Facultative

Facultative means "optional" or "discretionary" (antonym obligate), and is used mainly in biology. It is seen in topics including:

• Facultative anaerobe, an organism that can use oxygen but also has anaerobic methods of energy production.
• Facultative biotroph, an organism, often a fungus, that can live as a saprotroph but also form mutualisms with other organisms at different times of its life cycle.
• Facultative biped, an animal that is capable of walking or running on two legs as well as walking or running on four limbs or more, as appropriate
• Facultative carnivore, a carnivore that does not depend solely on animal flesh for food but also can subsist on non-animal food. Compare this with the term omnivore
• Facultative heterochromatin, tightly packed but non-repetitive DNA in the form of Heterochromatin, but which can lose its condensed structure and become transcriptionally active
• Facultative lagoon, a type of stabilization pond used in biological treatment of industrial and domestic wastewater
• Facultative necrophage, an animal that feeds on carrion but retains the traits needed to find and consume other food sources
• Facultative parasite, a parasite that can complete its life cycle without depending on a host
• Facultative photoperiodic plant, a plant that will eventually flower regardless of night length but is more likely to flower under appropriate light conditions.
• Facultative saprophyte, lives on dying, rather than dead, plant material
• Wetland indicator status for plants can include facultative (FAC), facultative wetland (FACW), and facultative upland (FACU) to distinguish types that appear in multiple regions

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