Glutamate dehydrogenase (GDH) and respiratory electron-transport-system (ETS) activities in Gulf of Mexico zooplankton

Abstract

Ratios of GDH [glutamate dehydrogenase] activity:NH4+ excretion and ETS [electron transport system] activity: O2 consumption were measured in western Gulf of Mexico zooplankton and averaged 18.7 .+-. 4.3 and 2.65 .+-. 0.55, respectively. These ratios were used to estimate NH4+ excretion and respiration rates of 2 natural zooplankton assemblages sampled quantitatively for GEH and ETS activity with a multiple opening and closing net and environmental sensing system (MOCNESS). Greater than 80% of the total GDH and ETS activity 0-200 m was concentrated in depth strata above the chlorophyll maximum, suggesting a strong zooplankton-phytoplankton grazing interaction. GDH activity per unit of zooplankton protein biomass was .apprx. 3-fold greater in the upper 100 m than between 100-200 m, while ETS activity per unit of zooplankton protein biomass showed no consistent pattern with increasing depth. O:N metabolic quotients were estimated for the zooplankton samples with the MOCNESS by ratioing GDH-excretion and ETS-respiration by atoms. Lowest O:N quotients occurred in depth strata above the chlorophyll maximum, suggesting a predominance of protein-based grazing and/or predation.