Role of intracellular carbonic anhydrase in inorganic-carbon assimilation by Porphyridium purpureum

Abstract

Air-grown cells of Porphyridium purpurem contain appreciable carbonic-anhydrase activity, comparable to that in air-grown Chlamydomonas reinhardtii, but activity is repressed in CO2-grown cells. Assay of carbonic-anhydrase activity in intact cells and cell extracts shows all activity to be intracellular in Porphyridium. Measurement of inorganic-carbon-dependent photosynthetic O2 evolution shows that sodium ions increase the affinity of Porphyridium cells for HCO 3 (-) . Acetazolamide and ethoxyzolamide were potent inhibitors of carbonic anhydrase in cell extracts but at pH 5.0 both acetazolamide and ethoxyzolamide had little effect upon the concentration of inorganic carbon required for the half-maximal rate of photosynthetic O2 evolution (K0.5[CO2]). At pH 8.0, where HCO 3 (-) is the predominant species of inorganic carbon, the K0.5 (CO2) was increased from 50 μM to 950 μM in the presence of ethoxyzolamide. It is concluded that in air-grown cells of Porphyridium. HCO 3 (-) is transported across the plasmalemma and intracellular carbonic anhydrase increases the steady-state flux of CO2 from inside the plasmalemma to ribulose-1,5-bisphosphate carboxylase-oxygenase by catalysing the interconversion of HCO 3 (-) and CO2 within the cell.

Carbon and Nitrogen Fixation byAnabaena fertilissimaunder Elevated CO2and Temperature

“…Uptake of N~H '~C O~ by A. fertilissima was estimated as the end-point study after Kumar et al (1982) using liquid scintillation counting. Cells were disrupted by application of 100 mPa pressure, and carbonic anhydrase activity in cell extract was assayed following a modified electrometric method (Dixon et al 1987).…”

Extra‐ and intra‐cellular carbonic anhydrase in relation to culture age in a high‐calcifying strain of Emiliania huxleyi Lohmann

“…for 30 min and the pellet resuspended in Veronal buffer pH 8 2. The resuspended pellet, the supernatant and intact cells were assayed for carbonic anhydrase by an electrometric method as described previously (Dixon, Patel & Merrett, 1987).…”

“…Fractions (1 ml) w'ere collected from the bottom of the gradient. Triose phosphate isomerase was assayed as described previously (Dockerty & Merrett, 1979).…”

Carbon dioxide availability, intracellular pH and growth rate of the coccolithophore Emiliania huxleyi

“…In microalgae capable of the direct utiliza-tion of HC03-as the exogenous inorganic carbon source for photosynthesis (Rees 1984, Pate1 & Merrett 1986, Dixon et al 1987, pHi is expected to be an important factor in maintaining the C02/HC03-equilibrium within the cytosol. In high-calcifying cells of E. huxleyi, H' fluxes at the Golgi/coccolith vesicle membrane will be important in relation to HC03-availability (Brownlee et al 1994b).…”