Utilization of dissolved protein and amino acids in the northern Sargasso Sea

The chemical composition and degradation of dissolved amino acids and glucose by bacteria were investigated in the northern Sargasso Sea. Multiple pools of amino acids were discerned based on microbial reactivity. Concentrations of dissolved free amino acid (DFAA) were highest in the surface waters, where rapid utilization of DFAA accounted for <20% of the measured bacterial growth requirements for C and N. Dissolved combined amino acid (DCAA) and protein (using the BCA assay) concentrations were 10- to 25-fold higher than that of DFAA. In surface waters, DCAA was largely identifiable as protein (similar to 100%), but most of the DCAA at depth (>100 m) did not respond to the protein assay (<50%). In surface waters, protein was the dominant form of organic nitrogen used to support bacterial growth, supporting 20 to 65% of the calculated bacterial N demand. In deeper waters, utilization of a modified form of protein (glucosylated protein) accounted for the highest portion of the bacterial growth demands. Free and combined amino acids did not meet the full bacterial N demand at depth, and other, unmeasured nitrogen sources were required. Similar to trends in utilization, maximum uptake rate (V-max) for protein was highest in surfaces waters. Modified protein had higher V-max in deep waters, consistent with the longer turnover time of this pool of DCAA and the likelihood that it is produced in the surface and transported to depth each winter during mixing events.