Glutamine Synthetase activity in limnetic phytoplankton

The seasonal distribution of potential Glutamine Synthetase (GS) activities was studied in eutrophic Lake Aydat (France, Massif Central). The validity of this index of N-assimilation in field studies was tested by a comparison with the coupling of distributions of inorganic nitrogen uptake rate based on¹⁴C incorporation into proteins and the amino acids to protein ratio together with several abiotic and biotic parameters. The potential GS activities recorded during the seasonal succession of phytoplankton (characterized by the dominance of diatoms) were in the same order of magnitude of those reported from the very poor literature on this subject. Over high N-NO ₃ ^- availability, the potential GS activities recorded were close to indirect estimations of N-uptake rates. During nitrogen depletion, high potential GS activities (which coincided with a development of small diatoms) were recorded suggesting that cell metabolism adapted to N-nutrient constraints. In addition, potential GS activity evolved concomitantly to other indicators of N-uptake and N-status. However, N-assimilation rates based on the enzymatic approach were lower than those obtained through indirect isotopic determination of uptakes rates. We suggest therefore that GS was not the unique pathway of ammonium assimilation in field populations.