Potential glutamine synthetase and glutamate dehydrogenase activities in the oligo-mesotrophic Lake Pavin: annual comparison with an indirect estimation of inorganic nitrogen utilization

Abstract

The annual distribution of potential glutamine synthetase (GS) and glutamate dehydrogenase (GDH) activities was studied in the oligo-mesotrophic Lake Pavin (France, Massif Central). The validity of these enzymatic activies as an index of N-assimilation in field studies was tested by a comparison with the coupling of distributions of inorganic nitrogen uptake rate based on 14 C incorporation into proteins together with several abiotic and biotic parameters. The potential GS and GDH activities recorded during the annual phytoplankton succession were in the same order of magnitude as those reported from the very scanty literature on this subject. Moreover, specific potential GS and GDH activities were significantly correlated with the external NO 3 - -N concentration suggesting that these enzymatic activities reflected nitrate utilization by algae. Specific activities of GDH were about 2.5 fold higher than GS specific activities suggesting that the GDH pathway may play a leading part in inorganic nitrogen assimilation in the oligo-mesotrophic Lake Pavin. However, N-assimilation rates based on the enzymatic approach were lower than those obtained through indirect isotopic determination of N-uptake rates. The reasons for such a discrepancy are discussed.