N-assimilation in the noxious flagellate Heterosigma carterae (Raphidophyceae):: Dependence on light, N-source, and physiological state

The capabilities of the diel vertically migrating flagellate Heterosigma carterae Hulburt for assimilating ammonium and nitrate into cell-N in light and in darkness were studied using cells of different N-status. Ammonium utilization in darkness, except by N-replete cells, attained >50% of use in the light. However, the capacity to use nitrate was poor in darkness, and less than 20% of nitrate-N that was taken up in darkness was then actually incorporated into cell-N. The ability to assimilate N in darkness improved as N-status (N:C) declined, concurrent with an increasing content of water-soluble carbohydrate. This carbohydrate was used to support dark N-assimilation. Cells held in darkness for over a day and that had halted nitrate-uptake were still capable of taking up ammonium. Furthermore, the act of taking up ammonium appeared to make available a source of C to support nitrate uptake that was previously unavailable. The implications of these results for the ecophysiology of this organism and for the construction of mathematical models of algal growth are considered.