Phototrophs (Gr: φῶς , φωτός = light, τροϕή = nourishment) are the organisms that carry out photosynthesis to acquire energy. They use the energy from sunlight to convert carbon dioxide and water into organic material to be utilized in cellular functions such as biosynthesis and respiration.
Most phototrophs are autotrophs, also known as photoautotrophs, and can fix carbon. They can be contrasted with chemoautotrophs that obtain their energy by the oxidation of electron donors in their environments. Photoheterotrophs produce ATP through photophosphorylation but use organic compounds to build structures.[1] Photoautotrophic organisms are sometimes referred to as holophytic.[2]
In an ecological context, phototrophs provide nutrition for all other forms of life (besides other autotrophs such as chemotrophs). In terrestrial environments, plants are the predominant variety, while aquatic environments include a range of phototrophic organisms such as algae (e.g., kelp), other protists (such as euglena), phytoplankton, and bacteria (such as cyanobacteria).
One product of this process is starch, which is a storage or reserve form of carbon, which can be used when light conditions are too poor to satisfy the immediate needs of the organism. Photosynthetic bacteria have a substance called bacteriochlorophyll, live in lakes and pools, and use the hydrogen from hydrogen sulfide instead of from water, for the chemical process. (The bacteriochlorophyll pigment absorbs light in the extreme UV and infra-red parts of the spectrum that is outside the range used by normal chlorophyll). Cyanobacteria live in fresh water, seas, soil, and lichen, and use a plant-like photosynthesis.
A photolithotrophic autotroph is an autotrophic organism that uses light energy, and an inorganic electron donor (e.g., H2O, H2, H2S), and CO2 as its carbon source. Examples include plants.
The depth to which sunlight or artificial light can penetrate into water, so that photosynthesis may occur, is known as the photic zone.