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WHY do deep-sea bacteria glow? Scientists in Poland say they’ve cracked this
long-standing biological mystery: the bacteria shine, it seems, to repair their
DNA after it has been damaged by ultraviolet radiation that penetrates deep into
the ocean.

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-François 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.”