70 The Economist June 4th 2 022
Science & technology
ConservationReef knots
I
t is notnews that coral reefs are in hot
water. Corals, which are sessile animals
related to sea anemones, host within their
tissues algae that provide them with both
food and their tourist-attracting colours.
But as temperatures rise, the photosyn-
thetic mechanisms of these algae go hay-
wire. Instead of molecular oxygen, the nor-
mal waste product of photosynthesis, they
start generating highly reactive and there-
fore toxic oxygen-rich compounds, such as
peroxides. If they do this to excess, their
hosts expel them, bleaching the coral con-
cerned white.
Bleached corals can linger for a while,
but starved of their symbionts’ supply of
nutrients they are vulnerable to disease.
Eventually, if the temperature does not
drop far enough to permit the algae to be
readmitted, they die.
And the world is getting warmer. As a
consequence, the amount of coral in it has
fallen since 1980 by between 30% and 50%.Locally, things can be yet more dramat-
ic. In just three years (between 2015 and
2018) Australia’s Great Barrier Reef lost
more than 30% of its corals to death-by-
bleaching. Surveys conducted this March
showed that another Barrier Reef bleach-
ing event is now under way. This instance
is particularly worrying because it is the
first to take place during a Pacific cooling
event called La Niña—rather than during
either its warming opposite, El Niño, or an
intermediate period between the two.
These losses affect more than just tour-
ists. Coral reefs are important parts of the
world’s ecology. They host a third of multi-cellular marine species, including many
commercially important fish. They also
provide free coastal defences. Cities such
as Cancun, Honolulu and Miami rely on
them to keep the waves at bay. According to
a study published in 2014 by Robert Cos-
tanza, an economist at University College,
London, such benefits have a value of up to
$10trn a year. Preserving reefs is thus of
practical as well as aesthetic importance.
So something needs to be done to stop
heat-induced bleaching.
One approach is to identify species that
are already heat-resistant and transfer
them to reefs which are at risk. Some of the
most spectacular examples of heat-resil-
ient coral come from sites in the Gulf of
Aqaba at the northern tip of the Red Sea,
which is one of the hottest places on the
planet. Several coral species found here
can weather heat that would lead to mass
bleaching elsewhere. A study published in
2021 by Romain Savary of the Swiss Federal
Institute of Technology in Lausanne
showed that a particular Red Sea reef-
builder called Stylophora pistillatawas able
to withstand rises in excess of 5°C above
the 27°C at which it normally lives—a
greater increase than Earth is expected to
face this century.
Similar pockets of heat resistance
might be expected to have evolved else-
where, too. Anne Cohen of the Woods HoleCorals are threatened by global warming. A bit of pre-emptive tinkering
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