Monday, May 22, 2006

Microbe gives clues to origin of life

Energy is critically important to origin of life theories, which is why so many of the newest theories postulate where life started (e.g. near thermal vents--a ready source of energy) in addition to how. After all, without the energy source to break down and combine molecules, life can't begin. A recent news story at Science provides a clue to the enegy problem:

One of the most vexing questions facing biologists is how life on Earth first emerged. Now, research on a methane-producing microbe has led to a novel theory that could breathe new life into the field and help two opposing theories find common ground.

On a simple level, the origin-of-life debate comes down to a question of how the first complex molecules came to be. The so-called heterotrophic hypothesis says that life arose from an organic soup of small molecules that were either brought to Earth by extraterrestrial objects or were produced through lightning-triggered reactions that combined gasses. Life originated when these smaller molecules assembled into larger molecules such as RNA and proteins. On the other hand, the chemoautotrophic hypothesis posits that iron sulfide reactions released hydrogen, which combined with carbon dioxide in the atmosphere to form organic compounds. These in turn gave rise to more complex molecules.

One problem with both hypotheses is that they don't have a reasonable source of energy for putting the larger molecules together. The heterotrophic theory ignores the issue, and the chemoautotrophic theory relies on extremely complicated enzymes that were unlikely to have already evolved.

An unusual microbe may give both sides hope. While microbial biochemist James Ferry and geomicrobiologist Christopher House of Pennsylvania State University in State College were studying the Methanosarcina acetivorans microbe, they noticed that it had a unique biochemistry: Many of the oldest bacteria on Earth can convert carbon monoxide into methane, but this microbe has the added ability to produce acetate from carbon monoxide as well.

Ferry and House suggest that the first metabolism involved two ancient enzymes found in this microbe that help produce the acetate. In a hypothetical ancient "protocell," excreted acetate would react with iron sulfides outside of the cell to form a sulfide-containing derivative known as an acetate thioester. The protocell would then take this in and break it down to acetate, completing the loop. A key part of the idea is that the formation of acetate inside the cell creates energy-rich molecules known as ATP that could provide the energy needed to combine small molecules into bigger ones. While the theory doesn't completely resolve the origin-of-life debate, it provides a common source of energy for both sides and could be a useful starting point for a compromise, the authors report in the June issue of Molecular Biology and Evolution.

Geochemist George Cody of the Carnegie Institution of Washington, D.C., agrees, but he notes the devil is in the details. "There's no reaction that I know of that converts acetate back to thioester," says Cody. Nevertheless, the work is a step in the right direction toward bridging the gap between the two old hypotheses, he says.