Mechanisms of CO fixation in bacterial photosynthesis studied by the carbon isotope fractionation technique.

1. The carbon isotope discrimination properties of a representative of each of the three types of photosynthetic bacteria Chlorobium thiosulfatophilum, Rhodospirillum rubrum and Chromatium and of the C-alga Chlamydomonas reinhardii were determined by measuring the ratio of CO to CO incorporated during photoautotrophic growth. 2. Chromatium and R. rubrum had isotope selection properties similar to those of C-plants, whereas Chlorobium was significantly different. 3. The results suggest that Chromatium and R. rubrum assimilate CO mainly via ribulose 1,5-diphosphate carboxylase and the associated reactions of the reductive pentose phosphate cycle, whereas Chlorobium utilizes other mechanisms. Such mechanisms would include the ferredoxin-linked carboxylation enzymes and associated reactions of the reductive carboxylic acid cycle. [ABSTRACT FROM AUTHOR]