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Metabolism and Enzymology

Glycolaldehyde Inhibits CO₂ Fixation in the Cyanobacterium Synechococcus UTEX 625 without Inhibiting the Accumulation of Inorganic Carbon or the Associated Quenching of Chlorophyll a Fluorescence ¹

Department of Biology, Queen's University, Kingston, Ontario, Canada, K7L 3N6

When studying active CO₂ and HCO₃^– transport by cyanobacteria, it is often useful to be able to inhibit concomitant CO₂ fixation. We have found that glycolaldehyde was an efficient inhibitor of photosynthetic CO₂ fixation in Synechococcus UTEX 625. Glycolaldehyde did not inhibit inorganic carbon accumulation due to either active CO₂ or HCO₃^– transport. When glycolaldehyde (10 millimolar) was added to rapidly photosynthesizing cells, CO₂ fixation was stopped within 15 seconds. The quenching of chlorophyll a fluorescence remained high ( 82% control) when CO₂ fixation was completely blocked by glycolaldehyde. This quenching was relieved upon the addition of a glucose oxidase oxygentrap. This is consistent with our previous finding that q-quenching in the absence of CO₂ fixation was due to O₂ photoreduction. Photosynthetic CO₂ fixation was also inhibited by D,L,-glyceraldehyde but a sixfold higher concentration was required. Glycolaldehyde acted much more rapidly than iodoacetamide (15 seconds versus 300 seconds) and did not cause the onset of net O₂ evolution often observed with iodoacetamide. Glycolaldehyde will be a useful inhibitor when it is required to study CO₂ and HCO₃^– transport without the complication of concomitant CO₂ fixation.

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