In our gut, for example, microbes produce methane - which is chemically nothing more than natural gas. They utilise hydrogen and carbon dioxide, gases produced by digestive processes and the decomposition of food by other microbes. Acetic acid-forming bacteria also process these two gases - in this case to acetic acid. Conversely, they can produce hydrogen: When they grow on sugars and proteins without oxygen, they break them down into hydrogen and acetic acid. This process is called fermentation.

 

Molecules such as sugar and proteins are also found in our intestines as food components. Microbes help us to break them down in the gut. For example, acetic acid producers produce hydrogen, carbon dioxide and acetic acid from sugar and methane producers use this "waste" to convert carbon dioxide and acetic acid into methane. In this way, they help to digest our food completely. There are also bacteria in our intestines that produce hydrogen sulphide from hydrogen, which smells like rotten eggs. These microbial gases escape from us in the form of "farting".

And these microorganisms are not only found in the gut, but also deep in the mud of lakes and oceans. Methane and acetic acid producers like to live together: The acetic acid producers feed on sugar and the methane producers utilise the hydrogen produced. They live together in a symbiotic relationship because bacteria can only digest sugar if methanogens eat the hydrogen produced. Too much hydrogen gas in the bacterial cells disrupts the necessary fermentation processes; the bacteria would not survive.

 

But what about bacteria that don't have any methane producers or other microbes nearby to absorb the hydrogen? What was it like at a time when there were no organisms on our planet that ate away the hydrogen? The answer is quite new: the acetic acid bacterium Acetobacterium woodii can reuse the hydrogen it produces itself. It uses the same metabolic pathway as the methane producers, but forms acetic acid instead of methane. This bacterium can thus grow all by itself by eating sugar or proteins and recycling the hydrogen formed to convert it into acetic acid together with the carbon dioxide present. This newly discovered form of hydrogen recycling gives researchers insights into how the first microbes might have lived billions of years ago, when there were not yet many different, complementary species.