Editorial: Curb your enthusiasm for aliens, NASA
Two chemicals that swiftly kill most living things may sustain weird organisms in harsh environments on Earth, or even on alien planets
THE outer limits of life just got stranger. Two chemicals that swiftly kill most living things may be harmless or even helpful to some unusual forms of life, suggesting that there are more ways of sustaining life than we realised.
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“Life as we know it could be much more flexible than we generally assume or can imagine,” says Felisa Wolfe-Simon of NASA’s Astrobiology Institute and the US Geological Survey in Menlo Park, California. Her team grew bacteria that are apparently able to substitute the deadly poison arsenic for phosphorus, one of the six chemical elements thought to be essential for life, even replacing the phosphate backbone of DNA with one based on arsenic (Science, ). The bugs could represent part of a “shadow biosphere” – a parallel form of life on Earth with a different biochemistry to all others.
“The bugs could represent part of a ‘shadow biosphere’ – a parallel form of life on Earth”
Whether or not the existence of an “arsenic bacteria” is confirmed – and some scientists are not convinced by the claim (see “Arsenic life”) – the publication of the paper has reinvigorated interest in alternatives to our kind of life.
An equally outlandish life form has now been suggested by at Western Michigan University in Kalamazoo. Haas calculated that if an alien microbe or plant used sunlight to knock electrons from chloride – producing chlorine gas as a waste product – it would be a more vigorous form of photosynthesis than the one we are used to, whereby light splits water molecules into oxygen, hydrogen ions and electrons.
Haas has proposed several alternative forms of photosynthesis, but in all of them the energy of sunlight is used to liberate electrons from chloride rather than water (see diagram). In both water-splitting and chloride-splitting photosynthesis, electrons are used to power the construction of sugars. Most life on Earth relies on photosynthesis either directly or indirectly for energy, but Haas says his reaction could provide more energy for sugar-making than the water-splitting one, potentially making chloride-based photosynthesis more profitable (Astrobiology, ).
, an astrobiologist at Washington State University in Pullman, says he finds the alternative photosynthesis idea fascinating and says it may well be realised on some life-hosting alien planets. “This is the type of research that really propels astrobiology,” he says.
of Washington University in St Louis, Missouri, who studies the origin and evolution of photosynthesis on Earth, is more sceptical. Although he is not prepared to rule it out chlorine-based photosynthesis from an energy point of view, he says its waste products, which include chlorine gas, are “incredibly corrosive and toxic to all forms of life as we know it”.
Chlorine gas was used as a weapon in the first world war, he points out. Because of the potentially deadly effects of the waste products of chlorine-based photosynthesis, he says he does not think it could sustain a biosphere. Haas counters that oxygen is a very aggressively reactive chemical too, and its rise in Earth’s atmosphere billions of years ago forced some microbes to flee into mud and other low-oxygen environments to survive. Other organisms, including our own ancestors, developed antioxidant compounds to protect against the damage and evolved to use oxygen to burn food for energy. Aliens might even breathe chlorine as we breathe oxygen, he says.
William Bains, CEO of biotech company Delta G, based in Cambridge, UK, who has also published papers on astrobiology, agrees. “Chlorine is associated with bad outcomes, but that is because we are not adapted to such environments,” he says. “For some terrestrial organisms today, oxygen is rapidly lethal, but humans survive OK in it.”
“Chlorine is linked to bad outcomes, such as rapid death, but that’s because we are not adapted to it”
Since chlorine is an efficient trapper of infrared radiation, it would act as a greenhouse gas. That could keep extrasolar planets warm enough for liquid water even if they were relatively far from their parent stars, where chlorine-deprived planets like ours would be frozen over, says Haas.
The absorption of infrared light by such an exoplanet’s atmosphere could be detected in its light spectrum. Though astronomers usually talk about looking for oxygen as a sign of life, they should also keep chlorine in mind, Haas says.
There may well be other important ways that alien biology could differ from ours, “things that didn’t happen to evolve here but are perfectly feasible”, says Haas. “We only have one data point for a biosphere and what kinds of biochemistry it has. We don’t really know how different it could be. We’re not sure what we’re going to find out there, and we need to be prepared for a great deal of variety.”
Arsenic life, taken with a pinch of salt
Felisa Wolfe-Simon of NASA’s Astrobiology Institute took arsenic-rich mud containing bacteria from Mono Lake in California (pictured) and grew them in ever-decreasing concentrations of phosphorus.
All known life is built around carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur – known as CHNOPS – which make up proteins, lipids and DNA. Wolfe-Simon’s rationale was that since arsenic is just below phosphorus in the periodic table, and shares many of its chemical properties – and is even used as a source of energy for some bacteria – the bugs would be able to swap one for the other. That, they report, is what happened, with arsenic replacing phosphorus even in the backbone of the DNA double helix itself. Some scientists who spoke to 91av, however, were far from convinced.
“I doubt these results,” says Steven Benner, a chemist at the in Gainesville, Florida. In order to measure the apparently modified DNA, it has to be put into a water-containing gel, which would rapidly dissolve any arsenic-containing chunks of DNA, but not those containing phosphorus. Since they found large chunks of DNA, it must contain phosphorus, not arsenic, Benner argues. “It remains to be established that this bacterium uses arsenic as a replacement for phosphorus in its DNA” or in any other biomolecule found in “standard” Earthly biology, he says.
at the University of British Columbia in Vancouver, Canada, says the paper does not present any convincing evidence that arsenic has been incorporated into bacterial DNA, calling the molecular biology methods used by Wolfe-Simon’s team “crude”.
“I’m not surprised by NASA’s publicity juggernaut, but I’m very disappointed that these scientists did not bring higher standards to their work, and that Science thought it fit to publish,” she says.
