Bioluminescence is a seemingly confusing adaptation for bacteria. Unlike large multicellular organisms, bacteria do not use it to catch prey or find mates. Recent research has even shown that under normal conditions, bioluminescent bacteria cannot compete with non-luminescent versions of themselves since they are wasting so much energy on the seemingly useless process. However, when exposed to UV light, the luminescent strains eventually came to dominate the cultures. This suggests that the bioluminescence is somehow related to preventing damage from the ionizing radiation. Research has demonstrated that bioluminescence requires radical initiators or reactive oxygen species in order to provide the energy required to release a photon. It is conceivable that bacteria are capable of channeling some of the reactive species produced by the ionizing UV radiation into their bioluminescent pathways in order to eliminate the harmful substances.
"The chemical reaction involved in bioluminescence must be sufficiently energetic to produce an excited singlet state molecule that will generate a visible photon as it relaxes back down into its ground state...Chemical oxidation reactions involving molecular oxygen fit this criterion, which may explain why the primary mechanism operating in bioluminescent reactions involves the breakdown of a peroxide bond." (Widder 2010:705)
"In mixed cultures of luminescent and dark mutants of Vibrio harveyi, the dark mutants rapidly overrun the culture unless the mixture is irradiated with ultraviolet (UV) light, in which case the balance tips the other way." (Widder 2010:707)
"[W]ithout a selective pressure, the luminescence is a disadvantage for bacteria, perhaps due to consumption of significant portion of cell energy. However, when the same experiments were repeated but cultures were irradiated with low UV doses, luminescent bacteria started to predominate shortly after the irradiation. Therefore, we conclude that stimulation of photoreactivation may be an evolutionary drive for bacterial bioluminescence." (Czyz 2003:140)