Enhanced extracellular enzymatic peptide hydrolysis in the sea-surface microlayer

The accumulation of dissolved organic matter (DOM) at the air-sea interface is controlled by dynamic physical processes at the boundary between ocean and atmosphere. Much of the DOM concentrated in the surface microlayer is t hought to be protein or glycoprotein. Enzymatic hydrolysis of these and oth er biopolymers is an important step in the microbial uptake of dissolved an d particulate organic matter in many aquatic environments. We employed a se nsitive fluorescence technique to investigate differences between extracell ular enzymatic peptide hydrolysis in the sea surface microlayer and corresp onding subsurface water from Stony Brook Harbor, NY. We separated the micro layer from its underlying water and thus measured hydrolysis potential rath er than an in-situ process. Peptide turnover was always faster in the micro layer than in subsurface waters. This was confirmed by allowing a new surfa ce film to form on subsurface water; hydrolysis was still faster in the new surface film. In a year-long study, we found the relative difference betwe en turnover times in the surface film and subsurface waters to vary greatly with season. While rate constants of peptide hydrolysis were generally hig her in both microlayer and bulk water samples in spring/summer than in fall /winter, the difference in activity between the two environments was greate st in winter. Enhanced hydrolysis in the sea surface microlayer is likely d ue to the greater concentrations of DOM in the microlayer. Seasonal changes in distribution of hydrolytic activity between surface film and subsurface water probably reflect seasonal variation in the mechanisms of DOM enrichm ent, which depend On water temperature, substance and energy fluxes across the water-air boundary; activity of aquatic organisms and other seasonal va riables. (C) 2001 Elsevier Science B.V. All rights reserved.