Geology Reference
In-Depth Information
Figure 33: A bobtail squid,
Euprymna scolopes
, whose internal light organs house biolumines-
cent bacteria.(
photo © Margaret McFall-Ngai
)
But it isn't the squid that generates the light—bacteria known as
Vibrio fischeri
do this
on its behalf. The bacteria live free in the sea, but, given the chance, love nothing bet-
ter than to colonise the cosy vaults of a squid's light organs where, safe from their own
predators, they consume a rich nutrient broth provided for them by their host. Wheth-
er in the sea or in a squid, each bacterium constantly produces small amounts of a sig-
nalling molecule known as AHL, which diffuses out of the cell and into the surrounding
environment. All
Vibrio fischeri
cells are also capable of detecting AHL in their sur-
roundings, using a receptor molecule known as LuxR. Out in the vast dilution of the sea,
only small amounts of AHL pass from cell to cell, triggering no response in LuxR. But
in the teeming bacterial populations inside the squid light organs, things are different.
Here, each cell absorbs so much AHL from its neighbours that LuxR and AHL lock in a
tight chemical embrace when the AHL exceeds a threshold concentration.
Together, the newly coupled molecules dance towards a specific region of the coiling
bacterial DNA strand, bonding with it in yet another tight chemical marriage which sets
into motion a beautifully choreographed chain of events ending in an astonishing feat:
the emission of the eerie greenish light known as bioluminescence. This trick is not ex-
clusive to squid; other animals have discovered that hosting dense light-emitting colon-
ies of
Vibrio fischeri
can be of considerable benefit, such as the fish
Monocentris ja-
ponicus
, which uses them to attract mates. The light organs of squid and fish are the only
places where the density of AHL becomes high enough to trigger light production. By
its use,
Vibrio fischeri
recognises whether it is inside a light organ or in the open sea,
and responds appropriately. It turns out that the use of AHL as a signalling molecule is
not restricted to
Vibrio fischeri
—it is used by a wide range of bacteria, including some
