Mitochondrial oxidative phosphorylation is required for ammonium assimilation in light in a marine diatom

Ammonium assimilation in plants occurs via the glutamine synthetase (GS, EC 6.3.1.2)/glutamine 2-oxoglutarate aminotransferase (GOGAT, EC 1.4.1.13 + 1.4.1.14 + 1.4.7.1) pathway. Rates of in vivo ammonium assimilation were measured in the marine diatom Phaeodactylum tricornutum by a recently developed technique that uses the protonophore carbonyl cyanide m-chlorophenylhydrazone to release unassimilated ammonium from the cells. In nitrogen-replete cells of P. tricornutum, there was a poor relationship between uptake and in vivo assimilation of ammonium, with the rate of uptake decreasing and the rate of assimilation increasing with time in the presence of ammonium. Ammonium uptake and assimilation were markedly light dependent, with assimilation inhibited by 77% in darkness. Oligomycin (5 micro g ml-1), an inhibitor of the mitochondrial ATPase, had no effect on the rate of photosynthesis, the maximum endogenous ammonium pool or GS activity in Phaeodactylum, but inhibited respiration by 24-27%. In the light, oligomycin inhibited ammonium assimilation by 55-70% and growth rate by 52%. One possible explanation for these results, namely that mitochondrial ATP is required to sustain activity of the cytosolic isoform of GS, is discussed.