Extracellular enzyme activity in a river-bay-shelf transect: variations in polysaccharide hydrolysis rates with substrate and size class

The microbial remineralization of high molecular weight dissolved organic carbon (DOC) begins with extracellular enzymatic hydrolysis, since macromolecules must be hydrolyzed to produce substrates sufficiently small to cross microbial membranes. In order to investigate substrate and size-class related patterns in extracellular enzymatic activity in the water column, potential hydrolysis rates of 4 polysaccharides (xylan, laminarin, pullulan, and fucoidan) were measured at stations in the Delaware River, Bay, and shelf. Potential hydrolysis rates of xylan and laminarin varied by station and season, but xylan hydrolysis rates typically exceeded those of laminarin by factors of 2 to 10. In contrast, pullulan and fucoidan hydrolysis rates were (with a single exception for fucoidan) essentially zero for all seasons and stations. Size fractionation experiments showed that most xylan- and laminarin-hydrolyzing activity was associated with the >0.45 μm size fraction. The contribution of free dissolved enzymes (the <0.2 μm fraction), however, was at times substantial. In September, when potential activities of both enzymes were at a maximum, the free-enzyme fraction contributed 48 to 69% of the xylan-hydrolyzing activity at the freshwater and midbay stations, and nearly 100% of the laminarin-hydrolyzing activity at the midbay and plume stations. These spatial and temporal variations in the contributions of free enzymes to laminarin and xylan hydrolysis may help to explain the decoupling of measurements of specific enzyme activities from large-scale measurements of total microbial populations. The contrasting behaviors of laminarin, xylan, pullulan, and fucoidan, all of which are soluble polysaccharides, suggest that certain types of polysaccharides may resist hydrolysis by planktonic microbial extracellular enzymes and, therefore, are relatively unavailable as substrates to the planktonic community.