AMMONIUM REGENERATION AND CARBON UTILIZATION BY MARINE-BACTERIA GROWN ON MIXED SUBSTRATES

We examined the impact of exposing natural populations of marine bacteria (from seawater collected near Woods Hole, Massachusetts, USA) to multiple nitrogen and carbon sources in a series of batch growth experiments conducted from 1989 through 1990. The substrate C:N ratio (C:N(S)) was varied from 1.5:1 to 10:1 either with equal amounts of NH4+ and different amino acids or an amino acid mixture, all supplemented with glucose to maintain the C:N(S) ratio equal to that of the respective amino acid, or with combinations of glucose and NH4+ alone. A common feature of the experiments involving amino acids was the concurrent uptake of NH4+ and amino acids that persisted as long as a readily assimilable carbon source (glucose in our case) was taken up. There was no net regeneration of NH4+, even though catabolism of amino acids occurred. Regeneration of NH4+ was evident only after glucose was completely utilized, which usually occurred at the end of exponential growth. The contribution of (NH4+)-N-15 to total nitrogen uptake by the end of exponential growth varied from approximately 60 to 80% when individual amino acids were present and down to approximately 24% when the amino acid mixture was added. These estimates are conservative because we did not account for possible isotope dilution effects resulting from amino acid catabolism. When NH4+ and glucose were the sole nitrogen and carbon sources, there was a stoichiometric balance between glucose and NH4+ uptake over a wide range of C:N(S) ratios, leading to a constant bacterial biomass C:N ratio (C:N(B)) of approximately 4.5:1. As a result NH4+ usage varied from 50% when the C:N(S) ratio was 3.6:1, to 100% when the C:N(S) ratio was 10:1. Gross growth efficiency varied from approximately 60% when NH4+ plus glucose were added alone or with the amino acid mixture, to 47% when the individual amino acids were used in place of the mixture. It is thus evident that actively growing bacteria will act as sinks for nitrogen when a carbon source that can be assimilated easily is available to balance NH4+ uptake, even when amino acids are available and are being co-metabolized.