The Role of Glycollic Acid in the Photoassimilation of Acetate byChlorella pyrenoidosa

Abstract

The kinetics of ³H-acetate assimilation by Chlorella pyrenoidosa in the light were examined. The primary products of assimilation were glycollate and succinate. After 10 sec glycollate contained 45 per cent and succinate 25 per cent of the tritium incorporated by the cells. The percentage of the total tritium in glycollate and succinate fell with time while that in citrate increased. Initially the specific activities (μc of ³H per μmole of acid) of succinate and glycollate were greater than citrate. When ³H-¹⁴C-2-acetate was added to the cells, total dpm for ³H and ¹⁴C in glycollate rapidly reached a steady state and gave a ³H/¹⁴C ratio of 10, compared with a ³H/¹⁴C ratio of 4 in the acetate. This ³H/¹⁴C ratio in glycollate is found because ³H is derived from ³H-¹⁴C-2-acetate and because the ¹⁴C is diluted with cold carbon from elsewhere. The addition of ¹⁴CO₂ at the same time as ³H-¹⁴C acetate decreased the ³H/¹⁴C ratio in glycollate but incorporation of ¹⁴C from ¹⁴CO₂ into glycollate was slower than incorporation from ¹⁴C-2-acetate. Although ¹⁴C from acetate rapidly appeared in glycollate, ¹⁴C-labelled glyoxylate was not detected. The ³H/¹⁴C ratio observed in glycollate rules out formation of glycollate from acetate via glycoaldehyde. The available evidence did not support glycollate formation via the Calvin cycle. ¹⁴C from ¹⁴C-Z-acetate appeared in glycollate before it did in phosphoglyceric acid. Total dpm for ³H, ¹⁴C, and ³H/^l4C ratio in Calvin cycle intermediates were not in equilibrium with glycollic acid.