Carbon dioxide fixation in Pyrobotrys stellata

The green alga Pyrobotrys stellata Korshik., an obligate phototroph, is unable to utilise carbon dioxide for growth, although assimilation of acetate is dependent on the photosynthetic process. The incorporation of ¹⁴CO₂ from ¹⁴C-bicarbonate into the cells of P. stellata is only 3% of that in Chlorella pyrenoidosa Chick. The activity of the key enzyme of the Calvin cycle, ribulose-1-5-diphosphate carboxylase, is very low in P. stellata, being only 7% of that in C. pyrenoidosa. The determination of the products of ¹⁴CO₂ fixation in intact cells confirms that ribulose-1-5-diphosphate activity is very low in P. stellata, since little carbon-14 is found in 1–3 diphosphoglyceric acid, the product of carboxylation of ribulose-1-5-diphosphate. It is concluded that the inability of P. stellata to utilize carbon dioxide for growth in the light is probably the result of the low ribulose-1-5-diphosphate carboxylase activity in the organism.