16 November 2019 | New Scientist | 45Some are more speculative than others.
Macalady points to an unusual breed of
microbes on Earth, the haloarchaea, which
use proteins called rhodopsins to feed on
light. “Some of those can absorb a photon and
directly pump a proton across a membrane,”
she says. They do this to produce adenosine
triphosphate, a molecule that carries energy
within cells, and don’t require the transport
of electrons. If an organism were able to
extract all of its energy from such processes,
it would have no need for the redox reactions
that dictated Aronson’s search.
“It’s always possible that you could
have life that has a totally different way of
surviving,” says Laurie Barge at NASA’s Jet
Propulsion Laboratory in California. “But
it’s really hard to make predictions when
you don’t have any examples.”
Back in the cave, the rhythmic dripping of
water and the metal clinks of climbing gear
echo around, and my toes are going numb
in my boots. As we eventually make our way
out towards the sunshine and leave the
subterranean darkness behind, I can’t help
wondering if I brushed shoulders with life of
the kind that might flourish on Mars. For the
moment, though, I’m just grateful to breathe
air that doesn’t stink of science. ❚could subsist without the need for water
(see “Does life need water?”, far left), or with
a chemistry built on something other than
carbon (see “Does life need carbon?”, below).
Moreover, alien life forms might be able to
harness electrons from the environment
rather than from redox chemistry inside
cells, or harvest energy directly from
electromagnetic radiation. That would make
their biology totally unique. Of course, says
Roger Buick at the University of Washington,
Seattle, “these are all very speculative ideas”.Donna Lu is a reporter at
New Scientist in London.
She tweets @donnadluSH
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TOCK“We suspect that there might be some new
tricks that life knows that we haven’t really
seen before,” says Macalady.
Aronson collects samples with tweezers,
dropping globules onto a strip of pH paper to
test their acidity. The pH comes up as 0. They
could be even more acidic, says Aronson, but
that is as low as the paper can measure. For
reference, the acids our stomachs use to break
down food score somewhere between 1.5 and
3.5 on the pH scale. This environment would be
inhospitable for us, but the life she is hunting
revels in the extreme acidity. Aronson won’t
know what she has found for sure until she
gets her samples genetically sequenced, but
her results could have major repercussions for
the search for life on even more hostile terrain.
One reason why Aronson’s work is so
exciting is that niches resembling the Frasassi
caves are thought to have existed on Mars. Acid
sulphate environments there, and associated
volcanic emissions containing sulphide,
would have provided the optimal conditions
for sulphur-producing life, says Macalady.
Europa, the smallest of Jupiter’s moons, is
also a candidate. Its ocean probably contains
sulphate and possibly sulphuric acid, says
Aronson. If we discover sulphide there as
well, she says, “it might make sense to begin
considering sulphur comproportionation as
a potential metabolism”.
But such organisms may ultimately seem
almost normal. More extreme redefinitions
of life might yet be possible. Some researchers
have suggested that any life on other worlds
Any organism you have
ever seen or interacted
with has been made from
carbon. This probably
isn’t an accident. “Carbon
is capable of the widest
range of chemical
structures,” says Roger
Buick, an astrobiologist
at the University of
Washington, Seattle.
“To have any sort of
complex life, it would
almost certainly have to
require carbon chemistry.”
The abundance of
carbon is also a factor inits favour, says Penelope
Boston, director of NASA’s
Astrobiology Institute
in California. “Carbon
compounds are not only
all over our own solar
system, but astronomers
see them in their
spectroscopic data. So we
know that our galaxy and
probably other galaxies
are carbon rich,” she says.
Carbon-based life
probably also requires
hydrogen, oxygen and
possibly nitrogen, says
Buick. But it is possiblethat other elements could
be substituted – because
of their structural
similarity. For example,
selenium could possibly
replace sulphur, and
arsenic could stand in for
phosphorus. In 2011, a
team of researchers said
they had discovered a
bacterium that could
replace phosphorus with
arsenic in its DNA – a claim
since widely discredited.
But that doesn’t mean it’s
impossible, says Buick. We
just need to keep looking.Does life need carbon?
The “snottites” of
the Frasassi cave
system could
harbour new life