Summary: CO fixation and uptake of sugars by were demonstrated by liquid scintillation and microautoradiographic techniques. The theoretical carbon content of a cell in the exponential and stationary growth phases was calculated from size measurements of images of acridine-orange-stained cells. The carbon content of a cell in the exponential phase was 1·25 × 10 mol and for a cell in the stationary phase it was 5 × 10 mol. was shown to obtain all of its cell carbon from CO fixation when it was cultured under aerobic gradient conditions in a mineral salt solution with iron sulphide. Uptake experiments were performed with 1·6 μM-[C]frucose, 1·6 μM-[C]fructose and 1·3 μM-[]sucrose. There was significant uptake of all three sugars. Measurements of respired CO showed that 48%, 25% and 32% of the total amount of incorporated sugar was respired for glucose, fructose and sucrose, respectively. The uptake of glucose increased when the glucose concentration in the growth medium was increased. At a glucose concentration of 10 μM or higher, the cell carbon was derived exclusively from glucose, within the errors of estimation. Mixotrophic growth with 20 μM-glucose decreased the CO fixation to 0·4 × 10 mol carbon per cell, compared to autotrophically grown cells with 1·0 × 10 mol carbon per cell. The addition of 20 μM-glucose gave an increase in cell number in the stationary phase from 1 × 10 to 5 × 10 cells ml.


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