The Bacterium
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The bacterium Moorella thermoacetica (nee Clostridium thermoaceticum) is found naturally at the bottom of stagnant ponds. It is a member of the Bacteria, one of the most ancient organisms know. Due to the simplicity of it's chemistry, it is tied closely with what is perhaps the first life-originating reaction, the formation of a carbon-carbon bond from methane and carbon dioxide:

CH4 + CO2 -> H3C-COOH

This genesis reaction happens only in "black smokers", undersea thermal vents where the two reactants above come in contact with iron, nickel and sulfur minerals which act as catalysts which makes the reaction run thousands of times faster than it would otherwise go.  Not surprisingly, since our bacterium can catalyze the same reaction, it has an enzyme with active sites composed of iron, nickel and sulfur atoms.

The bacterium is a mesophile, growing at 58C (140F). It is also anaerobic, so growth conditions must not allow oxygen into the growth medium. The medium must also have oxygen-reducing conditions, so we add sulfide (smells like rotten eggs) to mimic the growth conditions at the bottom of a warm stagnant pond.

M. thermoacetica is an acetogenic becterium, normally consuming sugar and excreting acetic acid in a modified citric acid cycle. But it can also grow autotrophically (consuming inorganic chemicals only) on CO2 and H2, utilizing the Wood-Ljungdahl pathway to synthesize acetyl coenzyme A (acetyl-CoA), which it then uses in place of the acetyl-CoA normally created when sugar is metabolized.  It grows better on sugar than carbon dioxide and hydrogen, so we grow ours on glucose and CO2 before harvesting the ACS enzyme.

2CO2 + 4H2 -> CH3COOH + 2H2O

The Wood pathway is illustrated by the above reaction: one carbon dioxide is first converted to CH3+ while the other is converted to CO. Both the methyl ion and the CO are combined on a nickel atom to form an acetyl group, which is hydrolyzed (combined with water) to form the acetic acid:

Harvesting is done in a Coy chamber, a large plastic bag filled with CO2 and H2, with gloves we can stick our arms in to work inside. The chamber has a vacuum airlock for moving supplies inside without introducing bacteria-destroying oxygen.

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Copyright 2003 Bruce Wilson
Last modified: 09/12/03