ENERGY-SOURCES FOR HCO3- AND CO2 TRANSPORT IN AIR-GROWN CELLS OF SYNECHOCOCCUS UTEX-625

Light-dependent inorganic C (C-i) transport and accumulation in air-gr own cells of Synechococcus UTEX 625 were examined with a mass spectrom eter in the presence of inhibitors or artificial electron accepters of photosynthesis in an attempt to drive CO2 or HCO3- uptake separately by the cyclic or linear electron transport chains. In the presence of 3-(3,4-dithlorophenyl)-1,1-dimethylurea, the cells were able to accumu late an intracellular C-i pool of 20 mM, even though CO2 fixation was completely inhibited, indicating that cyclic electron flow was involve d in the C-i-concentrating methanism. When 200 mu M N,N-dimethyl-p-nit rosoaniline 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 CO2 uptake was greatly reduced and the extracellular [CO2] e ventually increased to a level higher than equilibrium, strongly sugge sting that CO2 transport was inhibited and that C-i accumulation was t he result of active HCO3- transport. With 3-(3,4-dichlorophenyl)-1,1-d imethylurea-treated cells, C-i transport and accumulation were inhibit ed by inhibitors of CO2 transport, such as COS and Na2S, 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 Na2S but were inhibited by Li+. These results suggest that CO2 transport is supported by cyclic electron transport and that HCO3- transport is supported by linear electron transport.