Lichtgedanken 03

S C HW E R P U N K T 41 03 | LICHT GEDANKEN calcium eventually bind to form calcite. In this process, called »biomineralisa- tion«, bacteria cause solid carbonate to precipitate, forming small crystals of va- rious shapes, which settle as sediment in water, referred to as sinter rocks. It had previously been assumed that the pH value of the environment was the driving force for this process and that that this was how microbes could influ- ence the mineralisation process. When the pH is high (i.e. under alkaline condi- tions), carbonate precipitates and sinter rocks consisting of calcium carbonate, or calcite, form. If the pH is low (i.e. under acidic conditions), the carbonate remains dissolved in the water. Howe- ver, the scientists from the Institute of Microbiology in Jena made the surpri- sing discovery that even in an acidic en- vironment, bacteria ensure that carbo- nates are precipitated instead of being dissolved. Thus, the role of microorga- nisms in mineralisation may have to be re-evaluated. Do bacteria secrete minerals? »At the calcareous sites near Bad Kö- sen, we took samples of various bacte- rial strains from the groundwater, rocks and soil, and then investigated whether they could also achieve biomineralisa- tion under laboratory conditions,« says Prof. Erika Kothe. »More than 92 per cent of microorganisms do indeed form carbonates, usually calcite, and in rare cases also calcite containing magnesium or vaterite.« Approximately a third of them created an alkaline environment in the process, while most others had no influence on the pH of the surrounding medium. Six strains even produced aci- dic conditions. »The process by which the formation of minerals at a low pH is achieved is so far unknown,« explains Kothe. When they examined individual bac- teria more closely, the researchers no- ticed that it was not essential for the newly created minerals to be deposited directly on the bacterial cells; they also occurred at a distance from the cells. This means that there must be a diffu- sion mechanism at work, which trig- gers the mineralization, and substances secreted by the bacteria probably play a role. In addition, the external form of the precipitate varies. »We have found very different morphologies of the crystals, from round through elongated to rosette-shaped,« says microbiologist Kothe. And the microbiologists in Jena have also discovered one more astoun- ding connection: when extrapolated to one year’s worth of formation, the new minerals total around two grams per litre, which closely matches the annual growth of a stalactite. The process that has been observed is therefore already known from nature. The current results raise many new questions related to biomineralisation, which researchers want to answer in the years to come. »Our aim is to identify the process behind this mineralisation,« says Erika Kothe. »With one bacterial strain – a streptomycete – we have be- gun to switch off genes in order to find the genetic basis responsible for mineral formation. We have already obtained promising initial results.« Applied research is already drawing on the fundamental processes of this mi- neral formation: the information can, for example, advance the development of self-repairing concrete. Spores could be integrated into concrete or road surfaces, which, when water entered, would germinate and start to induce mi- neralisation that would seal the cracks from the inside. For Erika Kothe, the results demonst- rate once again the benefits of coope- ration between microbiologists and geoscientists. »We now know that mine- ral formation is far more complex than we thought, and that microbiological processes play a key role in it.« This would bring the two disciplines even closer together. At Friedrich Schiller University (FSU) in Jena, this is already happening. Coope- rative research initiatives have existed since 2000, and in 2005 these resulted in a BSc/MSc degree programme in Bio- geosciences. Since 2006, young resear- chers have been working together in the »Jena School for Microbial Communi- cation« (JSMC), which has been fun- ded by the Excellence Initiative of the German Federal Ministry of Education and Research and the German Research PhD student Hanka Brangsch from the Institute of Microbiology inspects bacterials cultures growing with heavy metals. F E AT U R E