44 | New Scientist | 16 November 2019
place more quickly, burning through all the
available energy in one glorious dash towards
chemical equilibrium, wouldn’t be able to
sustain organisms with longer lifespans.
“Life can only make money on reactions that
are far from equilibrium,” says Macalady.
Of these, there are hundreds of different
combinations of electron donor and acceptor
molecules that could theoretically power
life. The process of aerobic respiration
that provides energy in our cells, for
example, involves oxidising glucose to
carbon dioxide and reducing oxygen to
water. Photosynthesis, on the other hand,
sees carbon dioxide reduced into sugars
while water gets oxidised into oxygen.Life’s periodic table
“By mixing and matching different electron
donors and acceptors,” says Aronson, “you
can start to see where and how feasible certain
reactions might be.” Not all of these potential
reactions are equally interesting, however.
Some generate too little energy to power
life, some involve elements too rare to be
sustainable and others require pressures
and temperatures not found on Earth.
So far, in fact, only a small fraction of
the entries in this vast table of reactions
have ever been found. That leaves open the
possibility that a huge diversity of strange
new metabolisms could be sustaining life
in some hidden corner of the universe.
That corner could be surprisingly close to
home. A 2016 study estimated that Earth hosts
up to a trillion different microbial species, less
than 10 million of which have been catalogued.
All the rest – known colloquially as microbial
dark matter – remain tantalisingly enigmatic.
With so many unknown species waiting to
be discovered, chances are that a totally new
metabolism could be lurking under our feet.
For Macalady, the best chemical to start
with is sulphur. “Sulphur is one of the most
abundant elements in the universe,” she says.
Sometimes it is in its raw elemental form, but it
can also be bound with oxygen in a form known
as sulphate (SO 4 ) or in the molecule hydrogen
sulphide (H 2 S), known for its distinctive
smell of rotten eggs. Certain organisms have
already evolved to take advantage of sulphur’s“ We suspect life might have some new
tricks we haven’t really seen before”
abundance. In the same way that we breathe
oxygen, there are certain microbes that rely
on sulphate, says Daniel Jones, academic
director of the National Cave and Karst
Research Institute in New Mexico.
Some of the earliest identifiable life on
Earth probably got its energy from sulphur.
Organisms that reduced and oxidised
elemental sulphur into hydrogen sulphide
and sulphate have been traced back as far as
3.5 billion years ago, to the very beginning
of fossil records.
So what about doing it the other way
around? Just as photosynthesis in effect
reverses the chemistry of aerobic respiration,
does anything make a living by turning
hydrogen sulphide and sulphate back into
pure sulphur? No such organisms, known
as sulphur comproportionators, have ever
been found. But in theory, at least, the answer
is yes. “It’s likely that in 3 billion plus years
of evolution and the right pressures, some
species have developed the ability to do this,”
says Macalady.
They would need very specific conditions
to pull it off: an extremely acidic environment
high in sulphide and sulphate ions, and with
a temperature somewhere between 0°C and
25°C. This is why Macalady and Aronson
have come to Frasassi. Its chilly interior and
distinctive smell make it an ideal place to go
hunting for new sulphur-based life forms.
Thanks to the rope-rigging skills of two
Italian cavers, we abseil down, slippery with
mud, into a long cavern. The space adjoins
a grey-black pool of water and the walls are
covered with slimy, worm-like patterns called
biovermiculations, created by slow-growing
microbes. On the day I join them, Aronson’s
mission is to collect clear, teardrop-shaped
secretions that hang from the walls and
ceilings of the cave. Geologists call these
snottites, and their resemblance to the
dripping tip of a runny nose is uncanny.
Because the snottites are full of bacteria
and extremely acidic, Aronson hopes they
will contain the sulphur producers.
She has good reason for optimism. In the
15 years or so that Macalady and Jones have
been coming to these caves, they have found
that Frasassi snottites contain multiple strains
of bacteria that we have yet to grow in the lab.All known life relies on water to
survive. There are a number of
factors that make it so vital, but
chief among them is its power
as a solvent. When organisms
consume nutrients, or ferry
them within their bodies, those
processes are made easier if
the nutrients are in liquid form.
So many different chemicals
dissolve in water that it has
become central to all biology
on Earth, feeding and powering
organisms from bacteria to
blue whales.
“Water is a great solvent, but
it’s conceivable, on some weird
moon of some weird planet in
some weird solar system, that
ammonia perhaps could be
a substitute for water,” says
Roger Buick, an astrobiologist
at the University of
Washington, Seattle. But, he
cautions, “it wouldn’t work
terribly well”. Like water,
ammonia is abundant in the
universe, and it shares many
of the chemical properties that
make water a good solvent.
At the same time, it lacks many
of water’s additional life-giving
features. “It’d be a pretty junky
experiment in life,” says Buick.
Methane’s ability to dissolve
other substances has also
seen it floated as a potential
substitute for water. Large
lakes of liquid methane can
be found on Titan, the largest
of Saturn’s moons, which
has made this strange world
a priority for alien hunters.
But thus far, the search for life
has largely concentrated on
planetary bodies with evidence
of water, such as Mars, another
of Saturn’s moons, Enceladus,
and Europa, the smallest of
Jupiter’s main moons.Does life
need water?