INFLUENCE OF PH, LIGHT-INTENSITY AND OXYGEN ON THE SHORT-TERM EFFECT OF AMMONIUM ON NITROGENASE ACTIVITY OF CYANOBACTERIA FROM RICE FIELDS

The influence of pH, light intensity and anaerobiosis on the short-ter m effect of ammonium on nitrogenase activity was investigated in four cyanobacteria (Nostoc punctiforme UAM 205, Anabaena variabilis UAM 202 , Calohhrix marchica UAM 214 and Nodularia spumigena UAM 204) isolated from rice fields in Valencia, Spain. A fast switch-off of nitrogenase activity by ammonium was observed only when cells were incubated in t he light in the presence of oxygen at pH 10. A stabilizing effect of c arbohydrates, which was evident at pH 7, was not effective at pH 10. U nder these conditions ammonium caused a light-dependent oxygen uptake insensitive to cyanide, which could be related to inactivation of nitr ogenase.