Bacterial luminescence

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The production of visible light by bacteria; with very few exceptions this light is blue-green. The phenomenon is seen in many species of several genera, including Vibrio, Photobacterium, Alteromonas, and Xenorhabdus. Luminous bacteria are primarily marine, but there are some genera with terrestrial (Xenorhabdus) and fresh-water (Vibrio) species. In the marine environment the bacteria are found in various habitats, including planktonic (free-floating), saprophytic (on a variety of marine proteinaceous materials), parasitic (on a number of marine invertebrates), and symbiotic. The symbiotic habitat can take one of several forms. The symbiotic bacteria may be loosely associated as gut symbionts in many different marine organisms; they may be specifically and more tightly associated in the light organs of marine fishes and squids; or they may be very tightly associated as intracellular symbionts in luminescent pyrosomes (light-omitting organelles). When associated as light-organ symbionts, the bacteria are used by the host fish or squid as a biological light bulb. Under these conditions the bacteria are maintained in specialized organs where they are cultured by the host organism, kept free from contaminants, and continuously emit light. The actual light emission is then controlled physically by the host's use of shutters, chromatophores, or other mechanisms. These symbiotic relationships are probably the most common habitats in which bacterial luminescence is observed in the ocean.

The chemistry of bacterial luminescence is unique among luminous organisms. The enzyme that catalyzes light emission is luciferase; it combines with a riboflavinlike substance called flavin mononucleotide (FMNH₂). This complex then reacts with a long-chain aldehyde, and with molecular oxygen to form an excited state capable of emitting light. The molecule that actually emits the light is an altered form of the flavin. This unique biochemistry has been used as an indicator of the presence of luminous bacteria in cases where the symbiotic bacteria could not be obtained in pure culture or could not be grown free from their host. See also Bacterial physiology and metabolism; Bioluminescence; Riboflavin.