The intracellular pH and membrane potential were determined in the acidophilic algae Cyanidokchyzon merolae as a function of extracellular pH. The alp appear to be capable of maintaining the intracellular pH at the range of 635 to 7.1 over the extracellular pH range of 1.5 to 7.5. The membrane potential increase from -12 millivolts (negative inside) to -71 millivolts and thus ALH' decreased from -300 to -47 millivolts over the same range of extracellular pH. It is suggested that the A4H' may set the upper and lower limits of pH for growth. Photosynthetic performance was also determined as a function of pH. The cells appeared to utilize CO2 from the medium as the apparent Kco was 2 to 3 micromolar CO2 over the pH range of 1.5 to 7.5. C. merolac appear to possess a 'CO2 concentrating' mechanism. The growth of photosynthetic organisms living in hot, acidic environments may be limited by their capacity to control their intracellular pH, as well as by the availability ofinorganic carbon (C1) for photosynthesis. The latter is due to CO2 practically being the only C1 species present at the acid environment and the reduced solubility of CO2 at elevated temperature. The pH gradient between the cell interior and the acidic medium may favor the accumulation of Ci within the cell to a level determined, at equilibrium, by the pH gradient and the extracellular CO2 concentration. The intracellular CO2 level, which is the substrate for the carboxylation reaction, may be smaller than expected at equilibrium due to utilization in photosynthesis and limitations imposed by diffusion and permeability (12). Cyanobacteria and green algae growing under low CO2 level have been shown to possess a mechanism which enables them to concentrate CO2 within the cells and thus to perform high photosynthetic rates despite the low CO2 level (1,5).In the present study, we investigated the capacity of the acidiophylic green alga Cyanidioschyzon merolae to maintain its intracellular pH as well as the means by which CO2 is being supplied to the carboxylation site.
MATERIAILS AND METHODSCells of Cyanidioschyzon merolae (3) were grown in the medium described by Enami and Fukuda (4), pH 1.5 at 35°C in 500-ml flasks bubbled wih air. Continuous illumination at 6 mw-cm-2 (400-700 nm) was provided. Cells were harvested by centrifugation and resuspended in a medium containing citratephosphate buffer adjusted to the desired pH by changing the ratio between citric acid and KH2PO4. C02-dependent 02 evolution was measured using an 02 electrode (Rank Brothers, Bottisham, Cambridge, U.K.). Intracellular pH was determined from the distribution of acetyl (carboxy-'4C) salicylic acid at pH values below 4.5 and [14C]-5,5-dimethyloxazoladine-2,4-dione (DMO) at pH values higher than 4.5. The membrane potential, At, was calculated from the distribution of the lipophilic cation tetraphenylphosphonium (TPP*) as described elsewhere (6). The intracellular concentration of Ci was determined by the filtering centrifugation technique (5). Cells were centrifuged through a...
