These authors report the potentially astounding discovery that a novel bacterium can substitute arsenate for phosphorous in macromolecules such as nucleic acids and proteins (Wolfe-Simon et al., 2010). The authors used a range of techniques to study elemental composition in the cells, including, radioactive tracer analysis, NanoSIMS, and synchrotron X-ray studies. Basically the authors conclude that they observed intracellular arsenic in protein, metabolite, lipid, and nucleic acid fractions (Wolfe-Simon et al., 2010). As nucleic acids are the building blocks of life, if correct this paper has huge biological, evolutionary, and even philosophical consequences. However, it is not the purpose of this commentary to critically dissect the paper and look at the pros and cons of the publication; others in this issue and elsewhere are surely undertaking this task. Suffice to say many are surely citing Carl Sagan’s famous quote that ‘Extraordinary claims require extraordinary evidence’ (Sagan, 1980). The need for caution will be touched on toward the end of this article, however this commentary will focus on is if the data is correct what does that mean? What are the implications, and moreover what are the interesting follow-up questions?

One of the biggest questions is if indeed the microorganisms are substituting arsenic for phosphorus in major macromolecules, how are they doing it? With all of the myriad of biochemical reactions thus far known to involve phosphorous (many involving ATP and other cascade reactions), the only conclusion can be this microorganism has completely different enzymes and thus biochemical pathways to any known form of life. This suggests different evolutionary pathways to establish these metabolisms, yet sufficient pressure to ensure they were maintained and not lost in this microorganism (and potentially others?). For example, there would need to be an enzyme or enzymes akin to a phosphatase that could add or remove an arsenate moiety where needed, in addition to the already known arsenite oxidase and arsenate reductase (Silver and Phung, 2005). Thus an organism would have to possess a very different biochemical apparatus to facilitate the utilisation of arsenic in the way Wolfe-Simon et al., (2010) have proposed, and elucidating these pathways (if they exist) is critical to lend support to the authors’ conclusions. An obvious path to begin addressing and exploring these questions would be to utilise the recent advances in genomic, proteomic, and transcriptomic technologies (e.g. Rothberg and Leamon, 2008, Petrosino et al., 2009). Once the genome of this novel microorganism is sequenced (assuming it has not already been done), one can start to examine the genome for genes encoding novel proteins involved in arsenic metabolism. At the next level, predicted proteins will then need to be examined and one could employ techniques such as iTRAQ to study the overall proteomic response to changes in external arsenic concentrations employing isobaric tagging for relative and absolute protein quantification (Evans et al., 2007). Using X-Ray crystallography to examine macromolecules that supposedly have arsenic incorporated will also provide invaluable information as to the putative structural role of arsenic in these molecules.

Furthermore, if validated, another obvious evolutionary question is: is this the only example of this ‘form of life’ or are there are similar examples that have survived the ravages of evolutionary pressure? Is it possible that the other known essential elements for life (carbon, hydrogen, nitrogen, oxygen, and sulfur) could also be substituted in addition to phosphorous? The possibility that silicon could substitute for carbon as the basic building block of life has long been speculated, and if confirmed the discovery by Wolfe-Simon and colleagues adds weight to the search for ‘alternate life’ supported by different elements. The search for other hypothetical types of biochemistries and even alternate solvents other than water remains an intriguing quest for many scientists to truly understand the ‘limits for life’. Indeed a Committee set up by the United States Research Council commissioned a report in 2007 to explore this very question (USNRC, 2007). There are many obstacles these alternate biochemistries need to overcome, such as reduced stability of molecules, non-compatibility with water as a solvent, and restricted temperature ranges to optimally function (Pace, 2001). However as noted by Schulze-Makuch and Irwin (2006): "Life cannot be excluded from habitats very different from those of Earth, as a different set of complex chemical interactions requiring different molecular components, solvent systems, and energy sources become possible at temperatures, pressures, and chemical compositions very different from those on Earth".

Nevertheless, as alluded to earlier, many scientists have already criticized the findings of Wolfe-Simon (e.g., Prof. Rosie Redfield), including the contention that arsenic may have been sticking to the outside of the DNA and not actually incorporated into nucleic acids as claimed. One only needs to go back to early claims of microfossils evident in the Martian meteorite ALH84001 (McKay et al., 1996), to realise that bold claims in prestigious journals still have the potential to be challenged if not disproven. Indeed the consensus of opinion in the scientific community that ALH84001 is not of biological origin (Anders, 1996, Shearer and Papike, 1996, Bell, 1996, Becker et al., 1997, Bradley et al., 1997). Although the study by McKay et al., (1996) was rigorously conducted, all lines of evidence put forward suggesting the potential of microfossils of biological origin have long since been disputed, and some believe refuted. Potential issues of contamination and the fact that many of the ‘biological artefacts’ have now been shown to be abiotic clearly calls into question the conclusion of the original paper by McKay et al., (1996), and thus we do need to tread with caution when making conclusions from the claims made by Wolfe-Simon et al., (2010).

However, it should also be acknowledged the article has (presumably) gone through the correct and rigorous process of peer-review that any legitimate journal would demand. If true, then some of the harsh criticism of Wolfe-Simon and her team is unwarranted. The work by Wolfe- Simon and colleagues (2010) could indeed be shown to be wrong, but at the very least it has led to a very useful discussion on the topic between scientists and the need for sound evidence to back up conclusions. Any follow-up papers on this topic (by Wolfe-Simon or others) would then hopefully be even tighter and allow scientific peers and the general public to be completely convinced by the findings.

The implications of this report by Wolfe-Simon and colleagues are obvious and far-reaching. There needs to be rigorous follow-up studies subject to critical peer-review to prove or disprove the claims put forward, and to ensure a healthy debate in the field of astrobiology. Scientists and the general public have a right to question the claims reported in the paper, just as the authors have a right to defend them. It opens up many intriguing questions. Is natural selection universal? Could the universe be filled with failed life experiments based on different elements? Is intelligence inevitable?

Maybe life just is