PHOTOSYNTHETIC NITRITE REDUCTION AS INFLUENCED BY THE INTERNAL INORGANIC CARBON POOL IN AIR-GROWN CELLS OF SYNECHOCOCCUS UTEX-625

Photosynthetic reduction of NO2- was studied in air-grown cells of a c yanobacterium, Synechococcus UTEX 625. Addition of NO2- resulted in si gnificant amounts of chlorophyll a fluorescence quenching both in the absence and presence of CO2 fixation inhibitors, glycolaldehyde or iod oacetamide. The degree of NO2- quenching was insensitive to the O-2 co ncentration in the medium. Addition of 100 mu M inorganic carbon in th e presence of glycolaldehyde and O-2, leading to formation of the carb on pool within the cells, resulted in pronounced fluorescence quenchin g. Removal of O-2 from the medium restored the fluorescence yield comp letely, and the subsequent addition of NO2- quenched 36% of the variab le fluorescence. From the response to added 3-(3,4-dichlorophenyl)1,1- dimethylurea, the quenching by NO2- appeared to be photochemical quenc hing, and nonphotochemical quenching did not seem to be present. The r eduction of NO2- observed on its addition to inorganic carbon-depleted cells remained uninfluenced by O-2 or glycolaldehyde. The internal in organic carbon pool in the cells stimulated NO2- reduction, both in th e presence and absence of O-2, by 4.8-fold. An increase in NO2- reduct ion by 0.5-fold was also observed in the presence of O-2 during simult aneous assimilation of carbon and nitrogen in inorganic carbon-deplete d cells. Contrary to this, under anaerobiosis, NO2- reduction was supp ressed when carbon and nitrogen assimilation occurred together.