Opsin three domains of life: Archaea, Bacteria, and Eukaryota. In Eukaryota, type I opsins are found mainly in unicellular organisms such as green algae, and in fungi. In most complex multicellular eukaryotes, type I opsins have been replaced with other light-sensitive molecules such as cryptochrome and phytochrome in plants, and type II opsins in Metazoa (animals). Microbial opsins are often known by the rhodopsin form of the molecule, i.e., rhodopsin (in the broad sense) = opsin + chromophore. Among the many kinds of microbial opsins are the proton pumps bacteriorhodopsin (BR) and xanthorhodopsin (xR), the chloride pump halorhodopsin (HR) the photosensors
Algae Horaninow (1843), Agassiz (1859), Wilson & Cassin (1864), in further classifications, the "algae" are seen as an artificial, polyphyletic group. Throughout the 20th century, most classifications treated the following groups as divisions or classes of algae: cyanophytes, rhodophytes, chrysophytes, xanthophytes, bacillariophytes, phaeophytes, pyrrhophytes (cryptophytes and dinophytes), euglenophytes, and chlorophytes. Later, many new groups were discovered (e.g., Bolidophyceae), and others were splintered from older groups: charophytes and glaucophytes (from chlorophytes), many heterokontophytes (e.g., synurophytes from chrysophytes, or eustigmatophytes from xanthophytes), haptophytes (from chrysophytes), and chlorarachniophytes (from xanthophytes). With the abandonment of plant-animal dichotomous classification, most groups of algae (sometimes all) were