The large capacity for dark nitrate-assimilation in diatoms may overcome nitrate limitation of growth.

•  The ability of the diatom Thalassiosira weissflogii to assimilate inorganic N in darkness is compared with that seen in flagellates. •  Experiments were conducted with T. weissflogii grown in N-replete and in N-limiting cultures and the rates and capacity for ammonium and nitrate assimilation were determined. •  High daily growth rates in the diatom under high-light nitrate-replete conditions are only attainable by continuing nitrate assimilation in darkness using excess C accumulated in the light when nitrate assimilation cannot match C-fixation. The ability to use ammonium in darkness is greater than for nitrate but the ratio of dark to light assimilation for each N source is similar over a wide range of cellular N : C ratios. These capabilities are in strong contrast with those in the flagellates Heterosigma carterae and Heterocapsa illdefina, which are incapable of high nitrate use in darkness. •  While the possession of large capacity for dark nitrate-assimilation in diatoms may provide a mechanism that overcomes nitrate limitation of growth, the explanation for the lower capabilities exhibited by flagellates is less clear.