Occurrence of exo- and endopeptidases in dissolved and particulate fractions of coastal seawater

We analyzed potential activities of different proteolytic enzymes in size-fractionated seawater, and estimated the contribution of each size fraction (< 0.2, 0.2-0.8, 0.8-5, and > 5 mu m) to the bulk hydrolytic activity of each enzyme in the seawater. The activity of leucine-aminopeptidase was highly attributed to cell-associated size fractions, while the contribution of the dissolved fraction (< 0.2 mu m) to the bulk activity was only 10 to 30 %. In contrast, the contribution of the dissolved fraction to the activities of the trypsin- and chymotrypsin-type endopeptidases was as high as 40 to 80 % of their bulk activities measured in unfiltered seawater. These results indicated the potential importance of free proteolytic enzymes in seawater, especially for endopeptidases. Significant enzymatic activity in the dissolved fraction was also detected from experiments with isolated bacteria, suggesting that direct secretion of proteases from marine bacteria into surrounding water could be at least one of the sources of the dissolved proteolytic enzymes in seawater. Generally, the natural bacterial community of seawater was able to hydrolyze all of the 16 tested substrates, but at different rates. Selected members of the community (3 bacterial isolates and Synechococcus) hydrolyzed only one or a few of the applied substrates, and the substrate preference varied among the strains. These results suggest that natural bacterial communities are composed of a great variety of bacterial species with different (specific) enzymatic properties, including dissolved endopeptidases and cell-associated aminopeptidase. The combined activities of these enzymes are responsible for an effective degradation and re-use of high molecular weight organic matter at the community level.