4-Methylumbelliferyl-beta-N-Acetylglucosaminide Hydrolysis by a High-Affinity Enzyme, a Putative Marker of Protozoan Bacterivory

Hydrolysis of an artificial fluorogenic substrate, 4-methylumbelliferyl-beta-N-acetylglucosaminide, has been studied in a monoculture predator-prey system with either a flagellate (Bodo saltans) or a ciliate (Cyclidium sp.) fed upon pure bacterial culture (Aeromonas hydrophila or Alcaligenes xylosoxidans). Aeromonas hydrophila produced a low-affinity beta-N-acetylglucosaminidase-like enzyme (K(m), >100 mumol liter) but Alcaligenes xylosoxidans did not. Inoculation of both bacterial strains with bacterivorous protozoa induced the occurrence of another, high-affinity, beta-N-acetylglucosaminidase-like enzyme (K(m), <0.5 mumol liter). The latter enzyme showed significant, close correlations with total grazing rates of both B. saltans (r = 0.96) and Cyclidium sp. (r = 0.89) estimated by using uptake of fluorescently labelled bacteria. Further significant correlations between several protozoan parameters and kinetic parameters of this enzyme suggest its likely protozoan origin. If both types of enzyme occurred together, they could be satisfactorily distinguished by using kinetic data analysis. Hence, measurements of beta-N-acetylglucosaminidase-like activities might be promising to use to improve estimations of protozoan bacterivory.