The arsenic-eaters: An alternate biology?

Are these little bugs changing the very definition of life - or just expanding our understanding of it?

Posted: December 06, 2010

Do arsenic-tolerant bacteria redefine life, as some recent headlines pronounced? And why are people saying the finding increases the odds of finding aliens? After a much-ballyhooed NASA news conference Thursday, a number of biologists were also scratching their heads.

What, exactly, did NASA discover?

NASA fellow Felisa Wolfe-Simon and colleagues found some microbes living at the bottom of California's Mono Lake, which has a high level of arsenic. The scientists took the microbes out of the lake and infused them with more arsenic.

Then they claimed that the microbes were incorporating the arsenic into the structure of their DNA and other cellular machinery.

Is this the first example of non-carbon-based life?

No. Carbon is still in the leading role, along with oxygen, hydrogen, and nitrogen. The claim here is that one of the important supporting actors - phosphorus - has been taken out and replaced with arsenic, an unusual agent to be sure.

In all life as we know it, phosphorus atoms make up important working parts of cells, including the so-called backbone of DNA - the part that holds the genetic code together. The NASA scientists say these bugs are using DNA with an arsenic backbone instead of a phosphorus one.

Do these bugs use arsenic in their natural state?

That's not clear yet. They were able to survive in a lake that had lots of arsenic, but the scientists added more arsenic in the lab. When the arsenic is removed, the bugs go back to using phosphorus, which they seem to prefer.

Why did they choose arsenic? Why not something more benign?

Arsenic is the element of choice here because it's like an oversized version of phosphorus. Arsenic and phosphorus form similar bonds with oxygen and other atoms, leading to molecules or parts of molecules with roughly the same shape.

In the periodic table, arsenic is right below phosphorus, and this is important because elements in the same vertical column of the periodic table share bonding similarities.

Silicon, for example, is right below carbon, which is why science fiction writers are fond of speculating about silicon-based life.

If you think of these atoms as Tinkertoy connectors or Legos, arsenic and phosphorus would make the same connections. But arsenic is bigger and often acts differently.

That's what makes it such a deadly poison to most organisms - it fits in the spaces meant for phosphorus, but once there it alters the way the cellular machinery works.

Do scientists assume life must be based on the usual six elements: carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur? Why not life based on lead or molybdenum or plutonium?

Chemists say there's a reason life loves carbon. If carbon were a person, it would have the most friends on Facebook. Carbon is better than other elements at forming strong bonds with other carbons, linking them in chains, and adding oxygen, hydrogen, and other atoms around the edges.

"The thing about carbon is it so nicely forms long chains and large molecules that lead to wonderful things like proteins and membranes and DNA," said Everett Shock, a scientist at Arizona State University.

If the rules of chemistry and physics work the same way elsewhere in the universe, then carbon will act the same way and form the same kinds of molecules.

The other factor is abundance. Life here on Earth builds itself out of some of the six most common elements.

According to the scientists, there's a lot more phosphorus available in the galaxy than arsenic, so life elsewhere would probably use phosphorus.

Does this finding redefine life? What is the definition of life anyway?

Defining life has been a long-standing problem for biologists. Life is tough to define because life on Earth all runs on the same parts. It's like trying to define animals if we were the only animal on the planet. Would we think all animals needed hair? Or should walk on two legs?

In searching for life elsewhere, scientists have tried to be open-minded by allowing that life might not be made of the same stuff as here. On the other hand, if biologists started getting all new-age and declaring rocks and snowballs and everything else a possible life form, then they could say life was everywhere. But it wouldn't mean much.

So scientists today think of life as something that can access energy from the environment, replicate itself, pass information to the next generation, and evolve, said ASU's Shock. "We know life is defined in what it does and what it is capable of doing and not what it's made out of."

How excited are other scientists about the arsenic bugs?

Some say they would be excited if, and this is a big if, the NASA group can prove they've completely substituted arsenic for phosphorus in the working parts of the cell, including the DNA.

Penn biologist Mark Goulian said it would be surprising if the bugs could function the same way with arsenic because it forms weaker bonds with itself. But in any case, he said, this bacteria pushes the boundaries. "I would say it's demonstrating the plasticity of life - the range of environments life can live in."

Brandeis University biologist Greg Petsko said it would take more than just arsenic in the DNA to make this truly revolutionary. The arsenic would also have to replace phosphorus in another molecule that carries the genetic code - the RNA.

"The DNA and RNA need to be examined extremely carefully to see if they really contain only arsenic where the phosphate groups should be. The DNA maybe could - I bet the RNA cannot," he said. "Some really good, careful analysis needs to be done on all the molecules in this organism."

If they do confirm an arsenic-using biochemistry, that would be something new under the sun, said planetary scientist Christopher McKay of NASA-Ames research center.

Even in the most extreme environments on Earth, scientists have found only bacteria that look similar to the ones we find all around us. But this bug is different.

"This is a case where an organism is changing its fundamental biochemistry in order to live in an extreme environment," he said. "It's doing something we didn't think was possible in order to survive."

Why are some scientists talking about a shadow biosphere?

The term shadow biosphere refers to the theoretical prospect of unrelated life forms lurking here on Earth. So far, everywhere we look, life uses the same building blocks and the same genetic code and appears to be related through a common ancestor.

These organisms look to be part of the regular biosphere, so if there are any other biospheres, they remain in the shadows.

What does this have to do with aliens?

For years NASA scientists have been engaged in a field known as astrobiology, the study of life elsewhere in the universe. Since they don't yet have alien life to study, they've kept busy by exploring the limits of life here on Earth.

That's supposed to help guide the search for life elsewhere - pointing to possible habitable zones on planets and moons in our solar system and, more recently, planets orbiting other stars.

Here on our home planet, scientists have found organisms that can survive all sorts of extremes, from boiling hot springs to the frozen tundra. They've found microbes that live more than a mile underground and derive energy from hydrogen or other sources completely off the grid - separate from the food chain that starts with sunlight and supports most life on the planet.

Some extreme microbes survive in stuff that's as caustic as battery acid or bleach, while others can withstand blasts of radiation more than 1,000 times the lethal dose for us humans.

"That's all pretty outrageous," said ASU's Shock. "And we're still looking."


Contact staff writer Faye Flam at 215-854-4977 or fflam@phillynews.com.

See ARSENIC on E3

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