Flotillin-associated rhodopsin (FArhodopsin), a widespread paralog of proteorhodopsin in aquatic bacteria with streamlined genomes

Microbial rhodopsins are found more than once in a single genome (paralogs) often have different functions. We screened a large dataset of open ocean single-amplified genomes (SAGs) for co-occurrences of multiple rhodopsin genes. Many such cases were found among Pelagibacterales (SAR11), HIMB59, and the Gammaproteobacteria Pseudothioglobus SAGs. These genomes always had a bona fide proteorhodopsin and a separate cluster of genes containing a second rhodopsin associated with a predicted flotillin coding gene and have thus been named flotillin-associated rhodopsins (FArhodopsins). Although they are members of the proteorhodopsin protein family, they form a separate clade within that family and are quite divergent from known proton-pumping proteorhodopsins. They contain either DTT, DTL, or DNI motifs in their key functional amino acids. FArhodopsins are mainly associated with the lower layers of the epipelagic zone. All marine FArhodopsins had the retinal binding lysine, but we found relatives in freshwater metagenomes lacking this key amino acid. AlphaFold predictions of marine FArhodopsins indicate that their retinal pocket might be very reduced or absent, hinting that they are retinal-less. Freshwater FArhodopsins were more diverse than marine ones, but we could not determine if there were other rhodopsins in the genome due to the lack of SAGs or isolates. Although the function of FArhodopsins could not be established, their conserved genomic context indicated involvement in the formation of membrane microdomains. The conservation of FArhodopsins in diverse and globally abundant microorganisms suggests that they may be important in the adaptation to the twilight zone of aquatic environments. IMPORTANCE: Rhodopsins have been shown to play a key role in the ecology of aquatic microbes. Here, we describe a group of widespread rhodopsins in aquatic microbes associated with dim light conditions. Their characteristic genomic context found in both marine and freshwater environments indicates a novel potential involvement in membrane microstructure that could be important for the function of the coexisting proteorhodopsin proton pumps. The absence or reduction of the retinal binding pocket points to a drastically different physiological role.