Role Of Heterotrophic Bacteria In Regulating Ammonium Utilization, New Production And C0/sub 2/ Cycling

Ammonium cycling partly determines nitrate utilization by phytoplankton, new production, and indirectly the exchange of CO2 between the atmosphere and the oceans. trophic bacteria play i.n ammonium cycling is unclear. Investigations in the subarctic Pacific and the midcontinental Atlantic shelf indicate the heterotrophic bacteria consume or regenerate ammonium in response to the supply of dissolved free amino acids. Results of whole water incubations with 15NH4+ indicate that bacteria can consume approximately 30% of the ammonium taken up in subarctic Pacific waters. show that amino acid additions (0.3-1.0 p M ) inhibit uptake of ammonium by bacteria. In 0.8 pm size fractions dominated by heterotrophic bacteria, bacteria regenerated ammonium only when amino acid supply met or exceeded bacterial nitrogen demand. In coastal mid-Atlantic waters, amino acid additions (1.0 p M ) have been shown to inhibit uptake of 15N-Nitrate by phytoplankton. This inhibition is hypothesized to be the result of increased availability of ammonium due to either decreased bacterial utilization or increased bacterial utilization of amino acids and ammonium in part regulates nitrate utilization, and thus indirectly ei'fects new productions, flux of organic matter to the deep ocean, and CO2 cycling. The role hetero-