scanning electron micrograph image of bacteria GFAJ-1, which can grow on the legendary poison arsenic

Meet GFAJ-1, an otherworldly microbe that can grow on a legendary poison.
U.S. scientists are reporting the microbe, plucked from a California lake, can use arsenic to make proteins and other key molecules - including DNA.
They say the arsenic operates as a substitute for phosphorus, long considered one of six essential elements of life.
The find, which may have "profound evolutionary and geochemical significance," according to a report in the journal Science on Thursday, has the astrobiology world buzzing.
Bloggers had been speculating, based on NASA's plan to hold a news conference to coincide with the publication of the study, that scientists had found extraterrestrial life.
In reality, they've discovered a very strange form of life, right here on Earth.
"It's pretty exciting," says Lyle Whyte, of McGill University in Montreal. Whyte, who studies microbes living in sub-zero habitats in the High Arctic, said the discovery gives a glimpse of how odd extraterrestrial life might prove to be.
It also shows life can be much weirder than expected on this planet.
"If something here on Earth can do something so unexpected, what else can life do that we haven't seen yet," says Felisa Wolfe-Simon, lead author of the Science report and a researcher with the U.S. Geological Survey.
Wolfe-Simon is known for thinking outside the box and has speculated for years that some forms of life might be able to use arsenic instead of phosphorus.
To test the idea, she and her colleagues collected microbes from sediments in Mono Lake, located in the California desert, known for its high arsenic levels.
Wolfe-Simon took them back to the lab and put them on an arsenic diet. Over the next few weeks, microbes grew in the poisonous mixture and could be seen swimming around.
The scientists probed the tiny bacteria - dubbed GFAJ-1 - and found arsenic inside its cells. They then added a radioactively labelled version of arsenic, which revealed the arsenic was incorporated into the bacteria's proteins, fats and DNA. They also isolated some of the DNA, and report it contained arsenic.
The scientists say their work shows the bacteria "can grow by using arsenic instead of phosphorus," which is considered pretty profound.
"There are no prior reports of substitutions for any of the six major elements essential for life," the team reports.
Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur have until now been considered the key building blocks of DNA, proteins and fats.
The reason arsenic works is that it is chemically very similar to phosphorus. That is also why arsenic is so toxic - because cells attempt to use arsenic instead of phosphorus, gumming up the biochemical machinery in humans and animals, the researchers say.
GFAJ-1 is more adaptable, but Wolfe-Simon reports that bacteria do grow better when supplied with phosphorus.
But she and her colleagues say their experiments prove phosphorus-free life forms can exist, and suggest such organisms may have existed in hydrothermal vents when life first took hold on Earth billions of years ago.
Milva Pepi, at Italy's University of Siena, describes the results as "convincing and exhausting." Others are a bit more circumspect.
"It looks like there are a few holes to fill," says Whyte, who would like to see more conclusive proof the arsenic has replaced phosphorus in the microbe's DNA.
"But overall it's a game changer."
Like California's arsenic-laden Mono Lake, the Arctic is considered a proxy for the harsh conditions extraterrestrial life might be living in on Mars and elsewhere in the solar system.
NASA, which helped fund the research as part of its astrobiology program, says GFAJ-1 "has changed the fundamental knowledge about what comprises all known life on Earth."
"The definition of life has just expanded," Ed Weiler, NASA's associate administrator, says in a statement.
"As we pursue our efforts to seek signs of life in the solar system, we have to think more broadly, more diversely and consider life as we do not know it."