The subject grabbed Widdel’s attention years ago, in 1982, when an engineer friend gave him a tour of an oil storage site. There had been problems with a crude oil tank that was used to separate the oil/water mix from the drill holes; in the containers, a toxic hydrogen sulfide, stinking of rotten eggs, had accumulated. This “sour gas” is a corrosive substance that can even damage steel pipes.
Hydrogen sulfide is found in many oil reservoirs. It was generally known that it is generated by chemical reactions at high temperatures deep in the ground. However, why hydrogen sulfide would form in the lukewarm tank was a mystery. It soon became clear to Widdel that anaerobic bacteria must be responsible for the process in the anoxic tank, as hydrogen sulfide is a typical degradation product of anaerobiosis. But what were the bacteria feeding on?
At the beginning of the 1980s, it was still believed that crude oil could be degraded only by “aerobic” bacteria, organisms respiring oxygen. Crude oil consists of hydrocarbons, mainly so-called alkanes: long chains of carbon atoms with which only hydrogen atoms bond. The metabolism of aerobic bacteria, in which the alkanes are decomposed, is similar to the workings of a car engine. The long chains are torn into pieces and then react with oxygen, leaving only carbon dioxide and water.
The amount of energy released in the reaction with oxygen is enormous. This is why a liter of gas will get you pretty far. For aerobic bacteria, utilizing alkanes with the help of oxygen is thus a sumptuous meal. But what about the anaerobes? Many scientists assumed that anaerobic utilization of alkanes was impossible. Otherwise, surely the bacteria would have finished off the oil reservoirs of the planet during the course of the millennia?