Geomicrobiologists from the Centre for Applied Geoscience at the University of Tübingen led by Prof. Andreas Kappler, in cooperation with researchers from the University of Wisconsin (USA), the Federal Institute for Materials Research and Testing (BAM) and the Humboldt University in Berlin, have been able to add an important piece of information that further clarifies the existing state of knowledge of the respiration mechanisms in soil. They are the first in the world to show that microorganisms are able to use solid-phase organic soil particles, so-called humic substances, to replace oxygen when grown in the absence of oxygen. The humic substances are able to act as electron shuttles, thus serving as a medium for the flow of electricity. The scientists’ research results have been published in the journal Nature Geoscience.
Aerobic respiration is the release of energy from organic substances in the presence of oxygen. Electrons are released during the degradation of organic substances and are transferred to oxygen that is taken up by the cells and converted into water. It is easy to visualise this process by breathing on glass or a window, and watching the newly formed water condense. Some microorganisms are able to breathe under oxygen-deficient conditions by transferring electrons to nitrate or sulphate, rather than to oxygen. This leads to the release of nitrogen, or foul smelling hydrogen sulphide, a process that also takes place in individual compost heaps.
Prof. Andreas Kappler and his team of researchers at the University of Tübingen, in cooperation with researchers from the University of Wisconsin (USA), the Federal Institute for Materials Research and Testing (BAM) and the Humboldt University in Berlin, have been able to show that bacteria that occur in soil and sediments have another important reservoir of substances to which electrons can be transferred. These bacteria are able to transfer electrons to solid-phase material, so-called humic substances, which are produced from deposited, dying plants and other organisms. They represent the largest proportion of organic carbon in soil and sediments. Microorganisms are able to reduce such humic substances by using them as a replacement for oxygen to which the electrons are transferred under aerobic conditions. What makes this process even more important is the fact that the humic substances do not retain the electrons, but transfer them to iron minerals contained in soil and sediments, thereby functioning as an electron bridge or electron shuttle between bacteria and iron minerals. Since the transfer of electrons is nothing more than the flow of electricity, the humic substances thus transfer electricity from the bacteria to the iron minerals. Such processes were previously only known from dissolved organic compounds, and not from solid-phase organic soil particles.
Prof. Dr. Andreas KapplerUniversity of TübingenGeomicrobiology Work GroupCentre for Applied GeoscienceSigwartstraße 1072076 TübingenTel.: + 49 (0) 70 71/2 97 49 92Fax: +49 (0) 70 71/29 50 59E-mail: andreas.kappler (at) uni-tuebingen.de