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 (gamma)-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 gamma to a minimum (gamma(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 (theta) of soil microcosms was significantly lowered (0.78 x) 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 theta 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.