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Nitrogen isotopic response of prokaryotic and eukaryotic phytoplankton to nitrate availability in Sargasso Sea surface waters
Author(s) -
Treibergs Lija A.,
Fawcett Sarah E.,
Lomas Michael W.,
Sigman Daniel M.
Publication year - 2014
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2014.59.3.0972
Subject(s) - phytoplankton , nitrate , photic zone , environmental chemistry , incubation , seawater , nitrogen assimilation , oceanography , ammonium , nitrogen , surface water , prochlorococcus , chemistry , cyanobacteria , biology , ecology , nutrient , environmental science , synechococcus , bacteria , geology , biochemistry , organic chemistry , genetics , environmental engineering
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.