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MARINE PHYTOPLANKTON CHANGES FORM TO
PROTECT ITSELF FROM
DIFFERENT PREDATORS A
tiny single-celled organism that
plays a key role in the carbon cycle of cold-water oceans may be a lot
smarter
than scientists had suspected. In
a paper published June 11 in the
online version of Proceedings of the National Academy of Sciences,
researchers
report the first evidence that a common species of saltwater algae
– also known
as phytoplankton – can change form to protect itself against
attack by
predators that have very different feeding habits. To boost its
survival
chances, Phaeocystis globosa will enhance or suppress the formation of
colonies
based on whether nearby grazers prefer eating large or small particles. “Based
on chemical signals from
attacked neighbors, Phaeocystis globosa enhances colony formation if
that’s the
best thing to do for survival, or it suppresses the formation of
colonies in
favor of growing as small solitary cells if that’s the best
thing to do,” said
Mark E. Hay, Teasley Professor of Biology at the Georgia Institute of
Technology. “These changes in form made nearly a 100-fold
difference in the
alga’s susceptibility to being eaten. It’s
certainly surprising that a single-celled
organism can chemically sense the presence of nearby consumers,
identify those
consumers and change in opposing ways depending on which consumers are
present.” The
behavior could have implications
for global climate change because Phaeocystis blooms play a key role in
the
carbon cycle of cold oceans, accounting for up to 85 percent of local
productivity during some time periods. This complex defensive behavior
also
shows how environmental factors can affect even simple organisms, Hay
noted. Conducted
largely at Georgia
Tech’s marine lab in Phaeocystis
has two primary
predators: small grazers such as ciliates, which prefer to eat small
solitary
cells that are four to six microns in diameter, and the larger
shrimp-like
copepods, which prefer to eat large, ball-shaped colonies. When
copepods are attacking the
phytoplankton, therefore, the best survival strategy of Phaeocystis is
to form
solitary cells. When ciliates are attacking, the best strategy is to
form
colonies that are too large for those predators to consume. Lead
author Jeremy D. Long, along
with collaborators Gabriella W. Smalley, Todd Barsby, Jon T. Anderson
and Mark
Hay found that’s just what Phaeocystis does. Chemicals that
signaled attacks
from copepods suppressed the formation of colonies by 60 to 90 percent,
while
signals from ciliates enhanced colony formation by more than 25 percent. The
transformations took place
over periods of three to six days, and the overall size difference
could be
dramatic. “When one of these cells changes to the biggest
colony form, although
it takes a while, it’s like changing from a mosquito to 76
blue whales or 3,000
bull elephants,” Hay explained. “That’s a
pretty dramatic difference.” Defensive
responses are often seen
in higher plants, but this is believed to be the first report of such a
complex
and species-specific response in marine phytoplankton. Hay suspects
scientists
may find other examples of complex defensive strategies when they look
more
closely at other single-celled organisms. The
response of Phaeocystis could
be important to scientists studying climate change because the predator
that
ultimately consumes the phytoplankton determines the fate of the carbon
it
contains. If eaten by copepods, for example, the carbon becomes part of
fecal
packages that sink into the deep ocean where a portion of that carbon
is
sequestered – thereby reducing atmospheric carbon dioxide, a
leading greenhouse
gas. If consumed by smaller creatures like ciliates, less of the carbon
sinks
to the deep sea and more remains in the surface waters. “This
could alter the flow of
energy and nutrients from deep to shallow, depending on what might be
trying to
eat it, and how the organism responds to the chemical signals of
what’s
attacking it,” Hay said. Experimentally,
the researchers
attempted to separate the chemical signals from the actual predators.
They grew
Phaeocystis in the presence of either ciliates or copepods. They then
filtered
out both the phytoplankton and predators, leaving only water containing
the
chemical signals of attack. Water
samples containing signals
from the two predators were then separately introduced into Phaeocystis
cultures that had not been attacked. The scientists then studied how
the
different chemical signals affected the percentage of Phaeocystis
living in
colonies or as solitary cells. Finally, they examined whether this
response
affected how much the predators ate to determine if the change
conferred a
survival advantage. “We
found that these organisms
were making the right choice,” Hay said. “They were
shifting to the shape that
made them largely immune to whichever predator was attacking, and this
shift
suppressed either the feeding or growth and reproduction of the
consumer to
which they were responding.” The
role of this phytoplankton has
been controversial in the scientific community, with some arguing that
Phaeocystis makes a good food source for higher creatures in the cold
oceans,
while others contend its food role is small. While this paper
won’t resolve the
dispute, Hay believes it shows that both points of view could be
correct –
depending on which form the organism has taken. “It
depends on the environmental
context, which we are appreciating more and more in ecology and in
biomedical
research,” he added. “Some of these differences are
small, but they can have a
large effect.” ##
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