A couple of days ago I wrote about the exciting announcement out of NASA that scientists had discovered a lifeform that uses a slightly different set of building blocks than traditional critters.  Specifically, the microbes in question are alleged to use arsenic instead of phosphorus as a component when constructing their DNA.

Unfortunately, according to Rosie Redfield, a microbiologist out of British Columbia, the science was sloppy and proves nothing.  Writing on her personal blog, Redfield dissected the Science paper, explaining what the authors did and why it was both inconclusive and misleading.

Some excerpts:

In Fig. 1 (below), the correspondence between OD600 (Fig. 1 A) and cfu (Fig. 1 B) is not good.  Although the lines in the two graphs have similar proportions, OD600 is plotted on a linear scale and cfu/ml on a log scale (is this a shabby trick to increase their superficial similarity?).


The authors never calculated whether the amount of growth they saw in the arsenate-only medium (2-3 x 10^7 cfu/ml) could be supported by the phosphate in this medium (or maybe they did but they didn’t like the result). [...calculations removed for clarity...]  This value is just comfortably larger than the observed final density, suggesting that, although these bacteria grow poorly in the absence of arsenate, in its presence their growth is limited by phosphate.


The authors argue that the arsenate-grown cells don’t contain enough phosphorus to support life.  They say that typical heterotrophic bacteria require 1-3% P to support life, but this isn’t true.  These numbers are just the amounts found in E. coli cells grown in medium with abundant phosphate.   They are very unlikely to apply to bacteria growing very slowly under phosphate limitation, and aren’t even true of their own phosphate-grown bacteria (0.5% P).


The authors describe this fraction as DNA/RNA, but it also contains most of the small water-soluble molecules of the cell, so its high arsenic content is not evidence that the DNA and RNA contain arsenic in place of phosphorus.  The authors use very indirect evidence to argue that the distribution of arsenic mirrors that expected for phosphate, but this argument depends on so many assumptions that it should be ignored.

Bottom line: Lots of flim-flam, but very little reliable information.  The mass spec measurements may be very well done (I lack expertise here), but their value is severely compromised by the poor quality of the inputs.  If this data was presented by a PhD student at their committee meeting, I’d send them back to the bench to do more cleanup and controls.

There’s a difference between controls done to genuinely test your hypothesis and those done when you just want to show that your hypothesis is true.  The authors have done some of the latter, but not the former.

If you have a good head for science, I encourage you to read the whole thing for yourself.  Redfield is very methodical in her approach, and the blog article feels quite genuine and motivated for a desire to produce good science.

The entire things feels a little bit like the back-and-forth sparring between climate scientists that has played out in the media over the past few years.  Interestingly, CBC News is reposrting that the speculation as to how such sloppy work made it into the prestigious journal, Science, could be good old fashioned self interest:

Redfield acknowledged that the original paper was peer reviewed, but said that fact was “a puzzle.” She suggested that perhaps the reviewers may not have had an expertise in microbiology. Another possibility is that the reviewers raised some concerns, but the editors of Science didn’t think they were serious or were “motivated by the coup of getting this very high-profile article.”

It will be interesting to see where this one goes.  My gut feel is that the known recipe for life (hydrogen, oxygen, carbon, nitrogen, sulfur, and phosphorous) cannot be the only possibility in the infinite universe, however we still need proof that it isn’t.

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