MEASURING RATES OF AMMONIUM ASSIMILATION IN MARINE-ALGAE - USE OF THEPROTONOPHORE CARBONYL CYANIDE M-CHLOROPHENYLHYDRAZONE TO DISTINGUISH BETWEEN UPTAKE AND ASSIMILATION

A method for determining rates of ammonium assimilation in marine alga e is described. Ammonium assimilation is defined as the decrease in to tal (medium + cellular) ammonium. The protonophore carbonyl cyanide m- chloro-phenylhydrazone (CCCP) was used to distinguish between uptake a nd assimilation of ammonium. Ammonium uptake by nitrogen-replete and n itrogen-starved cells of the diatom Phaeodactylum tricornutum Bohlin a nd the green macroalga Enteromorpha sp. was completely (98%-99%) inhib ited in the presence of 100 mu M CCCP. In addition to inhibiting furth er uptake of ammonium, CCCP promoted the release of unassimilated ammo nium by nitrogen-replete and nitrogen-starved P. tricornutum and Enter omorpha that had been allowed to take up ammonium for a period. Most ( 97.5%) of preaccumulated C-14-methylammonium was released by nitrogen- starved P. tricornutum in the presence of CCCP. Specific rates of ammo nium assimilation in nitrogen-replete cultures of P. tricornutum were identical to the maximum growth rate, but specific rates in nitrogen-s tarved cultures were fourfold greater. Rates of ammonium assimilation in Enteromorpha during both the surge and the internally controlled up take phases were the same as the internally controlled rate of uptake, suggesting that the latter is a reliable measure of the maximum rare of assimilation.