14. We have recently learned about microbes that can use Arsenic in place of Pho

Question

14. We have recently learned about microbes that can use Arsenic in place of Phosphorus in biomolecules including DNA, proteins and ribosomes. Much of the media hype surrounding this finding has proposed that this is a “new life form!” How could you determine whether in fact this is a new form of life? What gene(s) would you use to build your tree? Is it likely that

this represents new life?

15. What do you think is most likely? A tree of life? A ring of life? A web of life? How does horizontal gene transfer influence your opinion?

Explanation / Answer

GFAJ-1 is a strain of rod-shaped bacteria in the family Halomonadaceae.

A phosphorus-free growth medium (which actually contained 3.1 ± 0.3 M of residual phosphate, from impurities in reagents) was used to culture the bacteria in a regime of increasing exposure to arsenate; the initial level of 0.1 mM was eventually ramped up to 40 mM. Alternative media used for comparative experiments contained either high levels of phosphate (1.5 mM) with no arsenate, or had neither added phosphate nor added arsenate. It was observed that GFAJ-1 could grow through many doublings in cell numbers when cultured in either phosphate or arsenate media, but could not grow when placed in a medium of a similar composition to which neither phosphate nor arsenate was added.

When the researchers added isotope-labeled arsenate to the solution to track its distribution, they found that arsenic was present in the cellular fractions containing the bacteria's proteins, lipids and metabolites such as ATP, as well as its DNA and RNA. Nucleic acids from stationary phase cells starved of phosphorus were concentrated via five extractions (one with phenol, three with phenol-chloroform and one with chloroform extraction solvent), followed by ethanol precipitation.

the best place to find that biological difference was in organisms that had evolved in a place where there was a LOT of arsenic, yet life still existed despite the presence of this nominal poison. Even in the absence of phosphorous, in an arsenic rich environment, a bacterium thrived. When they looked more closely, they discovered that the arsenic had substituted for phosphorus in part of the DNA of the bacterium.

Molecular analysis based on 16S rRNA sequences shows GFAJ-1 to be closely related to other moderatehalophile ("salt-loving") bacteria of the family Halomonadaceae. Although the authors produced a cladogram in which the strain is nested among members of Halomonas, including H. alkaliphila and H. venusta, they did not explicitly assign the strain to that genus.

In the Science journal article, GFAJ-1 is referred to as a strain of Halomonadaceae and not as a new species

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