Arsenic sandwiches on Mars? Not quite…

Arsenic isn’t generally thought of as a particularly friendly or positive element, unless of course you’re trying to kill someone, in which case it’s actually a great element, and one which I recommend if someone you really don’t like comes to tea (unless you live next door to Miss Marple, in which case, don’t try it, you will never outsmart Marps).
GFAJ-1 bacteria
However, a paper published in the journal Science has announced that a species of bacteria has been discovered lurking in the murky depths of Mono Lake in California that just loves arsenic. In fact, just to be different, it is so far the only organism that we know of that is able to use arsenic to replace phosphorus, which is the element that every other known form of life likes to use in DNA, proteins, cell membranes, and numerous other important biochemical type bits and pieces. This is pretty revolutionary, and if I may say so, really rather exciting. Not because, as some over-excited internet bods have been implying, this bacteria is an alien life form, it is very much not at all. The bacteria, known as GFAJ-1, definitely live on Earth and are definitely related to other bacteria on Earth; so sadly no ET just yet.
The reason I am excited however, beyond the fact I got to look at some gratuitous pictures of DNA for this post, is because this news disproves the theory that all life requires just six elements (carbon, hydrogen, oxygen, sulphur, phosphorus and nitrogen) to live. These bacteria can do without a decent supply of phosphorous.
DNA with a phosphate backbone
(phosphorus molecules looking stylish in pink)
Phosphorus is found, amongst other places, in the backbone of your DNA, in adenosine triphosphate (code name, ATP) which is the energy source inside your cells, in fats and in cell membranes (the barriers that stop the insides of your cells being… not inside your cells). The implication of this paper is that GFAJ-1 can replace phosphorus in all of these places with arsenic, which in most organisms would usually result in untimely death.
The reason for the whole untimely death thing is that arsenic is chemically very similar to phosphorous (it sits just below it in the periodic table, which usually means it’s quite closely related in chemical terms). It is arsenic’s similarity to phosphorus ironically, that makes it such an effective poison in most creatures. Because it is so similar, it can very easily get into your cells and take the place of phosphorus, the problem with this being that this replacement results in less stable products and, often, much slower reaction times. The products are therefore either produced too slowly, or they are too unstable to be used, and this eventually results in cell death, which ultimately leads to whole-body death.
The clever little bacteria in Mono Lake however, have found a way around this. The mechanism that they use hasn’t yet been fully explained, but what has been shown is that arsenic is present in the DNA backbone of the cells, in the fats and in the cell membranes. It was also noted that the bacteria grew up to 50% larger on arsenic than they did on phosphorous and they developed large compartments within the cell (called vacuoles), which the authors of the article suggested may act as a kind of storage space for unstable arsenic products, to stop them triggering the whole cell death bit.
This is pretty exciting, because new knowledge is always exciting (even when it doesn’t involve aliens).  However, it is important to remember that the experiments involved the scientists effectively starving the bacteria of phosphorus and continually adding arsenic, which will have put them in an unnatural situation. Mono Lake is extremely high in arsenic, but not necessarily as high as the Petri dish in which these experiments took place. In addition, there was still some phosphorus left to the bacteria, so this tiny amount of remaining phosphorus could, for all we know, be absolutely essential. Nonetheless, this is still a pretty cool bit of science. And it does have some interesting effects on what we thought we knew about life on this planet, if not necessarily other ones.
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