Abstract
Activity at acidic pH (4.5) of β-glucosaminidase (βGAM) has been suggested as a quantitative marker for biomass of bacterivorous protists in aquatic ecosystems. βGAM is an enzyme that cleaves peptidoglycan, a major component of bacterial cell walls. Measuring the rate of cleavage of the fluorochrome methylumbelliferone (MUF) from the fluorogenic substrate MUF- N-acetyl-β- d-glucosaminide (MUF-[GlcNAc]) is a simple assay for in situ activity of βGAM. However, this approach is seriously compromised by three characteristics of the enzyme: (1) all classes of marine microbes tested: bacteria, protists, and phytoplankton, exhibit βGAM activity, (2) the pH maximum for activity of βGAM is in the range of 6–8 for all classes of marine microbes, and (3) some species of marine phytoplankton have relatively high cell-specific and volume-specific βGAM 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 biomass of heterotrophic protists, although the method could be applied in defined culture experiments.
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