Rare earth metal essential element for methane-munching acid-volcano microbe

It all started with the observation that the bacterium, Methylacidiphilum fumariolicum, could only grow when the medium was supplemented with mudpot water from its original habitat in Italy. Which compound in the water was responsible for the growth? 'That eventually became a quest of six years' reports Huub Op den Camp, the principal investigator. 'We first started to elucidate if the compound was organic or inorganic in origin. After heating the mudpot water to very high temperatures the stimulating effect was still present. This strongly indicated that the compound was an inorganic mineral.'

Crucial enzyme contains mineral component

In the meantime, an important metabolic enzyme of the volcano bacterium was investigated. This enzyme (methanol dehydrogenase) showed unexpected characteristics that may explain the growth pattern of the bacteria from Op den Camp's study. And indeed, in cooperation with Thomas Barends, a crystallographer from the MPI Heidelberg, it was discovered that the methanol dehydrogenase contained an unknown mineral component. Methanol dehydrogenases from other bacteria contain calcium, but crystallographic analysis demonstrated that this element is too small to fit into the available space in the enzyme. The question remained, which mineral element replaces calcium? The microbiologists continued searching for possible candidates.

Mystery solved after six years

Op den Camp: 'I even brought vitamins and mineral preparation from home, all were without any stimulatory effects. Finally, metal-analyses of the purified methanol dehydrogenase provided the clue that lanthanides were the elements taking over the role of calcium. Lanthanides are a set of fifteen metals, also named rare earth metals. 'We have tested several lanthanides and could show that cerium was most stimulatory on growth. By adding the lanthanides to Nijmegen tap water, we could mimic the volcanic mudpot water that we needed to support good growth. The mystery was solved, after more than six years.'

To discover novel bacteria

Op den Camp: 'It is very special that cerium turned out to be an essential element for life for our volcano bacteria, but this finding can also help to discover new thus far unknown bacteria. In the past, when we tried to grow bacteria, lanthanides were never added as minerals which would have prevented bacteria that need these compounds to grow. Testing samples from many different ecosystems, now in media with lanthanides, may results in the discovery of many thus far unknown bacteria'.

Application in biomining

Rare earth metals like cerium are used in small but significant amounts in electronic products like cell phones and TV screens. Application of metal-using bacteria could be of help for metal-mining. According to Op den Camp these applications are still far away: 'The bacteria only need very small amounts of the metals. For an application in biomining major increases in the uptake would be necessary, and as such we first have to understand the transport systems involved'.