Metabolic regulation of amino acid uptake in marine waters’

To determine the relationships among the processes of uptake, intracellular pool formation, and incorporation of amino acids into protein, we measured the uptake of dipeptides and free amino acids by bacterial assemblages in estuarine and coastal waters of the southeast U.S. The dipeptide phenylalanyl-phenylalanine (phe-phe) lowerd V,,,,, of phenylalanine uptake when the turnover rate of phenylalanine was relatively high. When the turnover rate was relatively low, phe-phe either had no effect or increased V,,,,, of phenylalanine uptake. An analytical model was developed and tested to measure the turnover time of the intracellular pool of phenylalanine. When phe-phe was not added the turnover time of the intracellular pool was 0.89 min. Addition of 1.0 nM phe-phe increased the turnover time of this pool to 3.8 min in waters where phe-phe lowered V,,,,, of phenylalanine uptake. These results suggested that the size of the intracellular pool is regulated, which precludes high assimilation rates of both phenylalanine and phe-phe. In waters with relatively low phenylalanine turnover rates, bacterial assemblages appear to have a greater capacity to assimilate phenylalanine and phe-phe simultaneously. Marine bacterial assemblages do not substantially increase the apparent respiration of amino acids when concentrations increase. We conclude that sustained increases in uptake rates and mineralization by marine bacterial assemblages in response to an increase in the concentration of dissolved organic nitrogen is determined by the rate of protein synthesis. Metabolic regulation of amino acid uptake may prove to be the dominant intracellular mechanism by which total bacterial biomass production is controlled in aquatic environments. Recent studies have suggested that the uptake of free amino acids is a large fraction of bacterial biomass production (Lancelot and Billen 1984; Ferguson and Sunda 1984). Kirchman and Hodson (1984) showed that most of two amino acids (L-leucine and L-phenylalanine) assimilated by bacterial assemblages are incorporated into bacterial protein, which is about 50% of total bacterial biomass in culture (e.g. Ingraham et al. 1983) and in natural assemblages (Hagstrom et al. 1984). Bacteria in pure culture and natural assemblages (Kirchman et al. 1985) utilize extracellular amino acids for protein synthesis in preference to synthesizing amino acids de novo. Amino acid uptake consists of the active transport of an amino acid across the bacterial membrane into a free amino acid internal pool. The amino acid may then be incorporated into protein. We have been using dipeptides to examine the relationships among the processes of transport, possible pool buildup, and the incorporation of amino acids into protein (Kirchman and Hodson 1984). Dipeptides are assimilated by transport systems distinct from those transporting individual amino acids (see Payne 1980). Once transported across the membrane, dipeptides are quickly cleaved to the free amino acids. The addition of dipeptides to samples from aquatic environments is one way of manipulating the intracellular pool of a free amino acid in bacteria without affecting the extracellular concentration of the free amino acid. Peptide uptake probably has a role in the turnover of extracellular protein (Hollibaugh and Azam 1983). The concentration of combined amino acids in seawater, including proteins and low molecular weight peptides, is much higher than that of free amino acids (Lee and Bada 1977). However, * This research was supported by NSF grants OCE 81-17834 and OCE 84-16384 and NOAA Sea Grant high molecular weight proteins cannot be NA 80AA-D-009 1. A contribution of the Marine Inassimilated directly by bacteria (Law 1980). stitute, University of Georgia, Sapelo Island. Peptidases hydrolyze proteins to low mo2 Present address: College of Marine Studies. Unilecular weight peptides. Bacteria in pure culversity of Delaware, Lewei 19958. , ture (Payne 1980) and in mixed assemblages