Circulating Electric Currents Between Acid and Alkaline Zones Associated with HCO - 3 Assimilation inChara

Some charophyte species, like some other freshwater plants, have been shown to take up $HCO-3$ for photosynthetic fixation. Such plants show localized deposits of CaCO₃ associated with localized ‘OH⁻ excretion’. Chara corallina produces acid and alkaline regions in unbuffered external media, in the light, even when CaCO₃ is not being precipitated. These regions are associated with $HCO-3$ uptake and ‘OH⁻ efflux’ respectively. We have found that large currents circulate, in the light, between acid and alkaline zones of Chara internodes, by (i) direct measurement of the short-circuit current and (ii) measurement of potential differences in the solution around the cell. Both methods suggest currents of 0.5–1 μA per zone, with local current densities of about 75 mA m⁻², or local univalent fluxes of 0.75 μmol m⁻² s⁻¹. These circulating currents produce mean peak-to-peak potential differences of 7 mV in the external medium. The fluxes are believed to be of $HCO-3$ and H⁺ or OH⁻. While active transport of $HCO-3$ is strongly indicated, passive uniport of H⁺ or OH⁻ appears to be sufficient. Control systems for both fluxes would be required.