December 02, 2010

CHONPS: carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur. These six elements are the building blocks of all life.

The first four of these elements are each the least massive element in their respective periodic table group. Hydrogen is the simplest, a single electron orbiting a single proton: from it are all the other elements made. Helium comes next but it reacts chemically with nothing else. Fusion can build upon helium but life, never. Among all possible reactive elements born in the stellar fusion crucible: carbon, oxygen, and nitrogen are the easiest for a star to make.

The other two elements are used to create amino acids, the basic building blocks of life at the molecular level. Sulphur appears in cysteine and methionine, two of the essential amino acids we must ingest through our diet to sustain life.

But phosphorus: that is the very spine of all life. Adenine, guanine, cytosine, thymine, zippered together on a backbone of alternating sugars and phosphates: these are the essential components of DNA, upon which all life is based. The phosphate ion is also a key player in both adenosine triphosphate (ATP) and adenosine diphosphate (ADP): without which metabolism could not occur.

No phosphorus, no life.

Or so we thought.

The GFAJ-1 bacterium, a member of the common Halomonadaceae family, seemed to be just another extremophile at first: happy in high-saline, high-alkaline environments which would kill most other organisms. Mono Lake, a volcanic lake located high in the Bodie Hills of California, United States, provides both of those conditions. It also happens to have an unusually high concentration of arsenic.

When phosphorus is lacking, it seems that the GFAJ-1 bacterium is capable of using arsenic instead: in ATP, in glucose, in its proteins and lipids, even in its very DNA.

A successful elemental substitution has never been known before in a living organism. As a member of the nitrogen-phosphorus family in the table of elements, arsenic is chemically similar enough to phosphorus that our bodies often try to substitute arsenic for phosphorus, with disastrous results. This similarity to phosphorus is exactly why arsenic is so very toxic to us. Yet somehow, GFAJ-1 seems to have made it work.

Arsenic is known to be common in hydrothermal vent environments: which the depths of Mono Lake mimic quite well. After water diversions were ended in 1994, the deepest layers of Mono Lake ceased to mix in with shallower layers, and the lake reverted to its natural meromictic state. We have long known of entire ecosystems built upon sulphur, substituting chemical reactions based upon sulphur and the vent's geo-energy for photosynthesis and other photochemical reactions. The previously unsuspected ability of the GFAJ-1 bacterium to also use the arsenic eminating from these vents may thus have cast a light into the dimmest recesses of the very origins of life.

The research is still young. The results obtained by Felisa Wolfe-Simon have yet to be replicated: for it is on such replication that scientific evidence is built. Multiple dilutions built into the methodology make it unlikely that trace phosphorus could have contaminated the lab samples; yet any such contamination might be sufficient to maintain the phosphorus spine of the GFAJ-1 bacterium's DNA, with the arsenates used elsewhere in the bacterium's structure and metabolism. Upon this point much depends: for all life as we know it is based on DNA and RNA, and thus on phosphorus.

If the GFAJ-1 bacterium is indeed capable of replacing one of the core elements in its genetic structure with arsenic, it will force us to re-consider the nature of life and what is needed for life, not only on our own planet but in the entire universe.

To Mark Twain, Mono Lake was a "lifeless, treeless, hideous desert ... the loneliest place on earth". Will the GFAJ-1 bacterium native to Mono Lake turn out to be the loneliest organism in the universe ... or will it feel right at home?

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