麻豆传媒

WHY do deep-sea bacteria glow? Scientists in Poland say they鈥檝e 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. 鈥淏ioluminescence takes up several per cent of the cell鈥檚
energy,鈥� 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鈥檛 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鈥檛 a problem, as they use sunlight to fuel the
repair process, but it鈥檚 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. 鈥淭he development of an
internal source of light would be very important for them,鈥� says Wegrzyn.

The idea may also explain why populations of luminescent bacteria switch all
their lights on at a certain population density鈥攁 reaction called 鈥渜uorum
sensing鈥�. When bacteria are concentrated, waste products and metabolites
accumulate that could be harmful to the cells鈥� DNA, Wegrzyn says. 鈥淨uorum
sensing stimulates the DNA repair process.鈥�

Bioluminescence expert Jean-Fran莽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鈥檛 be simple. 鈥淢y guess is there
is a lot more going on.鈥�