A comparative study on the effect of O_2 on photosynthetic carbon metabolism by Chlorobium thiosulfatophilum and Chromatium vinosum

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The reductive carboxylic acid cycle appears to be the major carbon assimilation pathway in green sulfur bacteria, Chlorobium thiosulfatophilum. While cyanide was relatively ineffective in inhibiting the bacterial photosynthetic CO_2 fixation, photo-synthesis was strongly impaired in an O_2-containing atmospheric environment. No glycolate formation was detected in Chlorobium under an O_2 atmosphere. In the purple sulfur bacteria, Chromatium vinosum, however, photosynthesis was highly sensitive to cyanide, and in a short-term incubation (up to 10 min) photosynthetic CO_2 fixation was found to be relatively indifferent to an O_2-containing atmosphere of up to 100% O_2. Significant formation of glycolate was demonstrated upon a very brief exposure to O_2, whereas the total photosynthetic CO_2 fixation was slightly affected. However, ribulose-1,5-bisphosphate carboxylase activity in Chromatium extract was competitively inhibited by O_2 in a similar manner to the higher plant enzyme, K_i(O_2) value being 0.7 mM at pH 8.2. The percentage of incorporation of ^<14>CO_2 into glycolate and glycine under an O_2-containing atmosphere declined with increasing levels of bicarbonate concentrations in the medium. The Warburg effect and biosynthetic mechanisms involving glycolate in photosynthetic bacteria are discussed.