Increased growth of the microalga Chlorella vulgaris when coimmobilized and cocultured in alginate beads with the plant-growth-promoting bacterium Azospirillum brasilense

Abstract

Coimmobilization of the freshwater microalga Chlorella vulgaris and the plant-growth-promoting bacterium Azospirillum brasilense in small alginate beads resulted in a significantly increased growth of the microalga. Dry and fresh weight, total number of cells, size of the microalgal clusters (colonies) within the bead, number of microalgal cells per cluster, and the levels of microalgal pigments significantly increased. Light microscopy revealed that both microorganisms colonized the same cavities inside the beads, though the microalgae tended to concentrate in the more aerated periphery while the bacteria colonized the entire bead. The effect of indole-3-acetic acid addition to microalgal culture prior to immobilization of microorganisms in alginate beads partially imitated the effect of A. brasilense. We propose that coimmobilization of microalgae and plant-growth-promoting bacteria is an effective means of increasing microalgal populations within confined environments.

FIG. 1

Multiplication of C. vulgaris within alginate beads under batch and continuous growth conditions. Points on each curve denoted by a different lowercase letter differ significantly at P ≤ 0.05 in one-way ANOVA. Points denoted by different capital letters after each day of incubation differ significantly at P ≤ 0.05 in Student's t test. The experiment was repeated five times, and the results presented are from a representative experiment. Bars represent the standard error (SE). When the SE bar is absent, the SE is smaller than the point.

FIG. 2

Multiplication of C. vulgaris in the periphery and in the interior of alginate beads under continuous culture conditions. Points on each curve denoted by a different lowercase letter differ significantly at P ≤ 0.05 in one-way ANOVA. Points denoted by different capital letters after each day of incubation differ significantly at P ≤ 0.05 in Student's t test. The experiment was repeated five times, and the results presented are from a representative experiment. Bars represent the SE. When the SE bar is absent, the SE is smaller than the point.

FIG. 3

Multiplication of C. vulgaris and A. brasilense coimmobilized in the same alginate bead under continuous culture conditions. (A) Total number of cells; (B) number of alga clusters in the periphery; (C) number of alga clusters in the interior of the bead; (D) number of cells per cluster in the periphery; (E) number of cells per cluster in the bead interior. Points on each curve denoted by a different lowercase letter differ significantly at P ≤ 0.05 in one-way ANOVA. Points denoted by different capital letters after each day of incubation differ significantly at P ≤ 0.05 in Student's t test. The experiment was repeated five times, and the results presented are from a representative experiment. Bars represent the SE. When the SE bar is absent, the SE is smaller than the point.

FIG. 4

The effect of addition of IAA (10⁻⁴, 10⁻⁵, and 10⁻⁶ M) on C. vulgaris growth in batch culture. Points on each curve denoted by a different lowercase letter differ significantly at P ≤ 0.05 in one-way ANOVA. Points denoted by different capital letters after each day of incubation differ significantly at P ≤ 0.05 in Student's t test. Bars represent the SE. When the SE bar is absent, the SE is smaller than the point.