Science smells like an egg sandwich that has been forgotten about past its utilization by date and afterward abounded in the mud.

Exactly 300 meters over our heads is a rural Italian scene of moving sunflower fields, Verdicchio wineries, and winding mountain streets. Here in the Frasassi caverns, the air stinks of hydrogen sulfide and the dividers are disgusting with moderate developing microbial stores. My mind continues floating upwards, yet there is no place Aronson, a specialist at the University of Southern California, would prefer to be. Down here, a long way from the light, she is chasing outsiders.

Basically, anyplace you look on Earth, you discover life. It very well may be in destinations commanded by overwhelming metals that are dangerous to people or on the level of the Atacama desert, where soils are so dry they are Mars-like. It very well may be discovered benefiting from atomic waste, just as at the two limits of temperature and pH.

Yet, on the off chance that Aronson is correct, at that point, the Frasassi framework could be creeping with life, not at all like anything we’ve at any point seen: organisms that swallow down sulfur aggravates the manner in which we inhale air. These ultimate proof of a science drastically not quite the same as all other life on Earth.

Such a revelation would have sensational outcomes. A creature fit for producing vitality along these lines would not just shed light on the starting points of life all alone planet, it could likewise allude to the idea of being somewhere else known to mankind. To locate this new living thing, she simply needs to pursue the sulfurous stench.

The assorted variety of life on Earth is astounding. You needn’t bother with a cerebrum to be alive, or a heart, or a spine. You can get by without oxygen, without daylight, even without two cells to rub together. You can live without feeling the impacts of maturing, or in absolute confinement, or through hundreds of years of hibernation. When researchers think of the meaning of what brings home the bacon animal, it appears that something tags along to challenge it.

What is life, at any rate?
Yet, without a persuasive definition, there are a couple of things that most scholars concur with all living things must-have. They should all be able to do giving their hereditary material to another age of life forms through multiplication, just as having the option to develop and shed waste. Be that as it may, none of this would be conceivable without the most key prerequisite of all: that they are equipped for creating vitality from their condition and putting it to utilize. Living beings on Earth do this in three primary manners. “You eat light, you eat natural issue, or you eat rocks,” says Jennifer Macalady, an astrobiologist at Pennsylvania State University and pioneer of the Frasassi undertaking. Plants and different living beings that photosynthesize are controlled by the sun, while others get their vitality from synthetic concoctions put away in other living things or inside the geography of our planet. “Piece by piece, we’ve revealed a wide range of astonishing digestion systems that we didn’t envision,” says Penelope Boston, chief of NASA’s Astrobiology Institute in California.
These diverse metabolisms almost all rely on the same fundamental chemistry to generate energy. In each case, the power comes from the transfer of a single electron from one molecule to another. The molecule that donates the electron is said to be oxidized, while the electron receiver, paradoxically, is said to be reduced. These processes, known as redox reactions, can release energy and bring stability to the system.
One of the factors that may explain the importance of redox reactions to life is their extreme sluggishness. Reactions that take 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 oxidizing glucose to carbon dioxide and reducing oxygen to water. Photosynthesis, on the other hand, sees carbon dioxide reduced into sugars while the pool gets oxidized 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 certain feasible 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 large table of reactions have ever been found. That leaves open the possibility that a massive 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 cataloged. All the rest – known colloquially as dark microbial matter – remain tantalizingly enigmatic. With so many unknown species waiting to be discovered, the chances are that a totally new metabolism could be lurking under our feet.
For Macalady, the best chemical to start with is sulfur. Sometimes it is in its raw elemental form, but it can also be bound with oxygen in a way known as sulfate (SO4) or in the molecule hydrogen sulfide (H2S), known for its distinctive smell of rotten eggs. Individual organisms have already evolved to take advantage of sulfur’s abundance. In the same way that we use oxygen, there are certain microbes that rely on sulfate, 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 sulfur. Organisms that reduced and oxidized elemental sulfur into hydrogen sulfide and sulfate have been traced back as far as 3.6 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 sulfide and sulfate back into pure sulfur? No such organisms, known as sulfur 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 particular conditions to pull it off: an extremely acidic environment high in sulfide and sulfate 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 sulfur-based life forms.
Thanks to the rope-rigging skills of two Italian cavers, we abseil down, slippery with mud, into a long cavern. Space adjoins a grey-black pool of water, and the walls are covered with slimy, worm-like patterns called vermiculations, created by slow-growing microbes. On the day I join them, Aronson’s mission is to collect precise, 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 sulfur producers.
She has a 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. “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 significant 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 sulfate environments there, and associated volcanic emissions containing sulfide, would have provided the optimal conditions for sulfur-producing life, says Macalady.
Europa, the smallest of Jupiter’s moons, is also a candidate. Its ocean probably contains sulfate and possibly sulphuric acid, says Aronson. If we discover sulfide there as well, she says, “it might make sense to begin considering sulfur comproportionation as a potential metabolism.”

Does life need carbon?
The “snottites” of the Frasassi cave system could harbor new life
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 in
its favor, 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 possible that other elements could be substituted – because of their structural similarity. For example, selenium could possibly replace sulfur, 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.

But such organisms may ultimately seem almost reasonable. More extreme redefinitions of life might yet be possible. Some researchers have suggested that any life on other worlds could subsist without the need for water (see “Does life need water?”, far left), or with 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.” Some are more speculative than others. Macalady points to an unusual breed of microbes on Earth, the haloarchaea, which uses 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 doesn’t require the transport of electrons. If an organism were able to extract all of its power 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 the life that has a totally different way of surviving,” says Laurie Barge at NASA’s Jet Propulsion Laboratory in California. “But it’s tough 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 the 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.

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