Anoxic and sulfidic sediments formed during intensive milkfish farming in Bolinao Bay (Lingayen Gulf, Luzon, Philippines) showed concentrations of total proteinaceous matter roughly three times higher (up to16.9 mg cm−3) than those of less reduced sediments. Ectoprotease activities in these coastal sediments suggested an inhibition of enzymatic recycling of proteins by sulfide. Sodium sulfide proved capable of inhibiting protease activity produced by a bacterial isolate from a sulfidic sediment site (Ki = 20 mM Na2S · 9H2O). Reoxidation and removal of H2S from sulfidic sediments, however, proved inadequate to recover proteolytic activity. Hence, H2S formed during organic matter mineralization via bacterial sulfate respiration may have initiated an irreversible inhibition of the enzymatic degradation of proteinaceous matter from fish farming. Ectoprotease activity was positively correlated (r = .93) with redox potential values. Lowest proteolytic activities were found in strongly reduced sediments, that were predominantly fine grained (very fine sand and silt) and sulfidic at the same time. Low ectoprotease activity in these sulfide-rich sediments can be interpreted as a kind of negative feedback mechanism that would mitigate the immediate recycling of excessive amounts of feed borne protein-rich deposits. Proteins may be further protected from degradation by selective particle adsorption in the finest grained deposits of fish farming waste.

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