Anaerobic Metabolism in the N-Limited Green Alga Selenastrum minutum

Abstract

We have determined the flow of 15N into free amino acids of the N-limited green alga Selenastrum minutum (Naeg.) Collins after addition of 15NH4 + to aerobic or anaerobic cells. Under aerobic conditions, only a small proportion of the N assimilated was retained in the free amino acid pool. However, under anaerobic conditions almost all assimilated NH4 + accumulates in alanine. This is a unique feature of anaerobic NH4 + assimilation. The pathway of carbon flow to alanine results in the production of ATP and reductant which matches exactly the requirements of NH4 + assimilation. Alanine synthesis is therefore an excellent strategy to maintain energy and redox balance during anaerobic NH4 + assimilation.