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February 04, 1997 |
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Soaking up the Rays |
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A sponge uses optical fibers |
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By Marguerite Holloway |
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It is hard to see under the
sea-particularly if you are 120 meters down, lying beneath a thick covering
of ice during the endless nights of the Antarctic winter. Yet even in this
deep night, hoards of tiny algae live inside sponges, soaking up carbon
dioxide and, in turn, producing nutrients for their hosts. The mystery has
been where these minute green plants get the light they need to drive
photosynthesis. Taking inspiration from the age of
telecommunications, Italian scientists recently discovered the secret of the
symbionts. It turns out that some sponges have a system of fiber optics that
allows them to gather what little light reaches their murky depths and to
direct it to the algae. "We don't give sponges much credit. Most people
look at them and say 'this is a blobby lump,'" comments Mary K. Harper
of the Scripps Institution of Oceanography. "But considering how
primitive these animals are, it's amazing how adaptable they are." Like many sponges, the Antarctic sponge
that the team from the universities of Genoa and Perugia examined, Rossella
racovitzae, has a skeleton composed of little silica spikes called
spicules. They support the creature and keep predators away. In the case of R.
racovitzae, however, each spicule is capped with a cross-shaped antenna
of sorts. The flat spokes of the antenna capture light, which then travels
directly down the silica tube of the spicule to the garden of green thriving
at the base. (Harper suspects that this mechanism might allow larger numbers
of algae to thrive because there is more surface area in the internal folds
of the sponge than on its outer surface.) The researchers discovered R.
racovitzae's system after firing red laser light down a straight,
10-centimeter-long spicule and observing its unimpeded travel. They then bent
the spicule at various angles to see if it still successfully guided the red
light--and it did. Although they have tested just the one sponge--largely
because its spicules are long and easy to work with--the scientists think
others may use the same device. Group leader Riccardo Cattaneo-Vietti says
they will soon begin looking at cave-dwelling sponges for the same
adaptation. Sponges are not the only owners of a
natural light-guidance system. According to Jay M. Enoch of the University of
California at Berkeley, some copepods--another form of marine organism--have
light guides. And certain tropical plants have dome-shaped lenses on their
leaves, allowing them to collect and focus any light filtering down through
the thick rain-forest canopy. As Thomas Vogelmann of the University of
Wyoming described a few years ago, these leaf lenses lead to cells inside the
plant, which, in turn, guide light to needy cells at the base. --Marguerite Holloway |
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©
1996-2002 Scientific American, Inc. All rights reserved. |
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