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Abstract:
Six structurally distinct polysaccharides were fluorescently labeled and used to compare potential hydrolysis rates and substrate specificities of extracellular enzymes in bottom water and surface sediments. Potential hydrolysis rates differed by factors of 10–100 among the six polysaccharides. The relative order of potential hydrolysis rates in sediments was distinctly different from that of the bottom water samples. In surface sediments, pullulan hydrolysis was extremely rapid, and relative hydrolysis rates decreased in the order pullulan ≫ laminarin > chondroitin sulfate > xylan > arabinogalactan ≫ fucoidan. In bottom water, in contrast, pullulan, arabinogalactan, and fucoidan were barely hydrolyzed, whereas chondroitin sulfate, xylan, and laminarin were hydrolyzed relatively rapidly. Hydrolysis rates decreased in the order xylan > chondroitin sulfate > laminarin >>> arabinogalactan ≅ pullulan ≅ fucoidan. The differences among the relative hydrolysis rates might reflect fundamental differences in seawater and sedimentary microbial communities with disparate extracellular enzymatic capabilities. Carbohydrates are significant constituents of dissolved organic carbon in seawater and are detectable as molecularly distinct structures in sediments of significant geologic age. Slow hydrolysis may provide the time required for geochemical transformations that further increase resistance to remineralization.
