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

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Abstract
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.