Relations between d-ribulose-1,5-bisphosphate carboxylase,carboxysomes and CO2 fixing capacity in the obligate chemolithotroph Thiobacillus neapolitanus grown under different limitations in the chemostat

An adaptation of the d-ribulose-1,5-bisphosphate carboxylase (RuBPCase) activity to changing CO₂ concentrations in the growth medium in the chemostat was observed in the obligate chemolithotroph Thiobacillus neapolitanus. RuBPCase activity has been separated in a soluble and particulate fraction. The activity of the particulate fraction appeared to be associated with the carboxysomes.The total activity of RuBPCase of CO₂ limited cultures was about 5-fold higher than the activity of thiosulphate limited cultures grown in the presence of 5% CO₂ whilst the particulate activity and the soluble activity were about 8- and 1.5-fold higher, respectively. The fluctuation of the total and particulate RuBPCase activity correlated with the changes in volume density of carboxysomes in the cell.An inverse correlation between maximal CO₂ fixing capacity by whole cells and the volume density of carboxysomes was observed. The change in ratio of soluble RuBPCase activity to particulate RuBPCase activity paralleled the change in maximal CO₂ fixation by whole cells during the different growth conditions.