Dissolved free amino acids in the Sargasso Sea: uptake and respiration rates, turnover times, and concentrations

'Clean' techniques and high specific activity tritiated substrates were used to examine bacteria growth rates and uptake kinetics, and estimate turnover times and maximum concentrations (K, + S) of several dissolved free amino acids (DFAAs) in oligotrophic oceanic waters near Bermuda. Additions of a few nanornolar or less of a particular amino acid was generally but not always sufficient to generate uptake rates consistent with saturation-type kinetics. The proportion of label respired was generally 60 to 80 % of the tritium taken up for glutamate, glutamine, alanine, glycine, serine and glucose. For leucine and ornithine < 10 % of the tritium taken up was respired. The percentage of label respired was not dependent on the C : N ratio of the substrate or the DFAA concentration, over the range that we examined. Although there are a number of explanations for the high percentage of label respired, one possibility is that high C : N ratio substrates were supporting bacterial growth. If so, bacteria may not have been serving as N remineralizers in this system. There were significant differences in turnover times of individual DFAAs with alanine being shortest (3.6 to 13.2 h) and ornithine being longest (48 to 182 h). Maximum concentration estimates of DFAAs were in the lowand sub-nM range whether determined from uptake kinetics data or by HPLC. From the turnover times and concentration estimates, we calculated the flux into particulate matter for each of the amino acids studied. Tentative calculations indicate that individual DFAAs could supply as much as 20 % of the C and N required to support bacterial growth. Our results indicate that although DFAA pools are small they are rapidly recycled and could be significant sources of C and N for bacterial growth in the Sargasso Sea

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