Abstract :
[en] Unlike animal rhodopsins (Type II), which primarily mediate visual and other light-dependent signaling processes, microbial rhodopsins (Type I) are found in microorganisms, where they exhibit diverse functions, including light-driven ion transport and photosensory signaling. Dinoflagellate rhodopsins were first discovered through environmental metatranscriptomics, and homologs have since been identified in the harmful algal bloom-forming species Prorocentrum shikokuense, and Alexandrium catenella. We recently discovered that the coral endosymbiotic dinoflagellate Breviolum minutum possesses the XRs-like rhodopsin as a potential second engine to capture photoenergy and compensate for impaired photosynthesis under phosphorus limitation. To explore the photochemical diversity and ecological significance of microbial rhodopsins in coral symbiotic and harmful algal bloom-forming dinoflagellates, we here characterize the absorption spectra and photochemical properties of rhodopsins from two harmful algal bloom-forming dinoflagellates, Prorocentrum shikokuense (PsR) and Alexandrium catenella (AcR). We also focused on rhodopsins from two coral symbionts, Breviolum minutum (BmR), a well-established experimental model symbiont, and Cladocopium goreaui (CgR), the most widespread and ecologically important symbiont in reef-building corals. Rhodopsins were heterologously reconstituted in HEK293T cells and purified in the presence of all-trans retinal [5]. PsR–retinal complexes showed a λmax of 507 nm whereas the purified AcR–retinal complex exhibited a λmax of 515 nm, both absorbing in the green light spectrum. The ecological roles of dinoflagelatte rhodopsins are still unclear. In P. shikokuense it may enhance survival under low-light or nutrient-limited conditions during red tide formation.
The coral symbiont rhodopsin BmR exhibited a λmax of 543 nm, indicating sensitivity to yellow-green light while CgR absorbed predominantly blue-green light with a λmax of 492 nm. Interestingly, photochemical assays further evidenced that of all four tested rhodopsins, CgR is a bistable microbial rhodopsin, which can be reversibly photoswitched between two thermally stable UV and green absorbing states. This may be a naturally optimized photoreceptor with photochemical characteristics involved in alternative light-energy acquisition strategies in dinoflagellates.
