SIMULTANEOUS UPTAKE AND REGENERATION OF AMMONIUM BY MIXED ASSEMBLAGES OF HETEROTROPHIC MARINE-BACTERIA

Utilization of dissolved organic nitrogen (DON) and ammonium by natural assemblages of heterotrophic marine bacteria was studied in cultures of original seawater using N-15-ammonium. Increase of N-15 in particulate organic matter with time, i.e. newly formed bacterial cells, signified active assimilation of ammonium; however, the N-15 content of ammonium in the media was continuously diluted, indicating simultaneous mineralization of DON. Ammonium supplied up to 80% of nitrogen in new bacterial cells, yet more than half of the DON taken up, mostly in the form of dissolved combined amino acids, was converted to ammonium. This dual function of marine microbial populations is not in conformity with present ideas of microbial nitrogen metabolism and suggests that amino acid uptake is governed more by metabolic constraints than by substrate composition. These results give equal weight to the importance of bacterial heterotrophs as assimilators and regenerators of nitrogen in the marine environment.