Underground "highways" discovered for bacteria

Mushroom roots make it easier for the microbes to pass through the soil

Acquisition of the soil fungus Fusarium oxysporum © UFZ
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Mushroom roots play a greater role in spreading bacteria in the soil than previously thought. For the first time scientists have been able to prove that bacteria on the mucous membrane can migrate from living fungi through the soil. The investigations could help to clean up contaminated areas faster with the help of bacteria that reduce pollutants.

It looks like a giant green ball of yarn. With a little imagination, the photo reminds of a huge intersection with countless stratified roads and junctions. What the Leipzig microbiologist Lukas Y. Wick looks at intensively on his screen, however, is in reality the image of a fungal plexus with a confocal laser scanning microscope. The fine veins have a diameter of just ten microns, equivalent to only one-seventh of a human hair. Nevertheless, mushrooms are one of the largest biomass producers in the world. One gram of arable soil can contain up to 100 meters of fungus.

Soil microbes as pollutant-eaters

Wicks actual study objects are even smaller. He is interested in the bacteria in the soil. Bacteria can weaken the human organism - but they can also be useful helpers, for example by reducing pollutants. "For the bacterium, the pollutant is not a pollutant, " says Wick. "It simply decomposes the carbon compound, generating the energy and the materials it needs to live."

But they have to first come to their "food". Air or lack of moisture is an insurmountable obstacle. "That's why certain pollutants are so poorly degraded in the soil, and it's often not a lack of biochemical capacity but a lack of contact." That's why the scientists from the Helmholtz Center for Environmental Research study the migration pathways of bacteria.

Probably the largest motorway network in the world

A kind of highway for bacteria seems to be mushroom patches in the ground. The researchers around Lukas Wick come to this conclusion. In the laboratory experiment, they succeeded in demonstrating that the bacteria move through the soil on the fungal network. The ingredients: a pollutant, separating layers of glass balls, unloaded soil and a bacterium called Pseudomonas putida. Through all these layers, the bacteria had to fight through to get their "feed" phenanthrene. display

This polycyclic aromatic hydrocarbon is a common pollutant that appears in every combustion process: at gas stations, in car exhaust, in forest fires, in cigarette smoke or in old city gas works. "We deliberately let the bacteria go up against gravity, so you can not say it could be a bit of water that runs down and carries the bacteria, " says Lukas Wick. "We have tried to exclude all doubts and objections of any critics."

The bacteria made it to the top only where a fungus weave pulled through the ground. In the identical parallel experiment without fungus, however, the bacteria could not overcome the barriers. "With this article, we showed that there is an infrastructure."

Just follow your nose

The bacteria in this laboratory experiment are so-called chemotactic bacteria. That is, they measure the concentration of their "target chemical" and then move towards higher concentration - as with an autopilot. "The bacterium is just not the stupid thing but has adapted to its environment and goes to where there is food." In the model test, only one bacterial species was used. In nature, there are innumerable different ones and with that new questions arise: For whom does it have an advantage to be mobile and for whom not?

It will take some time until the processes in the soil are completely understood. The aim of the Helmholtz researchers is to present microbial landscapes in the future and to investigate what happens under what influences. A tool will be used, which has also helped to predict the spread of rabies or the spread of resettled animals.

In the future, ecological modeling can also provide prognoses for the distribution of bacteria. This knowledge makes it easier to clean up polluted areas. The "mushroom highway" is then perhaps not only the largest in the world but also the only one that helps nature return to its original state.

(Helmholtz Center for Environmental Research - UFZ, 09.02.2007 - NPO)