Is there alien life on earth?

The variety of life forms on earth is so large that some of them seem alien from the human perspective. In part this is because some of them may indeed be capable to live outside earth, since they thrive in environments hostile to other life, may they be extremely hot, cold, salt-rich or toxic. A great portion of these extremophiles are bacteria and in particular Archaea. Their habitats comprise deep water brine pools (regions of high salt and pressure), hot springs, ice, the cooling water of nuclear power plants or even the inside of rocks, just to name a few.[1]

Coming from this perspective it seemed almost reasonable that a group of microbiologists discovered a bacterium thriving in Mono Lake in California in 2010.[2] The lake offers a comfortable alkaline pH of about 10, high salt conditions of 1 M NaCl and as a kicker an arsenic concentration of about 200 µM. Intrigued by their discovery Wolfe-Simon and coworkers set out to unravel the survival strategy of the robust organism which they named GFAJ-1. While failing to explain how the bacterium detoxifies the arsenic they claimed that GFAJ-1 could grow in medium containing arsenic instead of phosphorous. The scientific community receipted the data mainly as uncertain, because all forms of life known so far were at least dependent on carbon, hydrogen, nitrogen, oxygen, sulfur and also phosphorous.[3] Omitting phosphorous would imply a life form who’s DNA, RNA and membrane-lipids consist of arsenate instead. Shaking the foundations of biology this discovery also had a tremendous impact on popular media.[4]

In 2012 the GFAJ-1-debate was fueled by Xu and coworkers, who investigated phosphate-arsenate substitutions in nucleotides and considered the interplay of DNA/RNA and proteins as too well optimized for phosphorus to work with arsenate.[5] A possible intracellular phosphate supply for GFAJ-1 was found by Basturea and coworkers, who exposed an E. Coli strain to an arsenate-rich phosphate-poor environment and observed ribosome breakdown, which provided for phosphate.[6] 18 months after the initial publication two groups independently disagreed with the findings of Wolfe-Simon and coworkers by claiming that GFAJ-1 does not integrate arsenate into its DNA.[7] The sophisticated strategy GFAJ-1 might use to extract phosphate out of an arsenate-rich environment could also be revealed recently.[8] This strategy may be valid since the phosphate concentration in Mono Lake is still several times higher than the one of arsenate.[9] However Wolfe-Simon still claims that her data may be right, since the bacterium might still incorporate traces of arsenate.[9] Herewith the question remains open: Is there alien life on earth, life that works sufficiently different from the one we know, that it could also do so off-earth? May it be that we are just too narrow-minded to notice it?

Felix Spenkuch

[1] R. Cavicchioli, Astrobiology 2002, 2, 281-292.

[2] F. Wolfe-Simon, J. Switzer Blum, T. R. Kulp, G. W. Gordon, S. E. Hoeft, J. Pett-Ridge, J. F. Stolz, S. M. Webb, P. K. Weber, P. C. Davies, A. D. Anbar, R. S. Oremland, Science 2011, 332, 1163-1166.

[3] a) S. Oehler, Science 2011, 332, 1149; b) P. L. Foster, Science 2011, 332, 1149; c) J. B. Cotner, E. K. Hall, Science 2011, 332, 1149; d) D. W. Borhani, Science 2011, 332, 1149; e) B. Schoepp-Cothenet, W. Nitschke, L. M. Barge, A. Ponce, M. J. Russell, A. I. Tsapin, Science 2011, 332, 1149; f) I. Csabai, E. Szathmary, Science 2011, 332, 1149; g) E. Pennisi, Science 2011, 332, 1136-1137; hS. A. Benner, Science 2011, 332, 1149; i) R. J. Redfield, Science 2011, 332, 1149.

[4] (last access 14.10.2012)

[5] Y. Xu, B. Ma, R. Nussinov, J. Phys. Chem. B 2012, 116, 4801–4811.

[6] G. N. Basturea, T. K. Harris, M. P. Deutscher, J. Biol. Chem. 2012, 287, 28816-28819.

[7] a) T. J. Erb, P. Kiefer, B. Hattendorf, D. Gunther, J. A. Vorholt, Science 2012, 337, 467-470; b) M. L. Reaves, S. Sinha, J. D. Rabinowitz, L. Kruglyak, R. J. Redfield, Science 2012, 337, 470-473.

[8] M. Elias, A. Wellner, K. Goldin-Azulay, E. Chabriere, J. A. Vorholt, T. J. Erb, D. S. Tawfik, Nature 2012 in print.

[9] Q. Schiermeier, Nature 2012, doi:10.1038/nature.2012.10971, (last access 14.10.2012)