THE RELATIONSHIP BETWEEN AMMONIA EXCRETION AND GDH ACTIVITY IN MARINEZOOPLANKTON

The relationship between the ammonia excretion rate and glutamate dehy drogenase (GDH) activity was studied in marine zooplankton over the co urse of a bloom and in different size classes (100-200, 200-500 and 50 0-1000 mu m). A weak correlation between GDH activity and ammonia excr etion rate was observed when all data were pooled. Better relationship s between the parameters were obtained by taking into account the subs trate being metabolized, as deduced from the experimentally determined O/NH4 ratio. There was also a positive correlation between the GDH/NH 4 and O/NH4 ratios, suggesting that the former ratio was lower when th e metabolic substrate being metabolized contained a high level of nitr ogen. High GDH/NH4 and high standard deviations were found when the in situ temperatures were low, while the ratio and standard deviations d ecreased at higher temperatures. Temperature probably had an indirect effect as a consequence of a better availability of nitrogen in the fo od 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 composi tion of the metabolic substrate affected the observed GDH/NH4 ratios b ecause it led to variations in cellular glutamate concentrations, whil e enzyme activities were measured at maximal rates (V-max). It is sugg ested that this methodological limitation is probably the most importa nt factor in determining the relationship between enzymatic activities and metabolic rates.