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Poison eaters: alternative life forms
Julian Calverley/Corbis
THIS WEEK
The poison eaters Things deadly to us may sustain weird organisms in harsh environments on Earth, or even on alien planets
David Shiga
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. “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, 8 | NewScientist | 11 December 2010
even replacing the phosphate to knock electrons from chloride – backbone of DNA with one based producing chlorine gas as a waste on arsenic (Science, DOI: 10.1126/ product – it would be a more Science.1197258). The bugs could vigorous form of photosynthesis represent part of a “shadow than the one we are used to, biosphere” – a parallel form whereby light splits water of life on Earth with a different molecules into oxygen, hydrogen biochemistry to all others. ions and electrons. Whether or not the existence of Haas has proposed an “arsenic bacteria” is confirmed – several alternative forms of and some scientists are not photosynthesis, but in all of them convinced by the claim (see the energy of sunlight is used to “Arsenic life”, right) – the liberate electrons from chloride publication of the paper has rather than water (see diagram). reinvigorated interest in In both water-splitting and alternatives to our kind of life. chloride-splitting photosynthesis, An equally outlandish life electrons are used to power the form has now been suggested “The bugs could represent by Johnson Haas at Western Michigan University in Kalamazoo. part of a ‘shadow biosphere’ – a parallel Haas calculated that if an alien form of life on Earth” microbe or plant used sunlight
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 watersplitting one, potentially making chloride-based photosynthesis more profitable (Astrobiology, DOI: 10.1089/ast.2009.0364 ). Dirk Schulze-Makuch, 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. Robert Blankenship of Washington University in
In this section n Quantum tool hones computers’ language skills, page 10 n Erasing memories of war, page 12 n Birth of an alternative internet, page 23
Alternative photosynthesis Sugar production could work using chlorine as a source of electrons instead of water – in theory
WATER-BASED PHOTOSYNTHESIS LIGHT
CHLORINE-BASED PHOTOSYNTHESIS CARBON DIOXIDE AND HYDROGEN
LIGHT
OXYGEN GAS (WASTE)
CARBON DIOXIDE AND HYDROGEN
CHLORINE GAS (WASTE)
WATER
CHLORIDE ELECTRONS
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 chlorinebased 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 chlorinebased photosynthesis, he says he does not think it could sustain –Does Mono Lake hold a secret?– a biosphere. Haas counters that
SUGAR
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 lowoxygen environments to survive.
“Chlorine is linked to bad outcomes, such as rapid death, but that’s because we are not adapted to it” 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
Arsenic life, taken with a pinch of salt Felisa Wolfe-Simon of NASA’s Astrobiology Institute took arsenicrich 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 New Scientist, however, were far from convinced. “I doubt these results,” says Steven Benner, a chemist at the Foundation for Applied Molecular Evolution 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 arseniccontaining 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. Rosie Redfield 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. Olivier Dessibourg
ELECTRONS
SUGAR
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.” 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.” n 11 December 2010 | NewScientist | 9
THIS WEEK
The poison eaters Things deadly to us may sustain weird organisms in harsh environments on Earth, or even on alien planets
David Shiga
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. “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, 8 | NewScientist | 11 December 2010
even replacing the phosphate to knock electrons from chloride – backbone of DNA with one based producing chlorine gas as a waste on arsenic (Science, DOI: 10.1126/ product – it would be a more Science.1197258). The bugs could vigorous form of photosynthesis represent part of a “shadow than the one we are used to, biosphere” – a parallel form whereby light splits water of life on Earth with a different molecules into oxygen, hydrogen biochemistry to all others. ions and electrons. Whether or not the existence of Haas has proposed an “arsenic bacteria” is confirmed – several alternative forms of and some scientists are not photosynthesis, but in all of them convinced by the claim (see the energy of sunlight is used to “Arsenic life”, right) – the liberate electrons from chloride publication of the paper has rather than water (see diagram). reinvigorated interest in In both water-splitting and alternatives to our kind of life. chloride-splitting photosynthesis, An equally outlandish life electrons are used to power the form has now been suggested “The bugs could represent by Johnson Haas at Western Michigan University in Kalamazoo. part of a ‘shadow biosphere’ – a parallel Haas calculated that if an alien form of life on Earth” microbe or plant used sunlight
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 watersplitting one, potentially making chloride-based photosynthesis more profitable (Astrobiology, DOI: 10.1089/ast.2009.0364 ). Dirk Schulze-Makuch, 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. Robert Blankenship of Washington University in
In this section n Quantum tool hones computers’ language skills, page 10 n Erasing memories of war, page 12 n Birth of an alternative internet, page 23
Alternative photosynthesis Sugar production could work using chlorine as a source of electrons instead of water – in theory
WATER-BASED PHOTOSYNTHESIS LIGHT
CHLORINE-BASED PHOTOSYNTHESIS CARBON DIOXIDE AND HYDROGEN
LIGHT
OXYGEN GAS (WASTE)
CARBON DIOXIDE AND HYDROGEN
CHLORINE GAS (WASTE)
WATER
CHLORIDE ELECTRONS
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 chlorinebased 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 chlorinebased photosynthesis, he says he does not think it could sustain –Does Mono Lake hold a secret?– a biosphere. Haas counters that
SUGAR
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 lowoxygen environments to survive.
“Chlorine is linked to bad outcomes, such as rapid death, but that’s because we are not adapted to it” 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
Arsenic life, taken with a pinch of salt Felisa Wolfe-Simon of NASA’s Astrobiology Institute took arsenicrich 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 New Scientist, however, were far from convinced. “I doubt these results,” says Steven Benner, a chemist at the Foundation for Applied Molecular Evolution 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 arseniccontaining 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. Rosie Redfield 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. Olivier Dessibourg
ELECTRONS
SUGAR
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.” 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.” n 11 December 2010 | NewScientist | 9