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Soil around power stations, chemical plants, oil refineries, airports, highways and gas stations is usually polluted with so-called polycyclic aromatic hydrocarbons (PAH). They can be found in oil, coal and tar deposits and are produced as byproducts of fuel burning (either fossil fuel or biomass). As a pollutant, they are of concern because some compounds have been identified as carcinogenic, mutagenic and teratogenic. Special bacteria that feed on such pollutants can theoretically be used to cleanse the soil of PAH, but in reality such a method does not work. The soil consists of billions of pores filled with air and water. The PAH clot in air pores and do not dissolve in water. Bacteria feeding on PAH live only in water pores. So, what can be done to feed them with pollutants?
Scientists at the Helmholtz Centre for Environmental Research suggested a creative way of solving the problem. Fungi and a ubiquitous soilborne pathogen, which cause damping-off and root rot on plants called Pythium ultimum, can transmit PAH to bacteria. Its mycelium is found throughout the soil with massive braiding of fine threads (one single gram of soil may contain up to 1,000 – 10,000 meters of fungal threads).
Only a few years ago researchers discovered that bacteria travel over the fungal threads through a labyrinth of soil pores, much the same as vehicles on a highway. This means that now Pythium ultimum can be used as pipelines delivering PAH to bacteria.
Researcher Lukas Y. Wick, in charge of the new study, told Technology Review that scientists were stunned with the speed of this process. In their experiments scientists placed Pythium ultimum into a Petri dish intended to simulate the air-filled pores in the ground. Researchers applied a polycyclic aromatic carbohydrate called Phenanthrene to a part of it and then investigated at regular intervals whether this substance could be detected in other areas of the test path. Within a few hours the carbohydrate had migrated from one end of the experimental dish to the other – ten to one hundred times faster than it could have by simple diffusion. Furthermore, it overcame the air gaps with no difficulty, which was not possible over the same path without hyphal networks. Experiments with other types of the PAH were also successful – the smaller the molecules of substances the faster fungi transported them. Now scientists are planning to test such a method of soil cleaning «in the field». They still have to study the safety of such a method, as pythium ultimum is detrimental to plants.Printable version