon fixation rates and taxonomic composition of two major photosynthetic picoeukaryote groups on a ∼7500‐km‐long latitudinal transect across the Atlantic Ocean (Atlantic Meridional Transect, AMT19). We show that even though larger cells have, on average, cell‐specific CO 2 uptake rates ∼5 times higher than the smaller ones, the average biomass‐specific uptake is statistically similar for both groups. On the other hand, even at a high taxonomic level, i.e. class, the contributions to both groups by Prymnesiophyceae , Chrysophyceae and Pelagophyceae are significantly different ( P