The Economist June 4th 2022 Science & technology 71Oceanographic Institution, in Massachu-
setts, is responsible for a newly launched
project which sets out to identify “super
reefs” of this sort around the world. Using
a mix of genetic analysis and hydrologic
modelling, she aims to find reefs that are
heat-resistant and genetically diverse, and
therefore potentially able to restore neigh-
bouring bleached sites to their former glo-
ry. She then hopes to expand protections
for these reefs, in order to increase their
chances of survival as heat-resistant re-
sources for the future.
The evolution of resilience to heat is
not, though, merely a matter of geography.
It has also been found around the world in
corals living cheek-by-jowl with more vul-
nerable specimens. This suggests its ori-
gins are complex. Christian Voolstra of the
University of Konstanz, in Germany (who
is also a co-author of Dr Savary’s paper), is
leading a project intended to identify the
responsible parts of a coral’s genome.
Some don’t like it hot
To do this he subjects a range of corals to
intense blasts of heat, to see how they fare.
While this 18-hour stress test, known as
cbass(Coral Bleaching Automated Stress
System), cannot capture the full effects of
long-term bleaching, his hope is that the
most bleach-resistant corals will neverthe-
less show their mettle in it.
Having established which corals are re-
silient, the next step is to search for genes
or genetic variants that are shared by such
corals but are absent from others. Dr Vool-
stra’s initial studies lead him to believe just
a few genes will indeed turn out to be re-
sponsible. And although some will be geo-
graphically specific, he expects others will
be found all over the world.
Further evidence points in that direc-
tion, too. In 2020 Phillip Cleves of Stanford
University published work which showed
that knocking out one particular gene in a
species called Acropora milleporasignifi-
cantly reduces its ability to withstand heat.
If resilience genes like these could be cata-
logued, and their presence identified in
the field, that would allow researchers to
identify resilient corals much more quick-
ly than cbasscan. This might be done us-
ing either some easily spotted biochemical
consequence of their presence (a so-called
biomarker), or one of the new generation
of hand-held gene-sequencing devices
now coming onto the market.
Even if the full genetic complexity of
heat resistance can be elucidated, though,
other mysteries will remain. Certain corals
are able to survive heat that kills their clos-
est cousins as well as their unrelated
neighbours. This has led to speculation
that heat-resistance can also be conferred
on corals by symbiotic organisms—either
particular types of their companion algae,
or perhaps the bacteria that collectively
constitute their “microbiome”.
It would make a lot of sense from both
the coral and the algal point of view for cor-
als’ algal symbionts to evolve more robust
mechanisms of photosynthesis, which do
not misbehave at high temperatures. Pre-
sumably, given time and continued global
warming, that would happen naturally. But
it might be possible to give the process a
helping hand. Indeed, in a paper published
in 2020, a group led by Madeleine van Op-
pen of Melbourne University showed it
was possible to make a palpable difference
to algal production of reactive oxygen-rich
compounds with just four years of selec-
tive breeding for heat tolerance.
Even if the algae cannot be pressed into
service in this way, though, other micro-
scopic organisms living within a coral
might be. Microbiomes—the collectives of
bacteria, fungi and viruses that cohabit
with most animals, especially in their
guts—are now taken seriously as physio-
logical influencers. The human micro-
biome has been connected, with various
degrees of plausibility, to conditions rang-
ing from obesity to Alzheimer’s disease,
and gut microbes are essential to the diges-
tive processes of animals as diverse as cat-
tle and termites. There is no reason for cor-
als to be exempt from their influence.
Raquel Peixoto of the King Abdullah
University of Science and Technology in
Saudi Arabia is investigating the matter in
collaboration with Dr Voolstra. In prelimi-
nary experiments, she and her team have
isolated several microbes shared by resil-
ient corals and then inoculated them into a
few dozen unresilient varieties that lack
them. The survival rate of the inoculated
corals, when exposed to a temperature rise
of 4°C, was 40% higher than that of the un-
inoculated ones.
Whatever cocktail of genetics and
germs is needed to produce resilience,
each of these factors suggests its own next
steps. If genetics is the key, then corals
with the relevant genes could be given pri-
ority by conservationists, transplanted to
new sites, or else induced to breed moreproductively, perhaps by crossing different
heat-resistant strains. If the microbiome is
responsible, then probiotic injections
could be developed. This would be excit-
ing. Breeding for heat resilience would
take generations. Probiotic injections
could transform the prospects of a coral
head doomed in the here and now. Some
experiments even suggest that individual
corals could be “hardened up” to adapt to
warmer climates within their own life-
times—and might then pass that tough-
ness to offspring via a process called epige-
netic inheritance, which allows certain ac-
quired characteristics to be handed down
for a generation or two via mechanisms
that control gene expression.
One last possibility is genetic tinkering
using crispr-Cas9 dnaediting or a similar
technique to insert or modify genes for
heat resilience. This is an approach that Dr
Cleves is exploring, though he has no in-
tention as yet of taking his experiments
outside a laboratory. The prospect of con-
ducting them on a reef remains controver-
sial, since it would mean letting genetical-
ly modified organisms loose in the wild.
But as the planet continues to heat up, he
believes there may come a point where
conservationists have no alternative. Be-
sides, it might be quicker than trying to
achieve similar results by crossbreeding.Know your enemy
The immediate priority, however, is to de-
velop a better understanding of what is out
there. This means doing several things.
These include creating standard heat-re-
sistance tests, so that species from differ-
ent sites can be compared; investigating
resistant corals to see which biomarkers
they share; interbreeding resistant corals
to find any undesirable characteristics that
are inherited along with thermal resil-
ience; and plumbing the transformative
potential of probiotics.
Further challenges await those seeking
to turn such observations to practical ef-
fect. The first is scale. The Great Barrier
Reef, admittedly the largest target, is the
size of Italy. By contrast, a restoration pro-
ject a few hectares in size would be regard-
ed as ambitious at the moment, so the first
targets are likely to be reefs of high value as
tourist attractions or natural sea defences.
In the longer term, automated dissemi-
nators of souped-up coral larvae or resil-
ience-encouraging probiotic bacteria
could help. So might identifying reefs that,
by virtue of local currents, play an outsize
role in propagating larvae to other sites—
for these could be the most useful places to
start. For Dr Cohen, recruiting these natu-
ral nodes will be crucial to engineering
change over sufficiently large areas. “We
have to let nature do its thing,” she says,
“because only nature can do it on the scale
that’s necessary.” AUSTRALIACoral SeaSource: “Emergent properties in the responses of tropical corals
to recurrent climate extremes”, by T. Hughes et al., 2021*145 reefs surveyed during five bleaching
events (1998, 2002, 2016, 2017 and 2020)G
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efll300 kmBleaching occurrences*
1998-2020
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