Arsenic-based life and Astrobiology

It’s been some time since the controvertial announcement that “arsenic-based life” had been discovered on planet Earth.  With time, however, the less-sensational reality of the discovery has been made more clear, and I think it is sensible to review the current state of the research as it relates to the biochemistry of life and the idea of “alternative” biochemistries.

An Imminent Announcement

The recap: Making some serious waves back in November of 2010, NASA released a media advisory stating that a news conference would be held days later that would reveal “an astrobiology finding” that would “impact the search for extraterrestrial life.”

The journal Science strictly embargoed details until the news conference was held.

Of course, the internet went bezerk.  NASA’s announcement, the first of its kind since the announcement of potential bacterial fossils in Mars meteoriete ALS-84001, seemed to hint to many that a rover had finally hit paydirt.  Signs of extraterrestrial biology had finally been found!

However, the rampant speculation that followed only fueled an initial spike of disappointment with the actual announcement: that young biogeochemist and astrobiologist Felisa Wolfe-Simon led a research team that discovered, as was popularly-reported, “arsenic-based” life here on Earth.

Wolfe-Simon’s discovery was  published in the journal Science and was actually met with a fair degree of sensationalism right out of the gate, followed by sharp criticism that survives to this day.

Magnified cells of bacterium GFAJ-1 grown in a medium containing arsenate. (Credit: NASA)

GFAJ-1: The Arsenic Experiment

A critical question of astrobiology is whether or not life is required to take advange of the same chemistry we do, i.e., that our biochemistry is the only biochemistry that works.  If other types of chemistry were available to life, (such as silicon-based life hypothesized on Saturn’s moon Titan,) then this implies that opportunities for life elsewhere in the universe are great in number.

If not, then life may be more rare; waiting for the perfect “goldilocks” conditions before it can arise.

Testing this hypothesis, astrobiology researchers have been pressing for evidence of so-called “shadow biospheres,” or examples of life taking advantage of different or exotic chemistries right under our own noses.  In other words, seeking out environments where life might have evolved out of necessity to take advantage of different, ordinarily toxic chemical elements is one strategy to investigate the question here at home.

With this objective in mind, Wolfe-Simon (and colleagues) proposed that instead of phosphate (PO4), life might find a way to substitute arsenate (AsO4, depicted in the header)  into its DNA.  Specifically, by isolating an extremophile (an exotic bacterium) from the bizarre ecosystem at work in the alakaline, salt-ridden, and arsenic-laden Mono Lake, Wolfe-Simon’s research team claimed success: the identification of an organism that was capable of substituting arsenic for a small percentage of its phosphorus!

Clarifying “Arsenic-based” 

Wolfe-Simon’s findings, which were obtained when the GFAJ-1 bacteria were grown in a culture doped with arsenate, are more accurately described as identifying a potential “arsenic-utilizing” as opposed to “arsenic-based” form of life.  Even so, the results were considered a boon for those proposing widening the technical search for extraterrestrial life.  In this view, should life be utilizing very different biochemistry than what we’re accustomed to, it is possible that the instruments on our rovers, etc., might not even detect it (or recognize what it was that was being detected).

However, the results have been hotly-debated since, and more recently, have been outright cast into doubt when researchers just this year used a separate analytical method and failed to detect arsenic in the GFAJ-1 bacteria.

The Take-Home

The jury is still out considering whether or not we’ve actually detected so-called “alien” biochemistry or hard evidence of a shadow biosphere.  That having been said, the justification and approach is still in my opinion a solid one.

It remains within the realm of possibility that extraterrestrial life (or terrestrial life under extreme conditions) might, due to opportunity or necessity, be chemically different from our own.

Food for thought.