Surfactants expressed by soil pseudomonads alter local soil-water distribution, suggesting a hydrological role for these compounds

Several biological roles have been demonstrated for surfactants expressed by soil and rhizosphere Pseudomonas spp., but the impact of these powerful surface-active agents on the local soil–water distribution within the partially saturated soil pore network has not been examined. To investigate this potential hydrological role, the liquid surface tension ()-reducing activities (LSTRA) of 72 pseudomonads isolated from a sandy loam soil by tensiometry of culture supernatants were characterized. Of these, 67% exhibited LSTRA, reducing to a minimum (Min) of 24 mN m−1 established by individual distribution identification analysis. Soil microcosms were then used to examine the impact of surfactant expression on the local soil–water distribution. The volumetric water content (θ) of soil microcosms was significantly lowered (0.78 ×) by Pseudomonas fluorescens SBW25 expressing the surfactant viscosin compared with a surfactant-deficient mutant (P<0.002). Six of 15 soil pseudomonad isolates examined were found to have a similar impact on θ when compared with sterile microcosms (P<0.05). These findings indicate that surfactant-expressing pseudomonads could modify the local soil–water distributions and that surfactants may therefore play a significant hydrological role in soils, in addition to their recognized biological activities.