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

AbstractRhodopsin-mediated photoreceptor currents, IP, 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, IP1, as a low- and high-intensity component, IP1a and IP1b, respectively. They peak between 1 and 10ms after the light-flash and are both likely to be carried by Ca2+. IP1a and IP1b exhibit half-maximal photon flux densities, Q1/2, of ∼0.14 and 58μE m−2, and maximal amplitudes of ∼4.9 and 38 pA, respectively. At acidic extracellular pH values (pH 3–5), both IP1 currents are followed by distinct H+ currents, IP2a and IP2b, with maxima after ∼5 and 100ms, respectively. Because the Q1/2 values of IP1b and IP2b virtually coincide with Q1/2 of rhodopsin bleaching, we suggest that the respective conductances G1b and G2b are closely coupled to the rhodopsin, whereas the low light-saturating conductances G1a and G2a reflect transducer-activated states of a second rhodopsin photoreceptor system