Polar Biol (1994) 14:195-204 9 Springer-Verlag 1994 L.M. Tupas 9 I. Koike 9 D.M. Karl' O. Holm-Hansen Nitrogen metabolism by heterotrophic bacterial assemblages in Antarctic coastal waters Received: 8 March 1993~Accepted: 27 September 1993 Abstract Field studies to examine the in situ assimila- tion and production of ammonium (NH2) by bacterial assemblages were conducted in the northern Gerlache Strait region of the Antarctic Peninsula. Short term incubations of surface waters containing 15N-NH2 as a tracer showed the bacterial population taking up 0.041-0.128 gg-atoms N1-1 d -~, which was 8-25% of total NH2 uptake rates. The large bacterial uptake of NH2 occurred even at low bacterial abundance during a rich phytoplankton bloom. Estimates of bacterial production using 3H-leucine and -adenine were 1.0pgCl-ld-1 before the bloom and 16.2#g C 1-1 d- 1 at the bloom peak. After converting bacterial carbon production to an estimate of nitrogen demand, NH~- was found to supply 35-60% of bacterial nitrogen requirements. Bacterial nitrogen demand was also sup- ported 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 min- eralized to NH~-. Bacterial production of NH~- was occurring simultaneously to its uptake and contributed 27-55% of total regenerated NH~- in surface waters. Using a variety of ~SN-labelled amino acids it was Luis M. Tupas (~) 1 . lsao Koike Marine Biochemistry Division, Ocean Research Institute, University of Tokyo, Minamidai, Nakano, Tokyo 164, Japan David M. Karl Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822, USA Osmund Holm-Hansen Polar Research Program, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA Present address: 1 Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, Honolulu, HI 96822, USA 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. Introduction Ammonium (NH2) has long been recognized as an important nitrogen source for the nutrition of phyto- plankton (Dugdale and Goering 1967; Eppley et al. 1977; Axler et al. 1981; Suttle et al. 1990). Recent reports of NH2 utilization by heterotrophic marine bacteria have given greater importance to studies of NH2 dy- namics in marine waters (Billen 1981; Wheeler and Kirchman 1986; Tupas and Koike 1990, 1991; Goldman and Dennett 1991). Coastal waters of the Antarctic Peninsula have been found to contain high concentrations of inorganic ni- trogen in the summer months (Olson 1980; Glibert et al. 1982; R6nner et al. 1983; Koike et al. 1986) and nitrogen is presumably not limiting in this environment. Another feature of the Antarctic coastal ecosystem is the bloom of phytoplankton which is often an order of magnitude higher than open waters of the Antarctic Ocean (E1- Sayed 1987; Holm-Hansen and Mitchell 1991). Despite the high concentrations of nitrate (NO~-) in the water, NH2 is the preferred and at times the principle nitrogen source utilized by phytoplankton (Olson 1980; Glibert et al. 1982; R6nner et al. 1983; Koike et al. 1986). If heterotrophic bacteria also routinely assimilate NH2, competition can be expected between the autotrophic and heterotrophic planktonic populations. Further- more because bacteria are also capable of regenerating nitrogen, their activity as NH~ consumers or regener- ators would influence the cycling of NH2 and conse- quently other dissolved substrates.