MEPS 196:195-206 (2000)  -  doi:10.3354/meps196195

ABSTRACT: The chitinolytic enzyme chitobiase (N-acetyl-β-d-glucosaminidase), which is expressed during the pre-moult phase of crustaceans, is released into the ambient water after shedding of the exuvium. We examined the potential use of chitobiase activity in water samples as a measure of secondary production of crustaceans. Chitobiase activity, expressed on the substrate 4-methylumbelliferyl-N-acetyl-β-d-glucosaminide (MUFNAG), was measured in experiments with the marine copepod Temora longicornis, with focus on the dissolved (Œfree¹, <0.2 µm) fraction. Significant activity of free chitobiase was found in the ambient water of individual copepodites when they moulted to enter the next stage. The activity of the chitobiase released was linearly related to body weight and to the increment of body weight. In bacteria-poor water (filtered through 0.2 µm), no significant decrease of free T. longicornis chitobiase occurred in the first 24 h. In non-manipulated water, the decay of free T. longicornis chitobiase by bacteria was exponential with time, with relative rates varying by 4 to 13% h^-1 at 15°C. The particle-bound chitobiase activity was related to the number of bacteria. Different sources of chitobiase could be distinguished by enzyme characteristics. The K_m value of T. longicornis chitobiase was 55 µmol l^-1, comparable to values published for Daphnia species. In Oxyrrhis marina, a heterotrophic dinoflagellate serving as the main food item in the T. longicornis culture, enzyme assays indicated a K_m < 0.2 µmol l^-1. Water predominantly containing bacteria showed a K_m > 100 µmol l^-1. The usefulness of free chitobiase as a natural tracer of secondary production was tested in a culture vessel of T. longicornis. The rate of chitobiase release was derived from the chitobiase activity in the ambient culture water over time and the rate of decay in water samples devoid of copepods. From the relation between release of chitobiase and increase of body weight of moulting copepodites, the total daily biomass increase in the culture vessel was calculated. These estimates compared well with classical monitoring of biomass changes. If this relation found in T. longicornis is representative of marine crustaceans in general, tracking of free chitobiase in water samples could be a biochemical tool for estimating in situ secondary production in natural waters.

KEY WORDS: Marine zooplankton · N-acetyl-β-d-glucosaminidase · Chitobiase · 4-methylumbelliferyl-N-acetyl-β-d-glucosaminide · Secondary production · Temora longicornis