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Comment by jpwagner | Hacker News Reader

I think this finding deserves a bit more respect.

Of course it does not answer ALL of our questions, but it proves DNA does not need phosphorus! AMAZING!

I agree that this is remarkable, but I think it's valid to criticize publicizing the finding as an astrobiology discovery, which is exactly what NASA (or its public relations department, at any rate) is calling it--and which it is not, despite its implications for astrobiology.

If we care about engaging the public, we should be concerned about sensationalism that detracts from the solid work of the scientists. If you cause people to think you're going to show them extraterrestrial life, fairly or not, you run the risk of them tuning out when the reality is less flashy, even if it's groundbreaking.

I couldn't disagree more. Astrobiology researchers made this finding. If you think it's not astrobiology take it up with NASA.

I'm surprised by the strong objection. I understand that the lead scientist is with the NASA Astrobiology Institute, but I still think that despite the important implications for astrobiology, calling it astrobiology is questionable. If neither NASA nor any astrobiologists had been involved, exactly the same results could have been obtained, and the astrobiology connection (though it might be played up because it is an interesting angle) would more clearly be incidental. In other words, I don't think you lose any accuracy or diminish the finding by referring to it as biological rather than astrobiological.

That said, I'm not sure I really want to get caught up on "biology" versus "astrobiology" as terms. I think the greater point (or at least the point I was trying to make) is about communicating clearly and managing expectations. When you say astrobiology or exobiology, invariably some people are going to cry, "What, no aliens? You tricked me!" If that indignance prevents them from trying to understand the importance of the story, should we even bother trying to cater to them? I don't have a good answer for that. Without raising the idea of extraterrestrial life, they might not have been engaged at all. It's just something for those involved in public outreach to be aware of.

Point taken. But I can't help but be reminded of this: http://xkcd.com/373/

Anyone who feels tricked that there were no aliens found doesn't have their head on straight :)

By your rationale, would anything be called Astrobiology?

I guess I tend towards thinking of astrobiology as the subset of biology dealing with life beyond Earth--a convenient and arbitrary definition, perhaps. In any case, under this definition, with no validated extraterrestrial life, astrobiology would be a largely theoretical science or a line of philosophical inquiry. But if that's too narrow an interpretation, maybe this indeed is an example of experimental astrobiology. Hence, I may have overstated my position when I said this is not astrobiology.

At this point, however, I fear I'm drawn towards philosophical digression. It seems the motivation of the scientist comes into play because she could end up with the same results whether she is a) looking for insight into how life could exist elsewhere in the universe or b) studying life in extreme conditions on Earth. The impetus and interpretation become the categorizers (in practice, I don't think it's too cynical to link this to funding source). On the broader question, I should perhaps defer to the epistemologists--does the nature of knowledge change depending on why it was sought or what is made of it?

Back to the question, what would I consider astrobiology? Well, if life or its precursors were found on Mars or a comet, that might work for me. But what happens when it falls to Earth? I don't know; I've never been particularly comfortable with the blurry divisions between disciplines anyway, so if I consider the dividing line between, for example, psychology and sociology to be tenuous, how am I supposed to feel in this instance? The thing is, knowing what to call it doesn't really affect our ability to understand or use it, it just changes the way we talk about it--which may be important, I grant. But this brings me back to the communication aspect. And as far as that goes, I feel like I've gone through a lot of words without saying much. Therefore, let me summarize like so: I understand why they call this astrobiology; I probably wouldn't call it that myself; most importantly, I think many people will be expecting something else.

So, exactly where in the paper did they show that there was DNA that had an arsenic backbone? They just showed that there was arsenic in the sample that they think is DNA, and not very much phosphorus. That could be contaminants. But also, how do they know that sample is the "DNA sample"?

They calculated that there's definitely not enough P in the bacterium to sustain all the processes it's running with regular DNA.

..based on their MS measurements. They can't accurately calculate for impurities that might be in the source medium, there's no negative control on that.

I agree. Note that the cells are not phosphorus-free. The paper states that the cells grown without phosphorus contained 0.02% phosphorus, presumably due to impurities in the reagents. Based on the numbers in the paper, it seems like this is enough phosphorus for one copy of the DNA genome.

My hunch is that the bacteria are able to use this low level of phosphorus, rather than replacing phosphorus with arsenic wholesale. It seems unlikely to me that the bacteria have evolved to use arsenic in place of phosphorus in the DNA, in ATP, in phospholipids, and in everything else. It seems even more unlikely that they can dynamically switch between arsenic and phosphorus for all their purposes, since all the enzymes would need to work with two different substrates. (In other words, changing an enzyme to work with As instead of P seems hard; changing an enzyme to work with As or P as needed seems much harder.)

That said, I think this is a cool discovery, and it will be interesting to see just how much of the biochemistry is actually using arsenic, since it seems that at least some is.

That's fair, they didn't prove an "arsenic backbone," but here's a couple of key lines...

"...the purified DNA extracted from +As/-P cells contained As. Our NanoSIMS analyses, combined with the evidence for intracellular arsenic by ICP-MS and our radiolabeled AsO4- experiments demonstrated that intracellular AsO4 was incorporated into key biomolecules, specifically DNA."

and

"...the level of PO4 impurities in the medium was insufficient to elicit growth in the control (-As/-P)"

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