THE ROLE OF BACTERIAL SURFACE-CHARGE IN THE ECOLOGY OF ROOT-NODULE BACTERIA - AN HYPOTHESIS

In an attempt to discover some characteristics of soil bacteria that may be of significance in their adaptation to soils, the electrophoretic mobilities (EPM) of Bradyrhizobium spp and Rhizobium spp have been measured and examined for correlations with several characteristics of the soils from which they were isolated. The magnitude of the EPM values of bradyrhizobia isolated from soils with clay contents that ranged from 4 to 70% were significantly correlated with the soil pyrophosphate-extractable Fe content (Fep) and with soil pH provided that the soil from which the strains were isolated had a clay content that was ≫: 20%. Multiple regression equations accounted for up to 84% of the variance for bradyrhizobia but there were no correlations with the EPM of the fast-growing Rhizobium spp. All rhizobia and most bradyrhizobia were net negatively-charged at the pH of the test soluton (5.5-6.0), however some Bradyrhizobium spp were net positively-charged. Invariably these isolates originated from soils that had a low pH or a high Fep or both. Postively-charged bradyrhizobia were only isolated from soils with clay contents that were < 20% or ≫: 40%. None were recovered from soils with clay contents between 20-40%. These results support the hypothesis that the surface charge characteristics of bradyrhizobia are matched to the soil from which they originated. The possible significance of these findings for both the ecology of root-nodule bacteria and as a tool for predicting the suitability of inoculum strains to particular soil types is discussed. © 1990.