The capacity of microorganisms to rapidly sense and adapt to environmental changes is an important factor for the stability of microbial ecosystem services, such as contaminant biodegradation in soil (de Lorenzo, 2008; Miller et al., 2009). Marx and Aitken (2000), for instance, have shown that bacterial chemotaxis, i.e. the ability to sense and move along chemical concentration gradients, enhances the biodegradation of naphthalene, a polycyclic aromatic hydrocarbon (PAH), in bioavailability-limited heterogeneous systems (for a review: Harms and Wick, 2006; Miller et al., 2009). Bacterial motility and chemotaxis in porous media, however, are still a field full of unknowns and experimental evidence for its importance is limited (Ford and Harvey, 2007). The efficiency of bacterial chemotaxis in bioremediation strongly depends on the effective mobility of bacteria within the system. As bacteria require high soil matric potentials or at least continuous liquid films for swarming and swimming (Or et al., 2007), directed chemotactic dispersal is restricted in water-unsaturated environments. This may affect the bioaccessibility of patchy, hydrophobic organic compounds (Semple et al., 2007), as average distances between bacterial microcolonies in soil are supposed to be in the range of 10−4 m (Bosma et al., 1997). Microbial dispersal and substrate mobilization are hence needed to overcome the distance between substrates and organisms. The strategy of filamentous fungi is to enlarge their external surface and to develop mycelia of high fractal dimension that optimally exploit the three-dimensional space containing the substrate (Nakagaki et al., 2004). Contrary to bacteria, the habitat of fungi is not restricted to water films. Fungal hyphae easily breach through air–water interfaces and form dense networks of up to 20 000 km length per cubic meter of soil (Pennisi, 2004). Importantly, by doing so they connect saturated and unsaturated soil pores (Wessels, 1997). Several reports on the role of fungal mycelia on ‘underground networking’ for nutrient translocation and provision to bacteria in the hyphosphere (Bending et al., 2006) and shaping of communities above and below the earth's surface (Whitfield, 2007) have been published. As both bacteria and fungi are important degraders of (anthropogenic) organic substances in soil, fundamental knowledge of bacteria–fungus interactions is also essential for the development of novel bioremediation approaches based on ecological principles. For instance, it has been shown that liquid films developing around hydrophilic fungi can be used by PAH-degrading bacteria to enhance their mobility in such a way that PAH-biodegradation in unsaturated soil is enhanced (Kohlmeier et al., 2005; Wick et al., 2007).
In the frame of our attempts to develop novel bioremediation approaches based on ecological principles we tested the hypothesis that fungal networks facilitate the movement of bacteria by providing continuous liquid films in which gradients of chemoattractants can form and chemotactic swimming can take place.