Amino acid and ammonium utilization by heterotrophic marine bacteria grown in enriched seawater

The nitrogen metabolism of natural bacterial assemblages was studied in seawater cultures enriched with the dissolved materials released by the mussel Mytilus edulis . The mussel released dissolved organic C, NH 4 + , and dissolved organic N (DON), mostly in the form of combined amino acids (∼70–100%) that constituted 30% of the total N released. 15 NH 4 + techniques showed that the contribution of NH 4 + to total microbial N demand during growth ranged from 50 to 88% even when large amounts of DON were utilized (∼70–260% of total bacterial N produced). An average of 80% of the DON used was converted to NH 4 + , showing simultaneous assimilation and regeneration of NH 4 + by the natural microbial population and contradicting the idea that NH 4 + assimilation occurs only when organic N is limiting.