Saturday, December 04, 2010

More on Those Arsenic-Eating Bacteria

I wrote my earlier post after cursorily reading a few of the news articles on those arsenic-eating (not arsenic-based!) bacteria. The questions that have come up inspired me to take a closer look. My main purpose in the earlier post was to recall some similar experiments that I had been close to.

I haven't read the full paper yet. I am usually satisfied to read the dead-tree version of Science magazine, and that won't be here for a few days yet. The paper is available on line, and I have an irrational aversion to registering for sites, especially when that registration failed, something that happened with Science some long time ago. Maybe I'll make a second try later today. The abstract and supporting online material are available, however, and they answer many of my questions while raising a few others.

This post summarizes the science nicely. A number of good questions are also raised in The Guardian's blog on the story. It appears that very little arsenic was incorporated into the bacteria's essential biochemistry. So I won't be seeing the fully-arsenic-substituted DNA any time soon. Not that I expected that.

Arsenic and phosphorus are probably more similar than any two up-and-down pairs of main-group elements in the periodic chart. That allowed the researchers to substitute arsenate for phosphate in the nutrient solutions. Even so, the bacteria bloated up, just as the algae did in deuterated water, a sign of stress. The bacteria would already have had mostly phosphorus in the places where phosphorus should be, with perhaps a few odd arsenic atoms substituted because they had grown up in the very bizarre environment of Mono Lake. The analyses of where the arsenic went are not totally unambiguous, and they show relatively little arsenic substituted for phosphorus at best.

So all the life we know about still consists primarily of carbon, oxygen, nitrogen, hydrogen, sulphur and phosphorus. Arsenic has not been shown to be able to function in the same ways phosphorus does, nor to be able to substitute completely for phosphorus in life containing all those other elements. For similar periodic-chart pairs, carbon-silicon interchange just isn't likely. Silicon can't do all the things carbon can, and its compounds have a nasty habit of being much more mineral-like than carbon's. Oxygen-sulfur is already in the group, although sulfur does different things in biochemistry than oxygen. Selenium has some trace functions. Substitution? Maybe, but probably not important. Nitrogen-phosphorus is part of the life elements, although, again, the functions are different. And we're looking at arsenic.

So, from the abstract,
Exchange of one of the major bioelements may have profound evolutionary and geochemical significance.
Um, yeah. If there's significant exchange, and not just tolerance of the occasional weird atom in an otherwise normal biochemical compound. The evolutionary significance is that extremophiles, the weirdos of the bacterial world, can grow to tolerate arsenic. If you take a chemical rather than a literary view, that's not too surprising because of the periodic-chart relationships I've been talking about. The evolutionary significance is that bacteria can evolve, which we knew anyway. And, in response to anonymous's question, 10,000 years is a long time in bacterial evolution. In just a few decades of antibiotic pressure, we've managed to evolve antibiotic-resistant bacteria. So living in an increasingly saline/arsenical environment could definitely improve tolerance of arsenic.

The Guardian blog has an example of what I think is going too far:
The microbe seems to be able to replace phosphorus with arsenic in some of its basic cellular processes — suggesting the possibility of a biochemistry very different from the one we know, which could be used by organisms in past or present extreme environments on Earth, or even on other planets.
Here's my correction:
The microbe seems to be able to replace phosphorus with arsenic in some of its basic cellular compounds - which could mean that arsenic is involved in cellular processes in place of phosphorus or that it is an obstacle that the microbe has found ways to work around.
I know, mine isn't as exciting.

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