5.11F: Facultative Phototrophy
- Recognize the traits associated with the classification of facultative phototrophy
An autotroph or “producer”, is an organism that produces complex organic compounds (such as carbohydrates, fats, and proteins) from simple substances present in its surroundings, generally using energy from light ( photosynthesis ) or inorganic chemical reactions (chemosynthesis). They are the producers in a food chain, such as plants on land or algae in water. They are able to make their own food, and do not need a living energy or carbon source. Autotrophs can reduce carbon dioxide to make organic compounds, creating a store of chemical energy. Phototrophs, a type of autotroph, convert physical energy from sunlight (in case of green plants) into chemical energy in the form of reduced carbon.
In terms of biology facultative means “optional” or “discretionary” the antonym of which is obligate meaning “by necessity”. Thus facultative phototrophy means an organism that can switch between phototrophy to make organix compounds and other means of getting cellular energy. Probably the best studied example of a facultative phototrophy is Chlamydomonas reinhardtii.
Chlamydomonas reinhardtii is a single celled green alga about 10 micrometres in diameter that swims with two flagella. It has a cell wall made of hydroxyproline-rich glycoproteins, a large cup-shaped chloroplast, a large pyrenoid, and an “eyespot” that senses light. Although widely distributed worldwide in soil and fresh water, C. reinhardtii is primarily used as a model organism in biology in a wide range of subfields. When illuminated, C. reinhardtii can grow in media lacking organic carbon and chemical energy sources, and can also grow in the dark when supplied with these. C. reinhardtii is also of interest in the biofuel field, as a source of hydrogen. As one can imagine switching energy sources under varying conditions allows facultative microbes to live in different conditions, in the case of a facultative phototroph it can rely of light other energy sources.
Key Points
- Phototrophs can obtain cellular energy from light as well as using light to fix carbon to make complex macromolecules on which to survive.
- Chlamydomonas reinhardtii is an organism that can rely on photosynthetic and chemical energy sources, depending on conditions.
- Facultative means optional, in terms of biology it refers to an organism that can switch energy sources for survival.
Key Terms
- autotroph : Any organism that can synthesize its food from inorganic substances, using heat or light as a source of energy.
- pyrenoid : any of several transparent structures found in the chloroplast of certain algae etc.; they are responsible for the fixation of carbon dioxide and the formation of starch