While scientists find ever more planets around other stars and contemplate missions to probe the far reaches of our own solar system, researchers are looking to the extremes of the Earth for clues about what kind of organisms could exist in the brutal conditions elsewhere.

There’s hardly a niche on Earth that hasn’t been colonised. Life can be found in scalding, acidic hot pools, in the driest deserts, and in the dark, crushing depths of the ocean. The very existence of these hardy organisms hints that life might be able to eke out an existence in the cold, dry climate of Mars, the icy, acidic conditions of Jupiter’s moon Europa, or in countless other spots beyond our solar system.

Somelike it hot

Steaming hot pools and scalding undersea hydrothermal vents provide a cosy habitat for heat-loving extremists. Such ‘thermophiles’ produce enzymes that are stable at high temperatures. Some have been isolated and put to work in everything from laundry detergents to food production.

The upper limit for life had been widely recognised as 113 °Celsius, thanks to a microbe called Pyrolobus fumari that was discovered in 1997 inside a single hydrothermal vent in the Atlantic Ocean, 3650 metres below the surface. However, a microbe collected from a vent in what’s known as the Faulty Towers neighbourhood, 2400 metres down in the Pacific Ocean, has upped the ante.

Cold comfort

The most frigid polar regions and the darkest depths of the ocean are home for a few organisms that like a good chill.

Many are bacteria or similar single-celled organisms called Archaea, but some lichens called cryptoendoliths go to extremes by colonising pores in Antarctic rock. There’s also an alga that creates reddish ‘watermelon snow’ – a phenomenon first described by Aristotle.

Cold-loving organisms, called psychrophiles, have specialised cell membranes that don’t stiffen in frigid temperatures, and many produce a kind of protein antifreeze.

Salt of the Earth

Despite its name, the Dead Sea does harbour life. It’s the saltiest body of water on Earth, but a few microbes thrive there, in water eight times saltier than the ocean. Scientists studying one of them, Haloarcula marismortui, discovered that it has specialised proteins that protect it from the effects of salt.

Scientists have theorised that any microbes living on Mars would have to be something like terrestrial halophiles in order to cope with the planet’s high salinity.

The acid test

Acidic hot springs and geysers that would eat away at human flesh are no match for some organisms that also make themselves at home in the acid runoff from mines.

The most extreme acidophiles known are microbes of the genus Picrophilus. They thrive at a pH of 0.7, and can grow down to a drain-clearing pH of 0. Both Mars, Europa and the clouds of Venus are thought to be acidic environments, so Earthly acidophiles intrigue scientists looking for life elsewhere.

Deep comfort

Recently, living cells – many of which are Archaea from the Pyrococcus andThermococcus genera – were found in a mud core taken from 1.6 km below the sea floor off the coast of Newfoundland. Though they represent the deepest life ever discovered beneath the sea floor, microbes of various kinds have been discovered at even greater depths under the continents. Communities of microorganisms have been found hunkered down in groundwater as far as 5 km below the surface of the land. Scientists think life exists even further down – to the point where the subsurface heat becomes unbearable for life.

Even the deepest part of the ocean, the Mariana Trench, which plunges 11 km below the surface of the Pacific Ocean near Guam, is inhabited. Globs of mud pulled from the trench have yielded an assortment of bacteria, fungi and primitive single-celled organisms. Video taken by an unmanned submersible that explored the area captured a sea cucumber, a scale worm and a shrimp in the trench, where the pressure would crush a human.

All dried up

In the most parched place on Earth, the Atacama Desert, which stretches nearly 1,000 km across South America, it rains only a few times a century. It’s no coincidence that the desert has been used by filmmakers as a stand-in for Mars, and by NASA to test instruments bound for the cold, dry planet.

Microbes like the bacterium Chroococcidiopsis have been found in the Atacama. But in its arid core, finally we think we’ve found an environment where it’s too dry for life.

Water is thought to be crucial for life because it provides a medium for nutrients to diffuse into cells and wastes to drift out, and a solvent for critical metabolic reactions.

Ageing well

Microbes can survive for many, many millennia, though scientists are still debating how long.

In 2007, active bacteria were isolated from permafrost samples drilled in Siberia, northwestern Canada, and Antarctica that were estimated to be up to half a million years old.

There are reports of microbes surviving for much longer in a kind of suspended animation. In 1995, researchers said they’d revived a bacterium taken from the gut of a bee preserved in amber for at least 25 million years.

In 2000, scientists made an even more astonishing claim – that they had brought to life a 250 million-year-ol bacterium dubbed Bacillus permians. According to the team, bacterial spores in a drop of water became trapped in a cavity inside salt crystals as they formed 250 million years ago. But other scientists countered that water often moves through salt deposits, potentially contaminating the ancient salt crystals with younger microbes.

Those reports remain controversial, but nevertheless, the ever growing list of long-lived microbes gives scientists hope that life may exist elsewhere in the solar system.

• Source: New Scientist, adapted by Crreate&Share