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

Abstract

Ammonium (NH 4 +) 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 NH 4 + 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 NH 4 + formation in crustaceans and consequently should be useful in estimating NH 4 + excretion by marine zooplankton. Here, we study the physiological rate of NH 4 + 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 NH 4 + excretion ratio (GDH/R NH 4 + ). We found a strong correlation between the R NH 4 + and the GDH activity (r 2 = 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 R NH 4 + scaled exponentially (b = 0.55) in well fed mysids, the GDH/R NH 4 + ratio increased with size. However, the magnitude of its variation increased even more when adult mysids were starved. In this case, the GDH/R NH 4 + ratio ranged from 11.23 to 102.41.