Algal Visual Proteins: An Evolutionary Point of View

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Abstract
Light is a major environmental signal controlling most life processes; hence, light perception is absolutely necessary for organisms that rely on light. In order to detect light, organisms evolved photoreceptors, which are found throughout all the kingdoms with enormous structural variations. Yet, the nature of the photoreceptor pigment is highly conserved, probably for the stringent conditions it has to satisfy. In prokaryotes such as the archaebacteria Halobacterium halobium and Natronobacterium pharaonis, the cell membrane provides an extensive surface on which membrane-spanning light-sensitive proteins are spread with a fixed bidimensional orientation for maximal effectiveness in photon capturing. In unicellular algae, a similar pigment-containing patch probably exists in the Chlorophyta, whereas more complex photoreceptors, such as three-dimensional crystals of membrane-spanning proteins, for example, may occur in the Euglenophyta and Chrysophyta. The superfamily of the seven membrane-spanning domains proteins is responsible for reception roles (chemoreception, mechanoreception, photoreception), and one of these proteins (i.e., rhodopsin) is responsible for photoreception and vision. Therefore, we could reasonably expect it or its homologues to be found in any group of living organisms that manifests photobehavior. Indications of the presence of rhodopsin in all the domains are emerging; therefore, we consider experimental evidence that bears out this theory on organisms other than algae and then provide a more exhaustive examination of the visual pigments present in algal phyla.

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