Characteristics and Diversity of beta-d-Glucosidase (EC 3.2.1.21) Activity in Marine Snow

Large amorphous aggregates commonly described as marine snow were sampled in the water column of the northern Adriatic Sea in August 1991. beta-d-Glucosidase activity associated with these aggregates was characterized. Enzymatic activity was measured with the fluorogenic substrate analog 4-methylumbelliferyl-beta-d-glucoside and found to be mainly associated with particles; on average, only 24% of the whole beta-d-glucosidase activity remained in the supernatant after centrifugation. Although the temperature optimum for beta-d-glucosidase activity was approximately 40 degrees C, incubation of the previously liberated particle-attached enzyme at 50 degrees C for 20 min caused a >90% reduction of enzymatic activity relative to the activity at 40 degrees C. The level of inactivation of beta-d-glucosidase was much lower, however, when whole marine snow was incubated, indicating qualitative modifications of beta-d-glucosidase in marine snow. Separation of beta-d-glucosidase by different approaches indicated that the diversity of isoenzymes is restricted. In samples taken from the pycnocline, only one major isoenzyme was present in noticeable amounts. This isoenzyme contributed up to 70% of the whole beta-d-glucosidase activity detectable by two different chromatographic separations (anion-exchange chromatography and size exclusion chromatography). Although the same isoenzyme was dominant in marine snow taken from surface waters (0.5-m depth), we found a second peak of activity which eluted at lower NaCl concentrations from the anion-exchange column. Generally, the diversity of isoenzymes exhibiting beta-d-glucosidase activity seems to be surprisingly small in amorphous pelagic aggregates.