78 Science & technology The Economist October 9th 2021
Symbiosis
Sea slugs and
chloroplasts
U
nlike their terrestrialcousins, sea
slugs are widely regarded as among the
most beautiful animals on Earth. In some
cases, part of that beauty comes from sub
cellular structures called chloroplasts,
which they extract intact from the algae
they eat and then sequester in gut divertic
ula for purposes of their own.
Chloroplasts are the distant descen
dants of photosynthetic bacteria that be
came symbiotic, a bit less than 2bn years
ago, with an organism ancestral to today’s
algae and plants. From their hosts’ point of
view, their purpose is to use energy from
sunlight to split water into hydrogen and
oxygen, and then react the hydrogen with
carbon dioxide to form molecular precur
sors of glucose. Further reactions turn
these basic materials into other biochemi
cals, too. Rearranging the carbon, hydro
gen and oxygen atoms and adding nitro
gen, for example, results in amino acids,
the building blocks of proteins.
The best guess, therefore, was that slugs
sequester chloroplasts in their diverticula
in order that they might continue photo
synthesising there. But the details were ob
scure. So, to shed more light, as it were, Só
nia Cruz of the University of Aveiro, in Por
tugal, and her colleagues collected some
sea slugs and ran a few experiments.
As they write in a paper in the Proceed-
ings of the Royal Society, they supplied the
slugs with simple chemicals (sodium bi
carbonate and ammonia) containing un
usual carbon and nitrogen atoms. Instead
of normal carbon, which has six neutrons,
the bicarbonate contained sevenneutron
carbon. Similarly, the ammonia contained
nitrogen with eight neutrons instead of the
regular seven. Molecules with these rare
isotopes can be detected using a technique
called mass spectrometry, so the fates of
the atoms involved are easy to follow.
Having thus treated their slugs, the
team exposed half of them to light for 36
continuous hours and kept the other half
in the dark. As they expected, they saw the
unusual isotopes of carbon and nitrogen
enter the chloroplasts of slugs exposed to
light. By contrast, little of the heavy nitro
gen and none of the heavy carbon entered
the chloroplasts of slugs kept in the dark.
What intrigued them, though, was the sub
sequent fate of those isotopes.
A naive assumption would be that the
products of the captured chloroplasts
would be scattered around a slug’s body,
and thus made available to all of its organs.
In fact, most of them ended up in a struc
ture called the albumen gland, which pro
duces nutrients that are then incorporated
in the animal’s eggs (the species con
cerned, Elysia timida, is a hermaphrodite,
so all individuals bear eggs). The rest mi
grated directly into the slugs’ gonads.
This result suggested that, rather than
helping with an animal’s general weal,
chloroplasts have been coopted by Elysia
for a specific role in reproduction. To
check that idea, the team conducted a fur
ther experiment. They kept pairs of slugs
for 28 days either in normal light or in con
ditions of greatly reduced illumination
and counted the number of eggs laid.
Pairs exposed to normal light averaged
238 eggs per week over the course of the ex
periment. Those kept almost in the dark
averaged 129. This prettymuch clinched
the idea that the ingested chloroplasts
have, somehow, become extensions of Ely-
sia’s reproductive system, and that chloro
plasts, those superbly adapted symbionts
of algae andplants, have been able to ad
just, too, tolifeas part of a very different
host indeed.n
One of nature’s strangest symbioses
gives up its secrets M
alariaisoneofmedicalscience’s
most formidable foes. The search
for a vaccine has been going on for dec
ades. Dozens of candidates have been
tested in recent years. But so far only one,
a jab called rts,s, made by GlaxoSmith
Kline, has proved effective in the final
stages of clinical trials. On October 6th
the World Health Organisation (who)
recommended rts,sfor use in childhood
vaccination in places with transmission
of Plasmodium falciparum, the deadliest
of the five parasites that cause malaria,
and the most common in Africa.
The whoreached its decision after
reviewing results from Ghana, Kenya and
Malawi, where more than 800,000 in
fants were vaccinated with a fourdose
regimen. In these countries rts,swas
included among the routine childhood
vaccines distributed by primary health
care centres. This implementation pro
gramme, in which rts,sreduced by 30%
the number of cases of severe malaria
whichledtohospitaladmissions,there
fore measured what kind of efficacy can
be expected if the vaccine is rolled out
widely across Africa.
Some may think that 30% efficacy is
not much. But in parts of subSaharan
Africa children contract malaria six
times a year on average. Each year more
than 260,000 African children die of it
before their fifth birthdays. Those who
survive often suffer lifelong harm, in
cluding stunting, a form of impaired
growth that affects the ability to learn.
The impact ofrts,swill thus be huge.
Crucially, the whosays that the vac
cine was found to be safe after more than
2.3m doses had been administered—
clearing the air on three “safety signals”
that had popped up in an earlier trial. It
also says the jab is highly costeffective.
The next step is therefore for gavi, an
international organisation that buys
vaccines for poor countries, to decide
whether it will add rts,sto its portfolio.
Malariavaccines
Ready to roll
The World Health Organisation approves the first vaccine for malaria
Hope