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Mineral-forming bacteria of great usage value

They’ve been active in the background for billions of years, but they have now come to the fore as potent helpers – we’re referring to iron-oxidising bacteria. Researchers from Tübingen have recently shown that iron-oxidising bacteria lead to rust-coloured, banded rock formations in countries like South Africa and Australia. When the bacteria form iron minerals they bind heavy metals, a characteristic that makes them interesting candidates for applications in the field of environmental technology.

Banded rock formations such as those in South Africa are the result of bacterial activity. © Prof. Dr. Andreas Kappler

Prof. Dr. Andreas Kappler from the Tübingen Centre for Applied Geosciences (ZAG) is one of many researchers who are great supporters of microbes, in particular of mineral-forming bacteria of genera that have been shown to be useful for humans. "Bacteria are everywhere and can do everything - that is the simple truth of the matter," said Kappler whose investigations into the development of early geological formations are aimed at revealing general geobiological relationships. He is particularly interested in banded iron formations that are predominantly found in Australia, but also in South Africa, the USA and other regions. Orange-red coloured rock was formed between 3.8 billion and 800 million years ago and, as Kappler's recent investigations have shown, is one of the oldest rock formations created by the activity of anoxygenic phototrophic bacteria.

Bacteria turn dissolved iron into rusty iron under anaerobic conditions

Iron oxidising bacteria form rust. © Prof. Dr. Andreas Kappler

"In principle, I believe that microorganisms are the sole plausible explanation why such bands - alternating layers of silicate and iron rock - have developed under the oxygen-deficient conditions that prevailed on the early earth," said Kappler who has been able to show in the laboratory what certain bacteria are able to do: "In an iron- and silicate-containing solution, green sulphur bacteria, purple sulphur bacteria and purple non-sulphur bacteria are able to oxidise iron (II) to iron (III) compounds at 20˚ C. One representative of these bacteria, Rhodobacter ferrooxidans, was not discovered until 1993 and it was several more years before further papers were published on this bacterial species. In Tübingen, the bacterium has become a kind of pet project for Kappler's team of researchers. The findings obtained with R. ferrooxidans are a geobiological and geomineral breakthrough, and are so important that they will be included in fundamental science books. "The results open up a completely new understanding of early earth development and the ‘evolution' of atmosphere and hydrosphere," said Kappler.

Iron-mineral-forming and converting bacteria are interesting for basic and applied research alike. Kappler is focusing on the practical use of the bacteria, for example the effect of iron-oxidising bacteria on drinking water filters. The pollution of drinking water with naturally occurring soil arsenic is a huge problem in some regions of China, Bangladesh and Vietnam.

Filters such as the one shown in the figure contain iron minerals and are used to remove arsenic from drinking water. © Dr. Michael Berg, EAWAG, Switzerland

The researchers are testing filters containing iron minerals and bacteria for their efficiency in withdrawing arsenic from drinking water. "We wanted to find out the effects aerobic microbial iron oxidizers have," said Kappler. Initially, the researchers were hoping to use the bacteria to naturally and quickly produce small iron (III) mineral particles that were able to bind high amounts of arsenic and would be withheld by the filters. However, the researchers' investigations led to surprising results: the  biogenic iron minerals produced by the bacteria are negatively charged, which is in contrast to chemically produced iron minerals. And this is why they are ineffective in binding negatively charged arsenic or neutral arsenite.

The addition of iron-oxidising bacteria proved to be counterproductive under the chosen conditions and the researchers needed to look for other solutions to simply and inexpensively filter arsenic from water. The addition of chemically produced iron (III) compounds to drinking water is rather costly, and therefore difficult to put in place in Bangladesh. A BMBF-funded project carried out by Kappler's team in collaboration with Osnabrück-based GEH Wasserchemie GmbH focuses on ways to prevent naturally occurring, iron-oxidising bacteria from settling on commercially available filters. It is likely to be very difficult to prevent microorganisms from entering the simple sand-filled household filters with no cover, such as those used in Vietnam or Bangladesh.

Bacteria “work” according to the penguin principle

Kappler is also focusing on other projects that deal with the removal of environmental toxins from water and soil using mineral-forming bacteria. In a project supported by the German Environmental Foundation (DBU, Deutsche Bundesstiftung Umwelt), the researchers are working on removing cadmium from soil using bacteria. “We know of some mineral-dissolving bacteria that are highly tolerant to cadmium. These bacteria dissolve iron minerals to which cadmium is bound,” Kappler explains. This enables plants to take up cadmium efficiently and to store it. The plants can subsequently be harvested and disposed of. The bacteria are far more efficient in soil than in the laboratory and manage to handle much higher cadmium concentrations. “In soil, the bacteria live in small particles or biofilm-like aggregations where only the bacteria that are close to the surface are exposed to high environmental concentrations of cadmium,” said Kappler. With this penguin strategy – penguins tend to group closely packed together during Antarctic winters where the animals on the exterior form a kind of phalanx against the cold – the bacteria manage to survive even in heavily contaminated soils. This is not only of great interest in terms of cadmium. Kappler believes that the stimulation of bacterial growth or the addition of larger bacteria populations can be used to remove other undesired metals from soils.

Kappler is always looking ahead and has other potential projects related to soil bacteria up his sleeve: he plans to use the bacteria to guide electron currents through soil. This idea is based on recent findings on humins. “Undissolved humins have long been regarded as inert material. However, we have now discovered bacteria that are able to use undissolved humins as electron acceptors,” said Kappler explaining that the bacteria and the redox-active humins can generate an electron conductance chain in soils and help reduce the quantity of organic pollutants and of some metal ions such as uranium at a certain distance, thereby reducing the negative effect on the ecosystem. “This means that bacteria do not necessarily have to be available at the polluted sites, but could be added to humin layers at the surface. An increase in electron conduction would then also lead to a reduction or immobilisation in the quantity of harmful substances in more remote soil regions,” said Kappler. This principle would open up completely new applications, which is one of the next projects Kappler and his team have in mind.

ERC Starting Grant 2012
The geomicrobiologist Prof. Dr. Andreas Kappler was awarded a Starting Grant 2012 with a purse of 1.4 million euros for his project “Microbial formation of minerals by communities of Fe(II)-oxidising bacteria in modern and ancient environments”. ERC Starting Grants are aimed at up-and-coming research leaders with 2-7 years of experience since completion of their PhD and a highly promising scientific track record.

Further information:
University of Tübingen
Centre for Applied Geosciences (ZAG)
Prof. Dr. Andreas Kappler
Sigwartstraße 10
72076 Tübingen
Tel.: +49 (0)7071 29-74992
E-mail: andreas.kappler(at)uni-tuebingen.de

Website address: https://www.biooekonomie-bw.de/en/articles/news/mineral-forming-bacteria-of-great-usage-value