Bioluminescent bacteria

Bioluminescent bacteria are any light-producing bacteria, present mostly in sea water, marine sediments, surface of decomposing fish and in gut of marine animals. While not as common, bacterial bio-luminescence is also found in terrestrial and freshwater forms. These bacteria may live free (example, Vibrio harveyi) or in symbiosis (example, Vibrio fischeri) with animals such as the Hawaiian Bobtail squid or terrestrial nematodes (example, Photorhabdus luminescens). The animals provide these bacteria a safe home and sufficient nutrition and use the light organs for camouflage, preying for food and attracting mates. In this sense, bioluminescent bacteria have evolved symbiotic relationships with other organisms in which both participants benefit. Additionally, bacteria may deploy luminescence reaction for quorum sensing - ability to regulate gene expression in response to bacterial cell density.

In most bio-luminescent organisms, the purpose of bio-luminescence is wide-ranged and somewhat clear or obvious. Such vital biological purposes include but are not limited to attraction of mates, defense against predators, and warning signals. In the case of bioluminescent bacteria, bio-luminescence serves as a form of dispersal. It has been hypothesized that enteric bacteria - especially those prevalent in the depths of the ocean - employ bio-luminescence as an effective form of distribution. After making their way into the digestive tracts of fish amongst other marine organisms and being excreted in fecal pellets, bioluminescent bacteria are able to utilize their bio-luminescent capabilities to lure in other organisms and prompt ingestion of these bacterial-containing fecal pellets. The bio-luminescence of bacteria thereby ensures their survival, persistence, and dispersal as they are able to enter and inhabit other organisms.

It is important that bio-luminescent bacteria decrease production rates of the enzyme called luciferase when the population is sparse in number in order to conserve energy. Thus, bacterial bioluminescence is regulated by means of quorum sensing. Essentially, certain signaling molecules named autoinducers with specific bacterial receptors become activated when the population density of bacteria is high enough. The activation of these receptors leads to a coordinated response of luciferase production that ultimately yields visible luminescence.

...
Wikipedia