Bacterial degradation of protein adsorbed to model submicron particles in seawater

We tested the hypothesis that protein adsorbed to submicron particles in seawater is more slowly degraded than the same protein freely dissolved. Bacterial hydrolysis of methyl-H-3-bovine serum albumin (H-3-BSA) dissolved or adsorbed to particles (polystyrene latex beads, diameters 0.126 to 1.5 mu m) was examined. Mixed bacterial assemblages cultured on BSA media, 2 bacterial isolates, and a natural marine bacterial assemblage hydrolyzed adsorbed H-3-BSA at much slower rates (1/10 to <1/200) than free H-3-BSA. We compared our hydrolysis rates with the predicted collision probabilities between bacteria and H-3-BSA freely dissolved or adsorbed to beads of different diameters. These comparisons suggest that the hydrolysis rate is influenced primarily by the transport rate of H-3-BSA to bacteria. Hydrolysis of adsorbed H-3-BSA differed greatly between 2 bacterial strains, which appears to be explained by differences in the affinity of bacterial proteases to H-3-BSA. Our results support the hypothesis that adsorption onto submicron particles greatly reduces degradation rate of proteins and suggest that the adsorption of dissolved organic matter (DOM) may be a mechanism that temporarily stores labile DOM in a slowly degrading, 'semi-labile' pool, which makes it available for export and further modification by geochemical processes.