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.