HCO3−efflux and the regulation of the intracellular Cipool size inSynechococcusUTEX 625

Abstract

Cyanobacterial cells of Synechococcus UTEX 625 were suspended in increasing extracellular inorganic carbon (Ci, up to 1 mM) under steady state photosynthesis conditions in a cuvette connected to a mass spectrometer. Photosynthetic CO2fixation, net CO2, and net HCO3−transport increased with increased extracellular Ciup to 100 μM [C1], with HCO3−transport contributing the most to the maintenance of the internal Cipool. When the extracellular [Ci] was increased above 100 μM, CO2, fixation and net CO2transport increased toward their maximum rates but net HCO3−transport progressively declined. At an extracellular [Ci] of 1 mM, the net HCO3−transport rate was only 15% of its value at 100 μM extracellular [Ci]. Under steady state photosynthesis conditions, the gross HCO3−transport rate remained at its maximum value at extracellular [Ci] above 100 μM either when CO2transport was precluded (i.e., in the presence of 200 μM Na2S) or when HCO3−transport was measured with H14CO3−and silicone oil filtration. As HCO3−efflux, calculated using the permeability coefficient determined at low [Ci], could not account fully for the decrease of the net HCO3−transport rate, Ciefflux and CO2, efflux were measured at high extracellular [Ci] during the initial period of darkness. With increasing intracellular [Ci] Ciefflux over intracellular [Ci] increased linearly but CO2, efflux over intracellular [Ci] was constant, hence the HCO3−efflux obtained by the difference between the Ciand the CO2efflux increased hyperbolically. Using HCO3−and CO2, efflux, we calculated that the permeability coefficient of the cell envelope for HCO3−but not for CO2increased with increasing intracellular [Ci]. When the HCO3−efflux was measured under steady state Ciflux conditions in the light, the permeability coefficient of the cell envelope for HCO3−also increased with increasing intracellular [Ci]. Reasons for the change in whole cell permeability to HCO3−at high extracellular [Ci] are discussed. Key words: Synechococcus UTEX 625, blue green algae, cyanobacteria, active HCO3−transport, HCO3−leakage, Ciconcentrating mechanism.