Glycolaldehyde inhibition of CO2transport in the cyanobacteriumSynechococcusUTEX 625

In studies of the inorganic carbon (C i ) concentrating mechanism of cyanobacteria glycolaldehyde is often used to inhibit photosynthetic CO 2 fixation. As a partial inhibition of active CO 2 transport by glycolaldehyde under steady-state C i flux conditions has been recently suggested, in this paper, using mass spectrometry, we investigate in detail the relationship between glycolaldehyde concentration ([GLY]) and CO 2 transport both during the initial period following illumination of a cyanobacterial cell suspension and during the so-called steady state when an internal C i pool has been allowed to develop. Carbon dioxide uptake following illumination of a cyanobacterial cell suspension was progressively reduced by increasing [GLY] in the medium, both in the absence or in the presence of carbonic anhydrase. The same features were found when either C i in the form of bicarbonate was injected to a pre-illuminated cell suspension in the presence of carbonic anhydrase in the medium or a pulse of CO 2 was provided to the cells in the light without carbonic anhydrase in the extracellular medium. Although photosynthesis was completely abolished with 10 mM glycolaldehyde, CO 2 uptake was only inhibited 20% but was decreased further by higher [GLY]. Double reciprocal plots of CO 2 uptake versus initial [CO 2 ] determined with a range of [GLY] showed the inhibition to be noncompetitive. To simultaneously study CO 2 uptake and CO 2 efflux, H 13 C 18 O 3 was used with cells in which CO 2 transport alone, or both CO 2 and HCO 3 - transport system were allowed to proceed. Cells were inhibited with either iodoacetamide or glycolaldehyde and the time course of the various CO 2 species in the medium demonstrated that CO 2 uptake was inhibited partially with glycolaldehyde resulting in a greater CO 2 efflux from the cells. When the cell suspension was again illuminated, in the presence of glycolaldehyde, initial CO 2 uptake was greatly reduced, and the extracellular [CO 2 ] eventually rose to a level higher than that which would be expected to be in equilibrium with the amount of C i present in the medium. Results are discussed in terms of glycolaldehyde use in C i transport mechanism studies in cyanobacteria. Key words: Synechococcus UTEX 625, blue-green algae, cyanobacteria, CO 2 transport, HCO 3 - transport, CO 2 efflux, inhibitor, C i concentrating mechanism.