Carotenoid pigments enhance rhodopsin-mediated phototrophy by light-harvesting and photocycle-accelerating

Summary Microbial rhodopsins, photoreceptor proteins widely distributed in marine microorganisms, receive large amount of light energy that sustains marine ecosystems. Although rhodopsins generally harbor retinal as their only chromophore, a recent study reported that carotenoid antennae transfer light energy to the retinal in proteorhodopsins, proton pump rhodopsins abundant in marine environments. Here, using marine bacterial isolates, we detected energy transfer from a carotenoid (myxol) to retinal not only in proteorhodopsin but also in the chloride ion-pumping rhodopsin. Carotenoid binding improved the light utilization efficiency of the proteorhodopsin by accelerating the photocycle, together with facilitating light-harvesting. The carotenoid-binding ability is conserved in rhodopsins of the phylum Bacteroidota , which are widely transcribed in the photic zone. These findings suggest that the distribution of carotenoid-binding rhodopsins is more taxon-specific than previously thought, thus underscoring the importance of carotenoid-binding rhodopsins that provide an extended light utilization strategy in the environmental adaptation of marine bacteria.