Vibrio fischeri

Ecology

Planktonic V. fischeri are found in very low quantities (almost udnetectable) in almost all oceans of the world, preferentially found in temperate and sub-tropical waters. These free-living vibrios subsist on organics within the water. They are found in higher concentrations in symbiosis with certain deep sea marine life within special light-organs; or as part of the normal enteral microbiota of marine animals. Additionally the bacteria can be pathogenic to certain species of marine invertebrates, some of which are commercially farmed in aquaculture. This disease is known as luminous vibriosis.

Symbiotic relationship

Symbiotic realtionships in monocentrid fisheses and sepolid squid appear to have evolved separately. the most prolific of these relationships is with the Hawaiian bobtail squid (Euprymna scolopes)

Free-living vibrios in the ocean waters innoculate the light organs of juvenile squid and fish. Ciliated cells within the light organs selectively draw in the symbiotic bacteria. These cells promote the growth of the sybionts and actively reject any competitors. Through quorum sensing the bacteria cause these cells to die off once the light organ is sufficiently colonised.

The light organ of certain squid contain reflective plates that intesify and direct the light produced, due to proteins known as reflectins.

Sepolid squids expel 90% of the symbiotic bacteria in its light organ each morning in process known as "venting". Venting is hypothesised to provide the free-living innoculum source for newly hatched squids.

Bioluminescence

The bioluminescence of V. fischeri is also caused by transcription induced by population-dependent quorum sensing. The luminescence is only seen when population density reaches a certain level.

The luminescence appears to follow a circadian rhythm, that is, it is brighter during the nightime than daytime.

The bioluminescence has also been shown to be proportionally related to both protection against ultraviolet radiation damage to genes and the pathogenicity of bioluminescent vibrios.

Genetics of bioluminescence

The bacterial luciferin-luciferase system is encoded by subset of genes labelled the Lux operon. In V. fischeri 5 such genes (LuxCDABE) have been identified to display activity with regards to the emission of visible light, and two genes (LuxR and LuxL) to be involved in regulating the operon. Several external and intrinsic factors appear to induce and inhibit the transcription of this gene set and produce or suppress light emission. More research is being done to better our understanding of these processes.

List of pseudonyms