The relationship between ammonia excretion and GDH activity in marine zooplankton

Abstract

The relationship between the ammonia excretion rate and glutamate dehydrogenase (GDH) activity was studied in marine zooplankton over the course of a bloom and in different size classes (100–200,200–500 and 500–1000 μm). A weak correlation between GDH activity and ammonia excretion rate was observed when all data were pooled. Better relationships between the parameters were obtained by taking into account the substrate being metabolized, as deduced from the experimentally determined O/NH4 ratio. There was also a positive correlation between the GDH/NH4 and O/NH4 ratios, suggesting that the former ratio was lower when the metabolic substrate being metabolized con tained a high level of nitrogen. High GDH/NH4 and high standard deviations were found when the in situ temperatures were low, while the ratio and standard deviations decreased at higher temperatures. Temperature probably had an indirect effect as a consequence of a better availability of nitrogen in the food when mixing took place in the water column. Differences in the GDH/ammonia ratio were also observed for different size fractions, largely because small animals had higher ammonia excretion rates. The composition of the metabolic substrate affected the observed GDH/NH4 ratios because it led to variations in cellular glutamate concentrations, while enzyme activities were measured at maximal rates (Vmax). It is suggested that this methodological limitation is probably the most impor tant factor in determining the relationship between enzymatic activities and metabolic rates.