Arsenic and phosphorus in bugs and superconductors
DOI: 10.1063/PT.5.010056
On Mondays I usually visit Science‘s press site, which contains links to papers that will appear in the journal four days later alongside enticing summaries of the most newsworthy papers. This Monday the paper listed first bore the title “A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus.” Its summary read:
Living off Toxic Waste -- Bacteria That Munch on Arsenic:
Can you imagine eating toxic waste for breakfast? Researchers have discovered a bacterium that can live and grow entirely off arsenic, reports a new study. The findings point for the first time to a microorganism that is able to use a toxic chemical (rather than the usual phosphate) to sustain growth and life. Arsenic is normally highly toxic to living organisms because it disrupts metabolic pathways, but chemically it behaves in a similar way to phosphate. Scientists have previously found organisms that can chemically alter arsenic; and these organisms have been implicated in ground water poisoning events in Bangladesh and other places in Asia when people have shifted to using borehole or well water to avoid cholera. Now, Felisa Wolfe-Simon and colleagues have found a bacterium able to completely swap arsenic for phosphorus to the extent that it can even incorporate arsenic into its DNA. The salt-loving bacteria, a member Halomonadaceae family of proteobacteria, came from the toxic and briny Mono Lake in California. In the lab, the researchers grew the bacteria in Petri dishes in which phosphate salt was gradually replaced by arsenic, until the bacteria could grow without needing phosphate, an essential building block for various macromolecules present in all cells, including nucleic acids, lipids and proteins. Using radio-tracers, the team closely followed the path of arsenic in the bacteria; from the chemical’s uptake to its incorporation into various cellular components. Arsenic had completely replaced phosphate in the molecules of the bacteria, right down its DNA.
I quote the summary in full so that you can remark for yourselves the utter absence of any hint of extraterrestrial life. On the other hand, the press release I received from NASA on Monday mentioned only the paper’s implications for extraterrestrial life:
NASA SETS NEWS CONFERENCE ON ASTROBIOLOGY DISCOVERY; SCIENCE JOURNAL HAS EMBARGOED DETAILS UNTIL 2 P.M. EST ON DEC. 2
WASHINGTON -- NASA will hold a news conference at 2 p.m. EST on Thursday, Dec. 2, to discuss an astrobiology finding that will impact the search for evidence of extraterrestrial life. Astrobiology is the study of the origin, evolution, distribution and future of life in the universe.
So if you awoke to news of poison-eating space aliens in California, credit NASA not Science. But how plausible is the link between the discovery of bacteria that can swap arsenic for phosphorus and extraterrestrial life?
Phosphorus is the ninth most abundant element in living organisms. Its compounds are found in teeth, bones, cell membranes, and a host of important biomolecules, including cells’ main source of chemical fuel, adenosine triphosphate. Phosphate groups also hold together the nucleotides in RNA and DNA.
Because arsenic belongs to the same group in the periodic table as phosphorus, it can readily replace phosphorus in biomolecules. But the arsenated compounds don’t work—hence the element’s toxicity. The newly discovered bacteria are remarkable in that they apparently possess a chemical means of mitigating the toxicity.
But before you start scanning the skies for arsenic-laced M-class planets , keep in mind that arsenic is cosmically rarer than its group V neighbor phosphorus. In Earth’s crust arsenic occurs at a concentration of about 1.5 parts per million. Phosphorus is 1000 times more abundant.
Life as we know it on Earth originated just once, a reflection of its low probability of getting started. I’m therefore skeptical that life forms based on a rare element such as arsenic evolved elsewhere.
I can’t resist ending this blog entry by pointing out another scientific substitution of arsenic for phosphorus. Three years ago in his quest to find semiconductors with interesting magnetic properties, Hideo Hosono and his team from the Tokyo Institute of Technology synthesized a compound with the chemical formula LaOFeP.
The material becomes superconducting at the unremarkably low temperature of 4 K. But the arsenic-substituted compound, when doped with fluorine, superconducts at 26 K, which is uncomfortably high for a normal superconductor. Hosono had discovered a new and exciting class of superconductor .
Despite spawning thousands of papers from excited physicists and chemists around the world, Hosono’s discovery barely registered in the mainstream media.
