Many species of marine bacteria produce light, a phenomenon known as bioluminescence. But the process takes a lot of energy, and it is unclear what they get in return. "Bioluminescence takes up several per cent of the cell's energy," says molecular biologist Grzegorz Wegrzyn from Gdansk University.

Wegrzyn and his colleagues studied a free-living luminescent bacterium called Vibrio harveyi. They found that strains carrying genetic mutations that reduced their ability to bioluminesce couldn't repair damaged DNA if they were left in the dark. But the mutants repaired the damage when exposed to an external light source.

The researchers then took light-emitting genes from V. harveyi and put them into a nonluminescent species, Escherichia coli. The same thing happened: only the light-producing bacteria could survive DNA damage when kept in the dark.

Wegrzyn thinks that the need to repair DNA could have been a strong selection pressure for bioluminescence in deep-sea bacteria. Bacteria usually use an enzyme called photolyase to mend DNA. But photolyase only works in visible light.

For most bacteria this isn't a problem, as they use sunlight to fuel the repair process, but it's a different matter for deep-sea bacteria. While UV rays may penetrate deep below the ocean surface, sunlight is easily absorbed.

This left them with no option but to create their own light. "The development of an internal source of light would be very important for them," says Wegrzyn.

The idea may also explain why populations of luminescent bacteria switch all their lights on at a certain population density--a reaction called "quorum sensing".

When bacteria are concentrated, waste products and metabolites accumulate that could be harmful to the cells' DNA, Wegrzyn says. "Quorum sensing stimulates the DNA repair process."

Bioluminescence expert Jean-Fran�ois Rees of the Catholic University of Louvain in Belgium is intrigued by the idea, but says that other mechanisms may also be at work.

Luciferase, the enzyme responsible for producing light, might be mopping up toxic chemicals in the cells and producing light in the process, he says.

Microbiologist Paul Dunlap at the University of Maryland, Baltimore, agrees that the final explanation won't be simple. "My guess is there is a lot more going on."
Source: Microbiology (vol 146, p 283)

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