Evidence for a light-induced H+ conductance in the eye of the green alga Chlamydomonas reinhardtii

Rhodopsin-mediated photoreceptor currents, I-P, of the unicellular alga Chlamydomonas reinhardtii were studied under neutral and acidic conditions. We characterized the kinetically overlapping components of the first, flash-induced inward current recorded from the eye, I-P1 as a low- and high-intensity component, I-P1a and I-P1b, respectively. They peak between 1 and 10 ms after the light-flash and are both likely to be carried by Ca2+, I-P1a and I-P1b exhibit half-maximal photon flux densities, Q(1/2), of similar to0.14 and 58 muEm(-2), and maximal amplitudes of similar to4.9 and 38 pA, respectively. At acidic extracellular pH values (pH 3-5), both I-P1 currents are followed by distinct H+ currents, I-P2a and I-P2b, with maxima after similar to5 and 100 ms, respectively. Because the Q(1/2) values of I-P1b and I-P2b virtually coincide with Q(1/2) of rhodopsin bleaching, we suggest that the respective conductances G(1b) and G(2b) are closely coupled to the rhodopsin, whereas the low light-saturating conductances G(1a) and G(2a) reflect transducer-activated states of a second rhodopsin photoreceptor system.