Inorganic and organic carbon and nitrogen uptake strategies of picoplankton groups in the northwestern Atlantic Ocean

Picoplankton populations dominate the planktonic community in the surface oligotrophic ocean. Yet, their strategies in the acquisition and the partitioning of organic and inorganic sources of nitrogen (N) and carbon (C) are poorly described. Here, we measured at the single-cell level the uptake of dissolved inorganic C (C-fixation), C-leucine, N-leucine, nitrate (NO₃⁻), ammonium (NH₄⁺), and N-urea in pigmented and nonpigmented picoplankton groups at six low-N stations in the northwestern Atlantic Ocean. Our study highlights important differences in trophic strategies between Prochlorococcus, Synechococcus, photosynthetic pico-eukaryotes, and nonpigmented prokaryotes. Nonpigmented prokaryotes were characterized by high leucine uptake rates, nonsignificant C-fixation and relatively low NH₄⁺, N-urea, and NO₃⁻ uptake rates. Nonpigmented prokaryotes contributed to 7% ± 3%, 2% ± 2%, and 9% ± 5% of the NH₄⁺, NO₃⁻, and N-urea community uptake, respectively. In contrast, pigmented groups displayed relatively high C-fixation rates, NH₄⁺ and N-urea uptake rates, but lower leucine uptake rates than nonpigmented prokaryotes. Synechococcus and photosynthetic pico-eukaryotes NO₃⁻ uptake rates were higher than Prochlorococcus ones. Pico-sized pigmented groups accounted for a significant fraction of the community C-fixation (63% ± 27%), NH₄⁺ uptake (47% ± 27%), NO₃⁻ uptake (62% ± 49%), and N-urea uptake (81% ± 35%). Interestingly, Prochlorococcus and photosynthetic pico-eukaryotes showed a greater reliance on C- and N-leucine than Synechococcus on average, suggesting a greater reliance on organic C and N sources. Taken together, our single-cell results decipher the wide diversity of C and N trophic strategies between and within marine picoplankton groups, but a clear partitioning between pigmented and nonpigmented groups still remains.