Evaluating the feasibility of using lignin–alginate beads with starch additive for entrapping and releasing Rhizobium spp.

Cell immobilization in polymers have proven successful in protecting the nitrogen-fixing bacteria Rhizobium. This study evaluated the feasibility of using lignin to develop lignin–alginate beads with a starch additive to immobilize and release Rhizobial cells. A lignin–alginate hydrogel was synthesized and cultured at different concentrations with 1 ml inoculum of Rhizobium meliloti and Rhizobium leguminosarum to determine the hydrogel's compatibility with the Rhizobium spp. The Rhizobium cells (3 ml inoculum) were then entrapped into the lignin–alginate beads (ratio of 2 g lignin to 1 g alginate) with starch additive and their entrapment efficiency, cell release and surface morphology investigated. The results suggest concentration of the lignin–alginate hydrogel had no effect on the survival of Rhizobium cells with time. Dried lignin–alginate beads increased the survival of Rhizobium cells from 61% to 73% while dried lignin–alginate beads with starch additive increased the survival of Rhizobium cells from 61% to 84% compared to only alginate beads. Light microscopy suggests alginate beads lost their sphericity without lignin and starch additive while fixed SEM images highlighted Rhizobium cells attached to starch granules. Overall, the results indicate the potential applicability of lignin as a component for the manufacture of carrier materials for entrapping Rhizobial cells.