RAPID BACTERIAL-DEGRADATION OF POLYSACCHARIDES IN ANOXIC MARINE SYSTEMS

Extracellular hydrolysis of organic macromolecules is often assumed to be the slow step in remineralization of organic matter. We tested thi s assumption by comparing the degradation of four polysaccharides (pul lulan, laminarin, and two polysaccharides isolated from the marine cya nobacterium Synechococcus WH7335) to determine whether size, linkage p osition, or anomeric linkage affected rates or mechanisms of carbohydr ate degradation by mixed cultures of anaerobic bacteria enriched from marine sediments. Gel permeation chromatography and nuclear magnetic r esonance spectroscopy (NMR) were used to follow the extracellular conv ersion of high molecular weight polysaccharides to lower molecular wei ght polysaccharides and oligosaccharides which were subsequently remin eralized. In all cases, substrate degradation was rapid. NMR spectra s howed that preferential hydrolysis occurred at specific chemical linka ges, and extracellular enzymatic hydrolysis of polysaccharides occurre d far more rapidly than bacterial uptake and remineralization of the l ower molecular weight oligosaccharides produced through enzymatic hydr olysis. Substrate size was not a significant determinant of reminerali zation rate: high molecular weight does not always correlate with slow degradation rate. The hypothesis that extracellular enzymatic hydroly sis is a slow step in the degradation of macromolecular organic matter in marine systems needs to be critically re-examined.