Evidence for a Light-Induced H + Conductance in the Eye of the Green Alga Chlamydomonas reinhardtii

Abstract

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 Ca 2+. I P1a and I P1b exhibit half-maximal photon flux densities, Q 1/2, of ∼0.14 and 58 μE m −2, and maximal amplitudes of ∼4.9 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 ∼5 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.