NITROGEN ASSIMILATION OF AN OCEANIC DIATOM IN NITROGEN‐LIMITED CONTINUOUS CULTURE<sup>1</sup>

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<jats:title>SUMMARY</jats:title><jats:p>The oceanic diatom<jats:italic>Thalassiosira pseudonana</jats:italic>Hasle and Heimdal (formerly<jats:italic>Cyclotella nana</jats:italic>) was grown with 12L:12D illumination cycles in nitrogen‐limited continuous culture with a mixture of ammonium and nitrate as the N source. Measurements included, at 3 different growth rates (degrees of N limitation), cell concentration, cell carbon, nitrogen, and chlorophyll<jats:italic>a</jats:italic>contents, cell volume, photosynthetic carbon assimilation vs. irradiance, short‐term uptake of ammonium and nitrate vs. their ambient concentrations, and<jats:italic>in vitro</jats:italic>activities of the assimilatory enzymes nitrate reductase and glutamic dehydrogenase.</jats:p><jats:p>The various parameters showed either an increase (pattern<jats:italic>a</jats:italic>) or a decrease (pattern<jats:italic>b</jats:italic>) with increasing N limitation. Those following pattern<jats:italic>a</jats:italic>were nitrate reductase activity and the capacity to assimilate nitrate and ammonium. Those following pattern<jats:italic>b</jats:italic>were glutamic dehydrogenase activity, photosynthetic rate, nitrogen:carbon and chlorophyll<jats:italic>a</jats:italic>:carbon composition ratios. Results are discussed in terms of the interpretation such measurement for natural phytoplankton and effects of circadian periodicity.</jats:p>

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