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 cyanobacterium, Synechococcus UTEX 625. Addition of NO2- resulted in significant amounts of chlorophyll a fluorescence quenching both in the absence and presence of CO2, fixation inhibitors, glycolaldehyde or iodoacetamide. The degree of NO2- quenching was insensitive to the O2 concentration in the medium. Addition of 100 [mu]M inorganic carbon in the presence of glycolaldehyde and O2, leading to formation of the carbon pool within the cells, resulted in pronounced fluorescence quenching. Removal of O2 from the medium restored the fluorescence yield completely, and the subsequent addition of NO2- quenched 36% of the variable fluorescence. From the response to added 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the quenching by NO2- appeared to be photochemical quenching, and nonphotochemical quenching did not seem to be present. The reduction of NO2- observed on its addition to inorganic carbon-depleted cells remained uninfluenced by O2 or glycolaldehyde. The internal inorganic carbon pool in the cells stimulated NO2- reduction, both in the presence and absence of O2, by 4.8-fold. An increase in NO2- reduction by 0.5-fold was also observed in the presence of O2 during simultaneous assimilation of carbon and nitrogen in inorganic carbon-depleted cells. Contrary to this, under anaerobiosis, NO2- reduction was suppressed when carbon and nitrogen assimilation occurred together.