We tested the hypothesls that proteln adsorbed to submlcron part~cles In seawater IS more slowly degraded than the same proteln freely d~ssolved Bacterial hydrolysis of methyl-3H-bov~ne serum a l b u m~n ( 3 H -B S~) dlssolved or adsorbed to particles (polystyrene latex beads diameters 0 126 to 1 5 pm) was examlned Mlxed bactenal assemblages cultured on BSA media, 2 bactenal ~solates, and a natural marlne bactelial assemblage hydrolyzed adsorbed 'H-BSA at much slower rates (1/10 to <1/200] than free 'H-BSA We compared our hydrolysls rates wlth the predicted collision probabilities between bactena and 3 H -B S~ freely dlssolved or adsorbed to beads of different diameters These comparisons suggest that the hydrolysls rate is ~nfluenced pnmanly by the transport rate of 'k-I-BSA to bacteria Hydrolysis of adsorbed 3 H -B S~ differed greatly between 2 bactenal strains whlch appears to be explained by differences In the afflnity of bactenal proteases to 3 H -B S~ OUI results support the hypothesls that adsorption onto subm~cron particles greatly reduces degradation rate of protelns and suggest thdt the adsorption of dissol\ied organic matter (DOM) may be a mechanism that temporarily stores lablle DOM in a slowly degrading, seml-labile pool, whlch makes 11 available for export and further modification by geochem~cal processes KEY WORDS-Bacterial degradation Dissolved organlc matter . Adsorption . Collolds INTRODUCTIONThe recent discovery of many detrital colloids and submicron particles in a wide variety of oceanic environments (Koike et al. 1990, Wells & Goldberg 1991 has renewed interests in the role of colloids in biogeochemical cycles of manne environments. These non-living organic collo~ds are much mol-e abundant (10"o 10'' particles ml-l, depending on size) than microorganisms including bacteria and viruses (Ko~ke et al. 1990, Wells & Goldberg 1991) and can represent a significant fraction (10 to 35%) of the organic carbon pool in the oceans (Benner et al. 1992). The chemical nature, sources, sinks, and turnover of colloids have yet to be clarified (Lee & Wakeham 1992, Nagata & Koike 1995).One potentially important role of colloids and submicron particles in oceanic material cycling is that they 'Present address Ocean Research Institute, The University of Tokyo 1-15-1 Mlnami-dal, Tokyo 164, Japan E-mail nagata@ori U-tokyo ac ]p provide large solid surfaces to which dissolved organic matter (DOM) may adsorb. In fact, prevlous studies have shown that adsorption of polymeric DOM such as protein to surfaces in seawater can occur very rapidly (within minutes; Kirchman et al. 1989) and that the adsorption can have substantial effects on degradation rates of organic material (van Loosdrecht et al. 1990, Fletcher 1991). Keil & Kirchman (1994 suggested that DOM adsorbed to colloids was less easily degraded than freely dissolved DOM. They found that dissolved protein aged in filtered seawater 1s abiotically modified to less b~ologically degradable protein in a short period (110 h), an observation that they could ex...
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