Riboflavin and lumichrome exuded by the bacterium Azospirillum brasilense promote growth and changes in metabolites in Chlorella sorokiniana under autotrophic conditions

Abstract

The effects of riboflavin and its degradation compound, lumichrome, produced by Azospirillum brasilense on the growth and production of carbohydrates, lipids, and pigments in Chlorella sorokiniana were analyzed. A. brasilense is able to produce and release riboflavin from the first 24h of incubation in Synthetic Growth Medium (SGM). Synchronic peaks of lumichrome and riboflavin suggest that the former originates from the photodegradation of the latter. The constant growth of bacterial population indicated that riboflavin and lumichrome are cell density-independent. In this study, riboflavin showed a higher effect on the population growth of Chlorella than did lumichrome. The effects of synthetic riboflavin and lumichrome on microalgae were dependent on the concentration of these compounds in the culture media. Lower concentrations of riboflavin (1ngmL−1 or 10ngmL−1) enhanced the growth of C. sorokiniana, while the highest amount of riboflavin (100ngmL−1) showed no effect. Synthetic riboflavin and lumichrome significantly increased the production of carbohydrates compared to the basal medium. Lumichrome showed the highest production of carbohydrates at 48h while riboflavin at 96h. Pigments were significantly more affected by the presence of lumichrome than by riboflavin. Enhanced growth and production of carbohydrates and pigments in C. sorokiniana grown in exudates of A. brasilense, suggests a synergistic effect of the riboflavin and lumichrome produced and exuded by this bacterium.