GDH activity and ammonium excretion in the marine mysid, Leptomysis lingvura: Effects of age and starvation

Ammonium (NH4+) release by bacterial remineralization and heterotrophic grazers is the largest recycled nitrogen source in the euphotic zone. It determines the regenerated fraction of phytoplankton productivity, so the measurement of NH4+ excretion in marine organisms is necessary to characterize both the magnitude and the efficiency of the nitrogen cycle. Glutamate dehydrogenase (GDH) is largely responsible for NH4+ formation in crustaceans and consequently should be useful in estimating NH4+ excretion by marine zooplankton. Here, we study the physiological rate of NH4+ excretion and the GDH activity in an important North Atlantic mysid, Leptomysis lingvura. We address body size and starvation as sources of variability on the GDH to NH4+ excretion ratio (GDH/RNH4+).We found a strong correlation between the RNH4+ and the GDH activity (r2 = 0.87, n = 41) during growth. Both variables were regressed against protein in order to obtain the allometric scaling exponent. Since GDH activity maintained a linear relation (b = 0.93) and RNH4+ scaled exponentially (b = 0.55) in well fed mysids, the GDH/RNH4+ ratio increased with size. However, the magnitude of its variation increased even more when adult mysids were starved. In this case, the GDH/RNH4+ ratio ranged from 11.23 to 102.41.

► We described effects of age and starvation on GDH activity and NH4+ excretion. ► The GDH assay was adapted to fluorometry to reduce the biomass required for analysis. ► Both age and starvation affect the physiology and the enzymology unevenly. ► Starvation causes NH4+ excretion and GDH activity to diverge more than does biomass.