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 fuel the algae with specific types of fungi. This causes bio-flocculation of the algae which allows for easier extraction. Often, the algae is dehydrated, and then a solvent such as hexane is used to extract energy-rich compounds like triglycerides from the dried material. Then, the extracted compounds can be processed into fuel using standard industrial procedures. For example, the extracted triglycerides are reacted with methanol to create biodiesel via transesterification. The unique composition of fatty acids of each species influences the quality of the resulting biodiesel and thus must be taken into account when selecting algal species for feedstock. An alternative