Magnetic bacteria: Filigree sewage treatment plants

A research team at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has succeeded in purifying uranium-containing water using a special type of bacteria known as magnetotactic bacteria, which owe their name to their ability to react to magnetic fields. They are able to incorporate heavy metals in solution into their cell wall. The research results (DOI: 10.1016/j.jhazmat.2022.129376) also shed new light on the interactions of uranium with bioligands.

„Our investigations are aimed at possible industrial applications in the field of microbiological remediation of waters that are contaminated with heavy metals in particular, such as those found as flood water in former uranium mines,

explains Dr Evelyn Krawczyk. Evelyn Krawczyk-Bärsch from the Institute of Resource Ecology at the HZDR. „For this project, we have enlisted the help of a very special group of organisms: magnetotactic bacteria“, adds her colleague Dr Johannes Raff and adds: „Due to their structure, they are virtually predestined for such a task.“

Because they have a special feature that distinguishes them from other bacteria: Magnetotactic bacteria form nanoscopically small magnetite crystals inside their cells. They are lined up like a string of pearls and are so perfect in shape that humans are currently unable to copy them synthetically. The individual magnetic crystals are each surrounded by a protective membrane. Crystals and membrane form the so-called magnetosomes, which help the bacteria to orientate themselves along the earth's magnetic field and thus orientate themselves in their habitat - and make them accessible for simple separation processes.

They are found almost everywhere in aqueous environments, from freshwater to salt water, even where there are few nutrients. The microbiologist Dr Christopher Lefèvre has even found them in the hot springs of Nevada. He and his colleague Dr Damien Faivre from the French Nuclear Energy Research Centre, the Commissariat for Atomic Energy and Alternative Energies (CEA), provided the Rossendorf team with their bacterial strain and expert tips on how best to keep them. Despite their frequent occurrence, there are a few things to bear in mind when cultivating them.

Stable heavy metal collector in a hostile environment

Magnetotactic bacteria are viable at neutral pH values even at higher uranium concentrations in aqueous solutions. They incorporate the absorbed uranium almost exclusively into their cell wall over a wide pH range - an excellent basis for coping with the conditions found in mining-relevant waters. No uranium enters the cell interior, nor is it bound by the magnetosomes.

It was already known that different types of bacteria bind heavy metals in their cell walls, although these can have very different structures. In the case of magnetotactic bacteria, the cell wall consists of a peptidoglycan layer only four nanometres thick, a macromolecule composed of sugars and amino acids, which is the main component of the cell walls of many bacteria. The cell wall of magnetotactic bacteria is sealed by a larger membrane consisting of sugars and fat-like components: potential docking sites for uranium.

„Our results show that peptidoglycan plays the main role in magnetotactic bacteria during uranium uptake. This finding is new and was not expected for this type of bacteria“, reports Krawczyk-Bärsch. The team even succeeded in determining three specific uranium peptidoglycan species and confirming the result with reference samples. The new findings were only made possible by a combination of microscopy and various spectroscopic techniques that is rarely available worldwide: „For example, in collaboration with the Institute for Ion Beam Physics and Materials Research at the HZDR we were able to use the electron beam microscope. The local concentration of our institutes at the site and the expertise of our colleagues are a great advantage for our work,

says Raff.

Importance for the remediation of contaminated waters

Magnetotactic bacteria can be easily separated from water using magnets due to their magnetic properties. „This is also conceivable on a large scale in the form of treatment directly in near-surface waters or by pumping the water out of underground mines and transferring it to pilot treatment plants,“ explains Krawczyk-Bärsch with regard to the development of innovative remediation strategies for contaminated water: The use of magnetotactic bacteria could be an effective alternative to expensive and conventional chemical treatments. This is because magnetotactic bacteria are inexpensive to keep, whereas the introduction of other biomass-based solutions into practice regularly fails due to the price, which is due to increased nutrient and energy requirements.

And another detail has piqued the researchers' interest in these bacteria: Their proteins are able to stabilise bivalent and trivalent iron in such a way that the synthesis of magnetite stored in the magnetosomes is successful. „We are therefore particularly interested in one question: How will these microorganisms interact with radionuclides of different oxidation states? We are thinking in particular of plutonium“, explains Raff. Because unlike uranium, it is conceivable that it utilises similar uptake pathways into the cell due to its chemical similarity to iron. How does this influence the migration behaviour of plutonium in nature and could plutonium also be removed from wastewater in this way? The topic is therefore also relevant for repository research: any results could then be incorporated into the safety assessment.

Publication:

10. Krawczyk-Bärsch, J. Ramtke, B. Drobot, K. Müller, R. Steudtner, S. Kluge, R. Hübner, J. Raff, Peptidoglycan as major binding motif for uranium bioassociation on Magnetospirillum magneticumAMB-1 in contaminated waters, Journal of Hazardous Materials, 2022 (DOI: 10.1016/j.jhazmat.2022.129376)

Press release of the "HZDR" from 09 May 2023