Influence of biofilms on the water repellency of urban soil samples

Water repellency is an important phenomenon in soil systems and is influenced by physical, chemical and biological factors. Studies on the influence of bacteria or surface-attached bacteria (biofilms) on soil water repellency are rare. In this study, we investigated the influence of hydrophilic and hydrophobic bacteria on soil wettability. Three different soil bacteria, Variovorax paradoxits, Bacillus sphaericus and an alpha-Proteobacterium, were investigated in different states (vegetative cells and spores in the case of Bacillus sphaericus). The bacteria, isolated from urban soils in the Berlin Tiergarten Park and from a sewage field (in Berlin-Buch), were grown in a bioreactor on sterilized soil samples and in batch cultures on four different synthetic materials with hydrophobic and hydrophilic surfaces, to form biofilms. Surface hydrophobicity of the overgrown material was determined by the water contact angle, and cell surface characteristics of bacteria were measured using the zeta potential and a hexadecane-two-phase-sy stem. The alpha-Proteobacterium and Variovorax paradoxus were classified as hydrophobic, and Bacillus sphaericus was classified as hydrophilic. Contact angles of the overgrown synthetic materials showed a significant influence of the respective material, but differences between the bacteria were not significant. The differences between the materials may be due to effects of the material on biofilm growth or EPS properties. EPS form biofilm matrices and are mainly responsible for aggregate cohesion. The bacterial EPS of the hydrophobic alpha-Proteobacterilan were more hydrophilic than the cell walls, while those of Bacillus sphaericus were less hydrophilic. In contrast to the artificial materials, differences in contact angle for the inoculated soil sample showed significant differences between the bacterial strains. The a-Proteobacterium was able to hydrophobize the soil sample, while Bacillus sphaericus showed significant hydrophilization. The results clearly demonstrate the effect of bacterial biofilms on soil wettability. Copyright (c) 2007 John Wiley & Sons, Ltd.