20(J.W.) 21 MerMAID1 as a model. Replacement of Cl − with Br − or NO 3 − resulted in negative reversal potential 131 shifts (Figs. 2d and S4b), thus revealing nonselective anion conductivity, as previously reported 132 for other ACRs 21,23 . In contrast, substitution of Na + with NMDG + , Ca 2+ , or Mg 2+ had only a slight 133 effect on reversal potentials ( Fig. 2d and S4b), thereby excluding a substantial contribution by 134 cations as charge carriers. 135Rhodopsin function often commonly involves de-and reprotonation of internal amino acids, and 136 pH changes can significantly affect photocurrent amplitude and kinetics 4,9,18,19 . We therefore 137 investigated the effect of extra-and intracellular pH (pH e and pH i ) changes on MerMAID1. 138 Variation of pHe between 6.0 and 8.0 slightly altered the photocurrent amplitude ( Fig. S4c) but not 139 the reversal potential ( Figs. 2d and S4d), thus excluding proton transport. Neither pH i nor pH e 140 (2002). 595