The Stoichiometry between CO₂ and H⁺ Fluxes Involved in the Transport of Inorganic Carbon in Cyanobacteria

Abstract

The pH of the medium during CO2 uptake into the intracellular inorganic carbon (CQ) pool of a high C02-requiring mutant (Ej) and wild type of Anacystis nidulans R2 was measured. Experiments were performed under conditions where photosynthetic CO2 fixation is inhibited. There was an acidification of the medium during CO2 uptake in the light and an alkalization during CO2 efflux after darkening. A one to one stoichiometry existed between the amounts of H' appearing in the medium and CO2 taken up into the intracellular Ci pool, regardless of the carbon species transported. The results indicate that (a) CO2 is taken up simultaneously with an efflux of equimolar H+, probably produced as a result of CO2 hydration during transport and (b) HC03-produced by hydration of CO2 in the medium was transported into the cells without accompanying net flux of Ho or OH-. The influx and efflux of Ci during Ci transport produced nonequilibrium between CO2 and HCO3-in the medium, with the concentration of HC03-being higher than that expected under equilibrium conditions. The nonequilibrium was present even under the conditions where the influx of Ci is compensated by its efflux. The direction of this nonequilibrium suggested that efflux of HCO3-occurs during uptake of Ci.Cyanobacteria are capable of accumulating Ci2 internally and thus possess a 'C02 concentrating mechanism ' ( 1, 3, 6, 9, 10 (8,15) and HC03-appears to be the species which arrives at the inner membrane surface, regardless of the species supplied (15). Since Ci accumulated within the cells is mostly HCO3-, uptake of CO2 during the accumulation of Ci would be accompanied by an influx of OH-(or H+ efflux). Stoichiometry between H+ flux and CO2 taken up into the intracellular C, pool, however, has not been measured. When HC03-is supplied to the cell suspension in a closed system, the photosynthetic utilization of C, leads to alkalization of the medium and a decrease of the extemal Ci concentration (2,9 gas-analysis system where CO2 is supplied to (or taken up from) the cell suspension, it is possible to keep the steady-state C, concentration of the medium constant while measuring the CO2 exchange of the system. Furthermore, the use of mutants or inhibitors enables one to separate CO2 uptake into the intracellular Ci pool from CO2 fixation into acid-stable products.In this study, we measured the flux of H+ during CO2 uptake in the light and CO2 efflux after darkening, using an open gasanalysis system. A high C02-requiring mutant (El) of Anacystis nidulans (7) and the wild type (treated with IAc, an inhibitor of CO2 fixation) were used in these experiments. Uptake of CO2 from the medium at air level of CO2 by this mutant (4, 7) or by the wild type in the presence of IAc (1 1, 12) represents the build up of the intracellular C, pool. The results demonstrated a one to one stoichiometry between the amounts of H+ appearing in the medium and CO2 taken up by the cells, even under the conditions where CO2 is transported. The concentration ratio of the different Ci specie...

Selective and Reversible Inhibition of Active CO2 Transport by Hydrogen Sulfide in a Cyanobacterium

“…Previous studies have suggested the participation ofa membrane-bound CA-like moiety in the transport of CO2 by Anabaena variabilis, Anacystis nidulans R2, and Synechococcus PCC 7942 (1,22,25,28). This conclusion was largely based on the observation that EZ inhibited CO2 transport (1,22,25,28) without apparent inhibition of intracellular CA (25).…”

“…This conclusion was largely based on the observation that EZ inhibited CO2 transport (1,22,25,28) without apparent inhibition of intracellular CA (25). Inhibition of CO2 transport by H2S in Synechococcus UTEX 625 further strengthens the view that CA and the CO2 transport system have properties in common.…”

“…This model differs from previous models (1,22,25,28) in that the CO2 and HCO3-transport systems are considered to be separate, specific, and independent. As discussed here and elsewhere (7,8,13,19) evidence for this arrangement is provided primarily from studies with selective inhibitors.…”

“…In Anabaena, the suggested role of the CA-like moiety is to convert CO2 to HC03-within the membrane for subsequent transport into the cell by the HCO3-transport system (1,22,28). In Synechococcus PCC 7942, however, it has been proposed that HCO3F from the medium is converted to CO2 within the membrane by a 'front end mechanism' and that this CO2 is then used as a substrate for the CO2 transport system (4,25).…”

Biomineralization in perforate foraminifera

“…The efflux of H + during calcification (Glas et al, 2012) may therefore help to constrain estimates of calcium pumping rates during calcification. Carbon dioxide uptake and proton efflux are also directly related in cyanobacteria (Ogawa and Kaplan, 1987). Ter Kuile et al (1989b) suggested that Ca 2+ is taken up by Ca 2+ -ATPase and this mechanism was subsequently used by Zeebe and Sanyal (2002) and Zeebe et al (2008) to show that H + removal is far more energy-efficient than Mg 2+ -removal during calcification.…”