Evidence for the occurrence of photorespiration in synurophyte algae

The fluxes of CO 2 and oxygen during photosynthesis by cell suspensions of Tessellaria volvocina and Mallomonas papillosa were monitored mass spectrometrically. There was no rapid uptake of CO 2, only a slow drawdown to compensation concentrations of 26 μM for T. volvocina and 18 μM for M. papillosa , when O 2 evolution ceased, indicating a lack of active bicarbonate uptake by the cells. Darkening of the cells after a period of photosynthesis did not cause rapid release of CO 2 , indicating the absence of an intracellular inorganic carbon pool. However, upon darkening a brief burst of CO 2 was observed similar to the post-illumination burst characteristic of C 3 higher plants. Treatment of the cells of both species with the membrane-permeable carbonic anhydrase inhibitor ethoxyzolamide had no adverse effect on photosynthetic rate, but stimulated the dark CO 2 burst indicating the dark oxidation of a compound formed in the light. In the absence of any active accumulation of inorganic carbon photosynthesis in these species should be inhibited by O 2 . This was investigated in four synurophyte species T. volvocina , M. papillosa , Synura petersenii , and Synura uvella : photosynthetic O 2 evolution rates in all four algae, measured by O 2 electrode, were significantly higher (40–50%) in media at low O 2 (4%) than in air-equilibrated (21% O 2 ) media, indicating an O 2 inhibition of photosynthesis (Warburg effect) and thus the occurrence of photorespiration in these species.