The Hydrogenophilalia are obligate thermophiles and include heterotrophs and autotrophs. About half of living dinoflagellate species are autotrophs possessing chloroplasts and half are nonphotosynthesising heterotrophs. All heterotrophs depend on autotrophs for their nutrition. Phototrophs can be either autotrophs or heterotrophs. Protozoa, like plants and animals, can be considered heterotrophs or autotrophs. Both heterotrophs and photoheterotrophs are found along with some facultative autotrophs. Members can be autotrophs, heterotrophs, or phototrophs using heliorhodopsin. The terms can be used to describe energy transfer in both autotrophs and heterotrophs. Some are autotrophs, heterotrophs, and a 2006 study even discovered chemoautotrophs. Oxygenic photosynthesis is a metabolic pathway facilitated by autotrophs, including plants, algae, and cyanobacteria. These fishes generally feed on autotrophs, algae and small invertebrates. Organisms that grow by fixing carbon are called autotrophs. Unicellular algae are plant-like autotrophs and contain chlorophyll. Zooxanthellae are autotrophs containing chloroplasts composed of thylakoids present in clusters of three. Heterotrophs are organisms that grow using the carbon fixed by autotrophs.