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While arsenic may have killed most of the guests at this cocktail party, some organisms actually require it to live. Researchers working in the incredibly salty Mono Lake in California have found bacteria that use arsenic in place of phosphorous, an essential compound in cell metabolic processes, questioning our understanding of what it means to be alive. Currently, our definition of life is largely based on the elements we know are necessary to it: carbon, hydrogen, oxygen, nitrogen, and phosphorous. Arsenic (As) is an element in the same group as phosphorus (P) and therefore shares similar chemical properties.

from the Washington Post | SOURCE: Prof. Ariel Anbar of Arizona State University. GRAPHIC: Bonnie Berkowitz and Alberto Cuadra / The Washington Post

In a paper in the International Journal of Astrobiology, the researchers, Felisa Wolfe-Simon, Paul Davies, and Ariel Anbar, explain that arsenic can be negatively charged over a range of physiological pH conditions and that “known life cannot easily distinguish” inorganic arsenic from inorganic phosphorus. They go on to discount certain objections against life favoring arsenic. For example, despite the instability of arsenic-based compounds in water, life could still exist given arsenic-rich conditions.

Their vision of life’s biochemical evolutionary path goes like this:

Once life ventured forth from its As-rich primordial home, the balance of advantage probably tipped in favour of P because P is typically 10,000 times more abundant than As at the Earth’s surface. However, As-based life could even survive today in restricted pockets where As is present in abundance, such as deep sea hydrothermal systems or seasonally relevant episodes at Mono Lake.

Such radically biochemically different life suggests the existence of a “shadow biosphere,” and the bacteria from Mono Lake represent “the tip of the iceberg,” according to Paul Davies.

Wolfe-Simon F, Davies PCW, Anbar AD. Did nature also choose arsenic? Int J Astro 2009;8(2):69-64.