**Abstract**

Microbial rhodopsins, which are photoreceptive membrane proteins consisting of seven α-helical structural apoproteins (opsin) and a covalently attached retinal chromophore, are one of the most frequently used optogenetic tools. Since the first success of neuronal activation by channelrhodopsin, various microbial rhodopsins functioning as ion channels or pumps have been applied to optogenetics. The use of light-driven ion pumps to generate large negative membrane potentials allows the silencing of neural activity. Although anion-conductive channelrhodopsins have been recently discovered, light-driven outward H<sup>+</sup> -pumping rhodopsins, which can generate a larger photoinduced current than a light-driven inward Cl-pump halorhodopsin, must be more efficient tools for this purpose and have been often utilized for optogenetics. There are abundant proton pumps in the microbial world, providing numerous candidates for potential practical optogenetic instruments. In addition, their distinctive features (that is, being accompanied by photoinduced intracellular pH changes) could enable expansion of this technique to versatile applications. Thus, intensive investigation of the molecular mechanisms of various microbial H+ -pumps may be useful for the exploration of more potent tools and the creation of effectively designed mutants. In this chapter, we focus on the functional mechanism of microbial H<sup>+</sup> -pumping rhodopsins. Further, we describe the future prospects of these rhodopsins for optogenetic applications.

**Keywords:** Microbial rhodopsin, Photocycle, Proton transfer, Neural silencing, Optical pH control
