Water Status of Rhizobia in Relation to their Susceptibility to Desiccation and to their Protection by Montmorillonite

Abstract

The greater susceptibility to desiccation of the Rhizobiurn legurninosarum group of rhizobia compared with the slow-growing rhizobia is related to the different amounts of water retained by these groups of bacteria at any relative vapour pressure, rather than differences in rates of water movement into or out of the bacteria. The higher retention of water by the R. legurninosarum group at any relative vapour pressure is related to greater availability of adsorptive surface area, and to higher surface energies resulting in a greater affinity for water. Although the internal osmotic pressure is greater in the slow-growing group than in the R. leguminosarum group, it is concluded that differences in internal solute concentrations cannot account for the different adsorption isotherms found for these groups.Montmorillonitic clay protects the fast-growing R. leguminosarum group from the effects of desiccation, but not the slow-growing group. Ca-montmorillonite retains greater quantities of water at any relative vapour pressure than either group of bacteria. Possible mechanisms of protection of the bacteria by the clay are discussed in terms of relative affinities for water under desiccation conditions.

Bacterial and Fungal Contributions to Carbon Sequestration in Agroecosystems

“…Bentonite clay, which includes montmorillonite and other smectite group minerals, protected Rhizobium from predation by protozoa in a liquid culture and in soil amended with the clay . Bacterial growth and activity were higher in the presence of montmorillonite clays compared with kaolinite clays and oxides (Bitton et al, 1976;Bushby and Marshall, 1977;Stotzky and Rem, 1966). This differential effect is related to the greater cation exchange capacity and surface area of 2:1 clay particles (Stotzky, 1966), which presumably confer greater protection against desiccation (Bitton et al, 1976;Bushby and Marshall, 1977) than 1:1 clays and oxides.…”

“…Several mechanisms of protection of microbial biomass in soils have been proposed: (i) clays promote microbial growth by maintaining the pH in the optimal range (Stotzky and Rem, 1966), (ii) clays adsorb metabolites inhibitory to microbial growth (Martin et al, 1976), (iii) clay-microbe interactions protect the organisms against desiccation (Bitton et al, 1976;Bushby and Marshall, 1977), and (iv) microbes reside in small pores in which they are protected against predation by higher trophic groups, notably protozoa (Elliott et al, 1980;Rutherford and Juma, 1992). Since all of these mechanisms are directly or indirectly related to the reactive surface properties of clays, the protective capacity of soils for microbial biomass has often been related to clay Fig.…”

“…The influence of clay type has received much less attention than clay content, and the few studies that have been done have most often been conducted under in vitro conditions with single bacterial species (Bitton et al, 1976;Bushby and Marshall, 1977). Bentonite clay, which includes montmorillonite and other smectite group minerals, protected Rhizobium from predation by protozoa in a liquid culture and in soil amended with the clay .…”

The Physiological Mechanisms of Desiccation Tolerance in Rhizobia

“…Previous studies have shown that slow-growing rhizobia such as CB1809 are less susceptible to desiccation stress than the fast-growing strains of rhizobia such as TA1 (Bushby and Marshall, 1977b;Deaker et al, 2007). This may explain the lack of significant improvement in survival of CB1809 as this strain of rhizobia has an inherently greater capacity for desiccation tolerance than TA1.…”

“…Previous studies have shown that slowgrowing rhizobia such as bradyrhizobia are less susceptible to desiccation stress than the fastgrowing strains of rhizobia such as R. leguminosarum sp. (Bushby and Marshall, 1977b;Deaker et al, 2007).…”