beta-glucosaminidase activity in marine microbes

Activity at acidic pH (4.5) of beta-glucosaminidase (beta GAM) has been sug gested as a quantitative marker for biomass of bacterivorous protists in aq uatic ecosystems. beta GAM is an enzyme that cleaves peptidoglycan, a major component of bacterial cell walls. Measuring the rate of cleavage of the f luorochrome methylumbelliferone (MUF) from the fluorogenic substrate MUF-N- acetyl-beta-D-glucosaminide (MUF-[GlcNAc]) is a simple assay for in situ ac tivity of beta GAM. However, this approach is seriously compromised by thre e characteristics of the enzyme: (1) all classes of marine microbes tested: bacteria, protists, and phytoplankton, exhibit beta GAM activity, (2) the pH maximum for activity of beta GAM is in the range of 6-8 for all classes of marine microbes, and (3) some species of marine phytoplankton have relat ively high cell-specific and volume-specific beta GAM activities at pH 4.5 and/or pH 7. Based on these results, enzymatic cleavage of the MUF-[GlcNAc] substrate does not appear to be useful as a specific assay for in situ bio mass of heterotrophic protists, although the method could be applied in def ined culture experiments. (C) 1999 Federation of European Microbiological S ocieties. Published by Elsevier Science B.V. All rights reserved.