INFLUX AND EFFLUX OF INORGANIC CARBON IN SYNECHOCOCCUS UTEX-625

The CO2 and HCO3- fluxes in air-grown cells of Synechococcus UTEX 625 at pH 8.0 were measured during dark to light and light to dark transit ions using a mass spectrometer and sampling of the reaction medium, Th e kinetic parameters for initial uptake of CO2 and HCO3- were determin ed during the initial period of illumination, The development of the i nternal C-i pool was followed up to steady-state photosynthesis, which occurred when the size of the internal inorganic carbon pool remained apparently constant for a limited period. The experimental procedure confirmed that only CO2 transport occurred with 100 mmol m(-3) Na+ and that both CO2 and HCO3- transport occurred with 25 mol m(-3) Na+. The K-1/2 values of initial CO2 and HCO3- uptake were 0.7 and 17.2 mmol m (-3) respectively and agreed closely with the K-1/2 values of net CO2 and HCO3- transport during steady-state photosynthesis, which were 0.6 6 and 17.1 mmol m(-3) respectively. Maximum rates of CO2 and HCO3- tra nsport were 423 and 219 mmol h(-1) g(-1) Chl. Maximum CO2 efflux obser ved upon darkening was 118 mmol h(-1) g(-1) Chl. A permeability coeffi cient of the cell for CO2 of 3 x 10(-8) m s(-1) was determined from th e dark CO2 efflux assuming an internal pH of 7.2 in the dark. Followin g the initial CO2 uptake in the light, the extracellular [CO2] steadil y declined when only CO2 transport was allowed, but an increase in the extracellular [CO2] when HCO3- transport was allowed to proceed sugge sted that an enhanced CO2 efflux occurred as a result of the larger si ze of the intracellular C-i pool.