Leopards may not be able to change their spots, but
corals can change their skeletons, building them out of
different minerals depending on the chemical composition of
the seawater around them.
That's the startling conclusion drawn by a Johns
Hopkins marine geologist, writing in the July issue of the
journal Geology.
Postdoctoral fellow
Justin Ries and his collaborators say this is the first
known case of an animal altering the composition of its
skeleton in response to change in its physical environment.
The aquatic animal's sensitivity to such changes poses
questions about its evolutionary history, as well as the
future of the ecologically important coral reefs that it
builds, Ries said, especially at a time when seawater is
changing in response to global warming and the buildup of
carbon dioxide in the atmosphere.
Ries, who received his doctorate from Johns Hopkins in
2005, collaborated on the research with his dissertation
advisers, Steven M. Stanley, now of the University of
Hawaii, and Lawrence A. Hardie, a professor in the Krieger
School's Morton K.
Blaustein Department of Earth and Planetary
Sciences.
Reefs are large underwater structures of coral
skeletons, made from calcium carbonate secreted by
generation after generation of tiny coral polyps over
sometimes millions of years of coral growth in the same
location. The team showed that corals can switch from using
aragonite to another mineral, calcite, in making the
calcium carbonate. They make that switch in response to
decreases in the ratio of magnesium to calcium in seawater,
Ries said. That ratio has changed dramatically over
geologic time.
"This is intriguing because, until now, it was
generally believed that the skeletal composition of corals
was fixed," he said.
Ries spent two months growing three species of modern
scleractinian corals (the major reef-building corals in
today's seas) in seawater formulated at six different
chemical ratios that have existed throughout the
480-million-year history of corals. He concocted this
"artificial seawater" using "recipes" provided by Hardie,
who several years ago discovered that the magnesium-calcium
ratio in seawater has vacillated throughout geologic
history between a low of 1.0 and today's 5.2, changing due
to chemical reactions between seawater brine and rising
magma along the ocean floor.
Ries placed his artificial seawaters in 10-gallon
glass tanks, then added fragments of the three species of
Caribbean reef-building corals. These were replete with
colonies of polyps, which had spent the previous month in
"equilibration tanks." Ries adjusted the chemistry of those
tanks over 30 days, until their magnesium-to-calcium ratios
were in line with the prescribed "ancient seawater"
chemistries.
Two months later, Ries removed the coral skeletons and
used X-ray diffraction to analyze their mineral
composition. He was surprised to find that corals grown in
the artificial seawater with a magnesium-to-calcium ratio
less than 2-to-1 began producing a large portion of their
skeleton as the calcite mineral, while those grown in
unmodified modern seawater produced exclusively the
aragonite mineral.
Though most scientists believed that corals were
programmed to produce only the aragonitic form of calcium
carbonate, he said, the team's work reveals that corals are
far more flexible and able to vary at least a portion of
their skeleton to growth favored by seawater chemistry. He
postulates that this "mineralogical flexibility" provides
corals with an "evolutionary advantage," as it would take
more energy for corals to produce skeletons that are not
favored by the chemistry of the seawater surrounding them.
Further, the calcite-producing corals grown in
artificial ancient water grew significantly more slowly
than did the aragonite-producing corals in modern water.
"The reduction in the corals' rate of growth that
accompanied their exposure to the chemically modified
seawaters is further evidence of corals' extreme
sensitivity to environmental change," Ries said. "This is
particularly significant given recently observed and
predicted future changes in the temperature and acidity of
our oceans--via global warming and rising atmospheric CO2,
respectively--that will presumably have a significant
impact on corals' ability to build their skeletons and
construct their magnificent reefs."
Corals are crucial to nearshore tropical ecosystems
because the reefs they build are inhabited by tens of
thousands of marine animals, plants, algae and bacteria
that make up the coral reef ecosystem, which is one of the
planet's most diverse, Ries said. But coral reefs also
serve a more practical purpose: They absorb wave energy
generated by hurricanes and other severe tropical
storms.
"Ironically, the same factor that is likely causing
such storms to increase in intensity--global warming--is
also causing the corals to bleach (lose their symbiotic
algae) and die, ultimately leading to the destruction of
the coral reefs, which protect the coasts from these
storms," Ries said. "All that being said, it is also
important to note that the magnesium-calcium ratio of
seawater changes only over millions of years and has no
direct relationship to recent global warming and ocean
acidification, which are believed to be at least partly
human caused."
His team's experiments do, however, have significance
with respect to global warming and ocean acidification,
Ries said, because they reveal that although corals can
adapt mineralogically to altered seawater chemistry, doing
so slowed the corals' rate of growth by nearly 65 percent.
"This provides us with further evidence that corals are
extremely sensitive to rapid environmental change, such as
global warming," he said.
The research was funded by the National Science
Foundation, the Petroleum Research Fund and the Howard
Hughes Medical Institute. Moody Gardens Aquarium of
Galveston, Texas, provided the coral fragments.