“‘Arsenic-life’ bacterium prefers phosphorus after all”,
written by Daniel Cressey, discussed the debunking of a previous article posted
in 2010. The article, “A Bacterium That Can Grow by Using Arsenic Instead of
Phosphorus”, written by Felisa Wolfe-Simon & others, claimed that one of
the bacteria in Mono Lake in California, which is very high in arsenic, could
metabolize without phosphate and instead use arsenate. After several subsequent
articles, this theory was proven to be false, and organisms need phosphate to
grow. Specifically, an article published in October of 2012, “The molecular
basis of phosphate discrimination in arsenate-rich environments” by Mikael
Elias & others, tested to see how these bacteria even differentiate between
the two extremely similar molecules. Being from the same group on the periodic
table, phosphorous and arsenic have practically identical electronegativities
and Van der Waals radii.
This was the foundation for Wolfe-Simon’s article, but much
of their results and findings were viewed subjectively because they wanted the
outcome to be true. In the research done by Elias and his colleagues, they
discovered that not only could these bacterial proteins differentiate between
phosphate and arsenate, they did so through one key chemical bond. They all
showed an extreme preference for phosphate, particularly the one bacterium that
began this controversy which filtered in phosphate at levels of 4,500-fold the
level of arsenate to phosphate. Arsenate was clearly still needed for these
bacteria due to the lengths they went to in order to bond with all of the
phosphates they could. However, some of the bacteria were able to bond with the
arsenate after depleting the supply of phosphate, though the hydrogen bond was
much weaker when the bacterium bonded with arsenate.
a. P. fluorescens
PBP bound to phosphate has a more direct connection
b. P. fluorescens PBP bound
to arsenate has an angled connection
Knowing that bonds could occur with arsenate when given no
other option opens the door to the possibility that there may be life on other
planets that survive without phosphate, or in fact use arsenate preferentially
over phosphate. It would be interesting to test these, and other bacteria, in a
pure arsenate solution, removing their ability to choose phosphate over
arsenate.
Sources:
‘Arsenic-life’ bacterium prefers phosphorus after all: http://www.nature.com/news/arsenic-life-bacterium-prefers-phosphorus-after-all-1.11520
The molecular basis of phosphate discrimination in
arsenate-rich environments: http://www.nature.com/nature/journal/v491/n7422/full/nature11517.html#supplementary-information
A Bacterium That Can Grow by Using Arsenic Instead of
Phosphorus: http://science.sciencemag.org/content/332/6034/1163.full
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