ALGAL VISUAL PROTEINS - AN EVOLUTIONARY POINT-OF-VIEW

Light is a major environmental signal controlling most life processes; hence, light perception is absolutely necessary for organisms that re ly on light. In order to detect light, organisms evolved photoreceptor s, which are found throughout all the kingdoms with enormous structura l variations. Yet, the nature of the photoreceptor pigment is highly c onserved, probably for the stringent conditions it has to satisfy. In prokaryotes such as the archaebacteria Halobacterium halobium and Natr onobacterium pharaonis, the cell membrane provides an extensive surfac e 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 pr obably 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 superfami ly of the seven membrane-spanning domains proteins is responsible for reception roles (chemoreception, mechanoreception, photoreception), an d one of these proteins (i.e., rhodopsin) is responsible for photorece ption and vision. Therefore, we could reasonably expect it or its homo logues to be found in any group of living organisms that manifests pho tobehavior. Indications of the presence of rhodopsin in all the domain s are emerging; therefore, we consider experimental evidence that bear s out this theory on organisms other than algae and then provide a mor e exhaustive examination of the visual pigments present in algal phyla .