Modelling suggests that optimization of dark nitrogen-assimilation need not be a critical selective feature in phytoplankton

Alternative strategies for the dark assimilation of ammonium and nitrate into microalgae are explored using a mechanistic model of algal physiology. The standard diatom strategy, continuation of N assimilation at high rates in darkness as long as reserve C remains, is the most advantageous. The flagellate strategy, incorporating ammonium but not nitrate at a reasonable rate in darkness, is best suited to organisms with high metabolic costs, inhabiting waters with relatively high concentrations of ammonium. The strategy of vertically migrating diatoms accumulation of nitrate in internal pools for assimilation after return to the photic zone - is best suited to slow-growing cells in low-ammonium environments. Differences between the strategies become less significant with increasing N-source limitation (the situation more typically encountered by flagellates and migratory species) because transport rather than post-transport assimilatory processes become most limiting. It is suggested that optimization of dark N-assimilation is not a critical selective feature; organisms with contrasting abilities in this regard usually inhabit different water bodies and have other more fundamental phenotypic differences (e.g. motility or silicon requirements).