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WWW.HZDR.DE 10 11 TITLE // THE HZDR RESEARCH MAGAZINE but researchers still do not know exactly which carbon compounds the microorganisms live on. In any case it doesn't look like they will be going hungry anytime soon: Approximately one percent of the clay is composed of carbon after all. We as human beings obtain energy in much the same way when we burn the carbohydrates from foods such as bread, potatoes, or fruit with oxygen. In many areas underground, however, pure oxygen is in short supply. Some bacteria adapted to this lack of oxygen long ago by burning carbon compounds using the abundance of nitrate compounds present, which are rich in oxygen. It is chemically bonded, however, and must be made usable for the bacteria through electrochemical reduction. But Andrea Cherkouk has found just such "nitrate reducers" in the connate water of the clay. A casing made of uranium The microorganisms in the clay also possess other characteristics that are of great interest for repository research: Apparently various elements of radioactive waste such as uranium, curium, and plutonium will adhere to the surfaces of the bacteria relatively quickly. "When this happens they presumably bond with the phosphate or carboxyl groups on the cell walls of the microorganisms," explains Henry Moll. This means that the bacteria could transport these radioactive substances throughout the underground environment and then deposit them again in very different locations. These processes have not yet been studied though. Is it possible that something similar would happen in the salt deposits from which Andrea Cherkouk has already isolated a few microorganisms? Since there isn't any oxygen inside clay or salt deposits, researchers are careful about ensuring that neither the drilling cores nor the cultures containing microorganisms come in contact with oxygen. Andrea Cherkouk has discovered a wide variety of different survival strategies among these isolated microorganisms. Some of them produce methane, others digest organic compounds and still others sulphate compounds, which are also quite abundant in stone. The archaeas living in salt also adsorb uranium, clumping together in the process. In the HZDR junior research group "MicroSalt", Andrea Cherkouk and her colleagues are currently researching how this process might influence the transport of radioactive elements through salt deposits. At home in gneiss So repository research would do well to take microorganisms into account. Not only the USA, but also the Netherlands and Poland are considering eventually establishing a repository in a salt deposit. In addition to Switzerland, Belgium and France are also investigating the possibility of storing their highly radioactive waste in clay rock. Microbiological research makes the strongest case for the creation of a repository in granite rock. There are indicators suggesting that the first final resting place for highly radioactive waste in the world, or at least in Europe, could be commissioned in the gneiss of Scandinavia. It is there, in the research repository Äspö, where Karsten Pederson has isolated the bacteria Pseudomonas fluorescens. His colleague Evelyn Krawczyk-Bärsch in Dresden is growing the microorganisms in the form of biofilm in her lab at the HZDR. These bacteria are able to cope very well with uranium and use it in their cells to form the mineral calcium uranyl phosphate. In this form the uranium is bound and is no longer free to move through the environment. Biofilm in fast forward Living together with Gallionella ferruginea in biofilms on the walls of Äspö is another bacteria. This microorganism obtains energy from the oxidation of iron(II) compounds to iron(III) compounds. Among bacteria this form of sustenance is actually quite common, since iron happens to be the fourth most common element in the Earth's crust. Once it has been produced, the iron(III) is quickly precipitated as ferrihydrite. "With the help of microorganisms this process occurs 60 times faster than it would otherwise," explains Evelyn Krawczyk-Bärsch. This yields large quantities of a rusty brown ferrihydrite sludge. A student took a closer look at this process in the HZDR labs during the course of completing her master's thesis: The bacteria first forms stalks on which little ferrihydrite beads develop. "This ferrihydrite offers a number of sites to which uranium and other radioactive elements such as neptunium can bond," as Evelyn Krawczyk-Bärsch explains the next reaction. Ferrihydrite will absorb almost all dangerous uranium(VI) and neptunium(V) compounds from a solution in this way. BACTERIA: Gallionella ferruginea lives off of iron, which precipitates as ferrihydrite after oxidation (picture taken with a scanning electron microscope).