NITROGEN-METABOLISM BY HETEROTROPHIC BACTERIAL ASSEMBLAGES IN ANTARCTIC COASTAL WATERS

Field studies to examine the in situ assimilation and production of ammonium (NH4+) by bacterial assemblages were conducted in the northern Gerlache Strait region of the Antarctic Peninsula. Short term incubations of surface waters containing N-15-NH4+ as a tracer showed the bacterial population taking up 0.041-0.128 mug-atoms Nl-1 d-1, which was 8-25% of total NH4+ uptake rates. The large bacterial uptake of NH4+ occurred even at low bacterial abundance during a rich phytoplankton bloom. Estimates of bacterial production using H-3-leucine and -adenine were 1.0 mug Cl-1 d-1 before the bloom and 16.2 mug Cl-1 d-1 at the bloom peak. After converting bacterial carbon production to an estimate of nitrogen demand, NH4+ was found to supply 35-60% of bacterial nitrogen requirements. Bacterial nitrogen demand was also supported by dissolved organic nitrogen, generally in the form of amino acids. It was estimated, however, that 20-50% of the total amino acids taken up were mineralized to NH4+. Bacterial production of NH4+ was occurring simultaneously to its uptake and contributed 27-55% of total regenerated NH4+ in surface waters. Using a variety of N-15-labelled amino acids it was found that the bacteria metabolized each amino acid differently. With their large mineralization of amino acids and their relatively low sinking rates, bacteria appear to be responsible for a large portion of organic matter recycling in the upper surface waters of the coastal Antarctic ecosystem.