Biochemical, physiological and ecological aspects of ammonium regeneration by marine crustaceans

Abstract

Laboratory investigations were carried out with the marine mysid, Praunus flexuosus, to assess the degree of coupling between glutamate dehydrogenase (EC 1.4.1.3; GDH) activity and the rate of whole animal ammonium excretion. GDH extracted from P. flexuosus showed the highest activities between pH 8.2-8.6 with a pH optimum near 8.5. A Hanes-Woolf transformation of the substrate kinetic data yielded an apparent Km of 2.5 mM glutamate. A strong correlation (r2 = 0.916) between GDH activity and the rate of ammonium excretion by this mysid confirmed that ammonium excretion rates could be accurately predicted by employing the enzyme assay presented here. GDH was strongly activated by ADP and inhibited by GTP. Such a sensitivity to purine energy charge implies that GDH may indirectly regulate growth and energy production as well as ammonium excretion in marine crustaceans. GDH activity surveyed in different taxa of marine crustaceans collected in the vicinity of Boothbay Harbor, Maine, was higher in muscle tissue extracts than in gill tissue extracts or in a whole animal homogenate prepared from the mollusc Mya arenaria. GDH extracted from Carcinus maenas muscle tissue had an apparent Km of 1.5 mM glutamate, and was saturated by NAD+ concentrations in excess of 1.2 mM. The localization of GDH in decapod crustacean muscle tissue suggests that the primary site for ammonium formation probably occurs in the muscle and not in the gills. GDH:NH4^+ excretion and ETS:oxygen consumption ratios surveyed among western Gulf of Mexico zooplankton averaged 18.7 ± 4.3 and 2.65 ± 0.55, respectively. These ratios were used to evaluate the NH4^+ excretion and respiration potential of two natural zooplankton assemblages sampled quantitatively with a Multiple Opening and Closing Net and Environmental Sensing System (MOCNESS). Greater than 80% of the total GDH and ETS activity 0-200m was concentrated in depth strata above the chlorophyll maximum, suggesting a strong zooplankton-phytoplankton grazing interaction. O:N metabolic ratios were estimated for the zooplankton sampled with the MOCNESS by ratioing GDH-excretion and ETS-respiration impacts by atoms. Lowest O:N ratios were found in depth strata above the chlorophyll maximum, suggesting a dominance of protein-based grazing and/or predation...