SURVIVAL OF ALGINATE-ENCAPSULATED PSEUDOMONAS-FLUORESCENS CELLS IN SOIL

Alginate-encapsulated and unencapsulated cells of Pseudomonas fluorescens R2f were introduced into soil microcosms with and without wheat plants to evaluate bacterial survival and colonization of the rhizoplane and rhizosphere. Encapsulation of cells in alginate amended with skim milk or with skim milk plus bentonite clay significantly enhanced long-term survival of the cells. There was a negligible effect on long-term bacterial survival when cells were encapsulated in alginate amended with TY medium or soil extract, as compared to water. Drying of beads resulted in a significant reduction in bacterial viability. After addition to soil, cells in dried beads increased in numbers and exhibited stable population densities, whereas cells added in moist beads showed stable dynamics at a higher level. Cells encapsulated in dried beads or fresh beads survived better than unencapsulated cells added to soil. Both cells in moist and dried alginate beads also survived a dry/wet cycle in soil, whereas unencapsulated cells were sensitive to these moisture fluctuations. Shortly after inoculation and 63 days after this, cells from moist beads colonized wheat roots at significantly higher levels than unencapsulated cells, whereas cells in dried beads did so at levels similar to unencapsulated cells. Cells in beads initially placed at different distances from a developing root mat were able to move towards and colonize the rhizosphere, at levels of roughly 10' to 10' colony-forming units of P. fluorescens R2f per gram of dry soil.