Photooxidative Damage in Photosynthetic Activities of Chromatium vinosum

Abstract

The capacity of photosynthetic CO2 fixation in the anaerobic purple-sulfur bacterium, C. vinosum is markedly impaired by strong illumination (9 .times. 104 lux) in the presence of 100% O2. In the absence of .**GRAPHIC**. decline in activity occurred gradually, with about 40% of the initial activity remaining after a 1 h incubation. The addition of 50 mM .**GRAPHIC**. to the incubation medium resulted in a measurable delay (about 30 min) of the inactivation process. Ribulose-1,5-bisphosphate carboxylase activity and light-dependent O2 uptake (electron flow) or crude extracts prepared after pretreatment of the bacterial cells with O2 and light were not affected but the photophosphorylation capacity of either bacterial cells or chromatophores was drastically reduced. The inhibition of photophosphorylation in the chromatophore preparations was significantly reduced by the addition of either an .**GRAPHIC**. scavenger, Tiron, or an 1O2 scavenger, .alpha.-tocopherol. The active O2 species, .**GRAPHIC**. or 1O2, might take part in the observed inactivation. The pretreatment of the bacteria with O2 and light inhibited CO2 assimilation through the Calvin-Benson cycle, while realtively stimulating the formation of aspartate and glutamate. It also inhibited the conversion of glycolate to glycine, resulting in a sustained extracellular excretion of glycolate. The inactivation of photosynthetic CO2 fixation by intact cells was enhanced by low temperature, KCN, or methylviologen addition during the pretreatment with O2 and light. The mechanism(s) of O2-dependent photoinactivation of photosynthetic activities in Chromatium are discussed in relation to the possible role of photorespiration as a means of producing CO2 in the photosynthetic system.