Nitrogen isotopic response of prokaryotic and eukaryotic phytoplankton to nitrate availability in Sargasso Sea surface waters

Taxon‐specific nitrogen (N) isotope data from the summertime Sargasso Sea have previously suggested reliance of prokaryotic phytoplankton on regenerated N but a greater importance of nitrate assimilation for eukaryotic phytoplankton. To investigate this further, particles collected in the summer from ∼ 100 m at the Bermuda Atlantic Time‐series Study site were incubated in particle‐free, unamended surface seawater (from 4 m and 30 m) with measured ambient nitrate and ammonium concentrations. Preincubation and postincubation particles were sorted using flow cytometry, and the δ 15 N of separated prokaryotic and eukaryotic phytoplankton was analyzed. In July 2009 and 2010, nitrate was undetectable throughout the euphotic zone (the upper ∼ 100 m). The δ 15 N of prokaryotic biomass was initially low and remained low upon incubation, with one exception out of six. Preincubation eukaryotic δ 15 N was higher than or similar to prokaryotic δ 15 N and decreased or remained low after incubation. These data confirm the expectation that, in nitrate‐deplete water, all phytoplankton shift toward a low δ 15 N characteristic of recycled N assimilation. In June 2010, the euphotic zone nitrate concentration was anomalously high (∼ 1 µmol L −1 ), and the δ 15 N of all populations varied as a function of depth, with highest δ 15 N in the surface where nitrate was lowest. After incubation in surface water, the δ 15 N of all sorted phytoplankton increased, suggesting nitrate assimilation by all groups, including Prochlorococcus , the nitrate assimilatory capabilities of which have been debated. This study supports the use of the δ 15 N of sorted phytoplankton as an indicator of the dominant N source fueling their growth.