SHEDDING LIGHT ON PHOTOTROPHIC PIGMENT EVOLUTION: RECONSTRUCTION OF ANCESTRAL RHODOPSINS

Rhodopsins, light-harvesting retinal-based pigment proteins, have likely provided a mechanism for microbial energy capture for billions of years and are thought to be important for understanding biosignatures of early life. One of the largest groups of rhodopsins are proteorhodopsins, which can be found in up to half of all surface photic-zone-dwelling marine bacterial species. Proteorhodopsins are classified into two major groups based on their visible absorption maxima: Green-light-absorbing proteorhodopsin (maximally absorb at 520 nm), and blue-light-absorbing proteorhodopsin (maximally absorb at 490 nm). Observed differences in spectral absorption are suggested to stem from sequence variability between the two proteorhodopsins. The two types of proteorhodopsin are not dispersed among marine bacterial taxa randomly but are observed in a depth gradient based on their spectral tuning to visible light availability in marine environments. For instance, green proteorhodopsins are dominantly expressed in organisms found at shallower depths, whereas blue proteorhodopsins are generally localized at greater depths where green light is unable to penetrate. Despite their global abundance, the phylogenetic relationships between blue and green proteorhodopsin are not well constrained. Furthermore, the ancestral absorption spectra of proteorhodopsins as well as the influence of proteorhodopsin color evolution on the long-term diversification of marine phototrophs are unknown. In order to explore the evolution of proteorhodopsin spectral absorption, we inferred ancestral proteorhodopsin protein sequences from phylogeny. Our results support the idea that proteorhodopsin genes are distributed amongst diverse marine bacterial taxa, including Proteobacteria and Bacteroidetes. Our evolutionary analyses provide evidence that proteorhodopsins are a monophyletic family of rhodopsins, and that there have been multiple independent events of blue/green color diversification within the proteorhodopsin superfamily. Unmasking the ancestral color-state of proteorhodopsins may shed light on the evolution of pigment-based biosignatures and prove beneficial in determining the succession of marine phototrophs in the future.