Dissolved free and combined amino acids in nearshore seawater, sea surface microlayers and foams: Influence of extracellular hydrolysis

Dissolved combined (DCAA) and free (DFAA) amino acids in sea surface microlayers and foams are enriched compared to their subsurface waters in coastal areas of Long Island, NY, and the Caribbean Sea. The similarity between compositions of microlayer and subsurface DFAA and DCAA is consistent with bulk water being the main source of proteinaceous matter concentrating in the microlayer. Enrichment factors in the microlayer varied widely with time and place, but, as others have found previously, DFAA enrichment was almost always greater than DCAA enrichment. The reason for this is not obvious as the greater surface activity of DCAA should lead to their greater enrichment by most concentration mechanisms. Extracellular hydrolysis of naturally occurring DCAA to DFAA by microlayer and foam biota would be a plausible explanation of the higher DFAA: DCAA ratios observed in the microlayer relative to subsurface waters. However, we observed no change in DFAA concentration when DCAA were consumed during incubations of microlayer samples in the laboratory. We added protein to microlayer samples at different times to simulate input of proteinaceous matter from bulk water to the microlayer. The surprising absence of any change in DFAA concentrations while protein concentrations decreased suggests that DFAA production by hydrolysis and its subsequent uptake must be very tightly coupled, and that higher DFAA enrichment in the microlayer has other causes. For example, higher enrichment of DFAA could be due to a greater adsorption of DCAA than DFAA in the microlayer by particles, which concentrate in the microlayer.