Indirect Effects of α-Hydroxypyridinemethanesulfonate and Cyanide on Photosynthesis and Glycolate Excretion in Chlorella

Abstract

Summary The effects of two inhibitors of photosynthetic carbon metabolism (HPMS and cyanide) were studied in Chlorella pyrenoidosa . Cultures were grown either with air or high CO 2 (5% CO 2 in air). The effects of these two inhibitors on glycolate excretion and 14 CO 2 fixation were determined and compared with the effects of the carbonic anhydrase (CA) inhibitor, ethoxyzolamide (EZA). Air grown Chlorella had high carbonic anhydrase activities and released small quantities of glycolate when incubated with 100% O 2 with saturating light intensities. Identical cultures treated with 10 mM HPMS or 100 μM NaCN doubled the rate of glycolate excretion while reducing carbon-fixation by 31 and 40% respectively. Saturating concentrations (≥ 0.5 mM) of EZA caused a greater than five-fold increase in glycolate excretion and a 73% inhibition of carbon fixation. HPMS when used with EZA further increased glycolate excretion compared to HPMS alone. Saturating concentrations of EZA with 100 μM cyanide caused a 60% increase in glycolate excretion compared with cyanide alone. Cultures grown with high CO 2 responded to the inhibitors very differently. They had no detectable carbonic anhydrase activity and excreted glycolate at more than five times the rate of the air grown cells with no inhibitor treatment. The addition of 10 mM HPMS either with or without EZA produced only relatively small changes in both glycolate excretion and carbon-fixation. Saturating levels of EZA caused only a 34% increase in measured glycolate, and HPMS with EZA showed a 24% increase compared with the control. Cyanide inhibited glycolate synthesis in the high CO 2 -grown cells by 27% and by 19% when tested with EZA; cyanide inhibited carbon-fixation by 29% alone and by 37% when used with EZA. Consistent with the low levels of CA in these high CO 2 grown cells is the small effect that EZA has on carbon-fixation and glycolate excretion either by itself or with HPMS or with cyanide. The inhibition of glycolate dehydrogenase by HPMS or cyanide does not appear to cause significant changes in glycolate excretion or carbon- fixation. The activity of glycolate dehydrogenase is low and is not altered by the growth conditions, as is carbonic anhydrase. It is concluded that both HPMS and cyanide do significantly alter photosynthetic carbon metabolism in air-grown Chlorella pyrenoidosa by inhibition of carbonic anhydrase. Cells grown with high CO 2 do not show these effects because very little CA activity occurs in such cells.