Are Bacteria the Major Producers of Extracellular Glycolytic Enzymes in Aquatic Environments?

In aquatic microbial ecology, it has been considered that most extracellular enzymes except phosphatases are of bacterial origin. We tested this paradigm by evaluating the relationship between bacterial cell number and the activity of three glycolytic enzymes from 17 fresh waters and also from a laboratory experiment. Our large sets of pooled data do not seem to support such a simple explanation, because we found only a weak correlation of bacterial number with activity of α ‐glucosidase (r s = 0.63), β ‐glucosidase (r s = 0.45), and β ‐ N ‐acetylhexosaminidase (r s = 0.44). We also tested relations of the enzymatic activities to potential sources of natural substrates: dissolved organic carbon (DOC) and phytoplankton (as chlorophyll a ). Their correlations with the enzymatic activities tested were very weak or insignificant. On the other hand, we found evidence for distinct producers of extracellular enzymes by analysing enzyme kinetics. The kinetics usually did not follow the simple Michaelis‐Menten model but a more complex one, indicating a mixture of two enzymes with distinct affinity to a substrate. In combination with size fractionation, we could sometimes even distinguish three or more different enzymes. During diatom blooms, the diatom biomass tightly correlated with β ‐ N ‐acetylhexosaminidase activity (>4 μm fraction). We also documented very tight relationships between activity of both glucosidases and dry weight of Daphnia longispina (r s = 1.0 and 0.60 for α ‐ and β ‐glucosidases, respectively) in an alpine clear‐water lake. Our data and evidence from other studies indicate that extracellular glycosidic activities in aquatic ecosystems cannot generally be assigned only to bacteria. Also invertebrate animals and other eukaryotes (fungi, diatoms, protozoa etc.) should be considered as potentially very important enzyme producers. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)