Wednesday, October 15, 2008

Life in the center of the earth (almost)

It really is remarkable how little we know about life on earth, even with scientific data pouring in constantly. This is particularly true for the smallest forms of life; microbes and organisms that are only a little bigger than microbes. But since we are used to seeing and hearing things, we rarely even think of life that is smaller than say an insect. Yet over 90% of all life is microbial, and we don’t even know how many species of mammals there are on earth, leave alone microbes.

Microbes though couldn’t care less. They have always ruled earth (though we would love to think we rule this planet) and will, in all probability, continue to do so. It is easy to forget that the earliest forms of life were microbial, single-celled organisms capable of only the simplest functions: survival and reproduction. And it is in these two functions that microbes have excelled. In places where you would think life would be impossible, you are more likely than not to find some microbes chugging along. Put it this way, if there was a hell, there probably wouldn’t be any humans there, but there would be plenty of extremophiles having an orgy. It is always fascinating to see what creature can survive where no other can, and every now and then there is a discovery of yet another creature (usually a bacteria or archaea) that defies all probability of life and thrives.

So, let us say there was a world without light, where the temperature was over 60 degrees centigrade (140F), where the pH was over 9 (an extremely alkaline environment), and there was little or no oxygen. Would you think there would be life possible? Apparently, if you literally dig deep enough, the answer to that is yes. So what kind of life is it?

Some researchers collected fracture fluid from a depth of almost 3 kilometers within the earth’s surface, from a South African gold mine. Within it, they surprisingly found a single dominant species of bacteria, which they called Candidatus Desulforudis audaxviator (link). Now, it looked like these bacteria were perfectly happy living all alone in a vent where there was no light, and therefore no photosynthesis, as well as next to no oxygen, and an extremely alkaline and hot environment. Everything about its life seems wrong. Yet it lives, doing all things that living things need to do. It fixes nitrogen and carbon. It divides (albeit all so slowly, taking a few hundred years to divide). All life needs energy to drive it. Yet there is no light here, so this bacterium actually gets energy from the radioactive decay of uranium. This allows the generation of an electrochemical gradient from hydrogen to sulfate. It was thought that for all life you need a diverse ecosystem (which provides nutrients for each other, or helps break down compounds and so on). And here we have this bacteria happily being an ecosystem of one, in true US Army style. Yet, this bacterium is not all that different from other bacteria, or just all other living cells in general, and has all the genes used to make amino acids, or metabolizing carbon and nitrogen, with a few tweaks here and there.

This story is probably best told by one of the people who discovered it and then analyzed its genome, in this absolutely fascinating podcast. If life fascinates you, this podcast will amaze you.

Hear here.

If we do some day travel in space, and explore new worlds, we probably will not see any four eyed green web-fingered aliens. If we do find something, it will probably be closer to this bacterium.

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