Energy Sources for HCO(3)(−) and CO(2) Transport in Air-Grown Cells of Synechococcus UTEX 625

Light-dependent inorganic C (C(i)) transport and accumulation in air-grown cells of Synechococcus UTEX 625 were examined with a mass spectrometer in the presence of inhibitors or artificial electron acceptors of photosynthesis in an attempt to drive CO(2) or HCO(3)(−) uptake separately by the cyclic or linear electron transport chains. In the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the cells were able to accumulate an intracellular C(i) pool of 20 mm, even though CO(2) fixation was completely inhibited, indicating that cyclic electron flow was involved in the C(i)-concentrating mechanism. When 200 μm N,N-dimethyl-p-nitrosoaniline was used to drain electrons from ferredoxin, a similar C(i) accumulation was observed, suggesting that linear electron flow could support the transport of C(i). When carbonic anhydrase was not present, initial CO(2) uptake was greatly reduced and the extracellular [CO(2)] eventually increased to a level higher than equilibrium, strongly suggesting that CO(2) transport was inhibited and that C(i) accumulation was the result of active HCO(3)(−) transport. With 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated cells, C(i) transport and accumulation were inhibited by inhibitors of CO(2) transport, such as COS and Na(2)S, whereas Li(+), an HCO(3)(−)-transport inhibitor, had little effect. In the presence of N,N-dimethyl-p-nitrosoaniline, C(i) transport and accumulation were not inhibited by COS and Na(2)S but were inhibited by Li(+). These results suggest that CO(2) transport is supported by cyclic electron transport and that HCO(3)(−) transport is supported by linear electron transport.