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

The microbial remineralization of high molecular weight dissolved organic c arbon (DOC) begins with extracellular enzymatic hydrolysis, since macromole cules must be hydrolyzed to produce substrates sufficiently small to cross microbial membranes. In order to investigate substrate and size-class relat ed patterns in extracellular enzymatic activity in the water column, potent ial 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 sea son, but xylan hydrolysis rates typically exceeded those of laminarin by fa ctors 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 lamin arin-hydrolyzing activity was associated with the >0.45 mum size fraction. The contribution of free dissolved enzymes (the <0.2 mum 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, an d nearly 100% of the laminarin-hydrolyzing activity at the midbay and plume stations. These spatial and temporal variations in the contributions of fr ee enzymes to laminarin and xylan hydrolysis may help to explain the decoup ling of measurements of specific enzyme activities from large-scale measure ments of total microbial populations. The contrasting behaviors of laminari n, xylan, pullulan, and fucoidan, all of which are soluble polysaccharides, suggest that certain types of polysaccharides may resist hydrolysis by pla nktonic microbial extracellular enzymes and, therefore, are relatively unav ailable as substrates to the planktonic community.