Comparison of algae growth and symbiotic mechanisms in the presence of plant growth promoting bacteria and non-plant growth promoting bacteria

Culturing algae with bacteria has been explored as a means of promoting algal growth and altering algal biochemical composition. Plant growth promoting bacteria (PGPB) are of particular interest given their ability to produce phytohormones that are bioactive in microalgae. However, many non-plant growth promoting bacteria also promote algal growth. The objective of this study was to test the benefits of a model PGPB versus non-PGPB in a side-by-side study, evaluating improvement in algal growth rates and impacts on biomass composition. Three green microalgae strains were tested with three strains of bacteria: Azospirillum brasilense (a model PGPB), Escherichia coli (a non-PGPB), and Bacillus megaterium (a potential PGPB). A. brasilense produces the auxin phytohormone indole-3-acetic acid. A. brasilense promoted growth in two of the three algae strains by 39-69% but did not promote growth in Chlorella sorokiniana (strain UTEX 2805). E. coli promoted growth in all three algae strains by 16-64%. That the non-PGPB, E. coli, performed as well as the PGPB, A. brasilense, suggests the importance of symbiotic mechanisms that are ubiquitous among many bacteria. One such mechanism, cell-to-cell O-2-CO2 exchange, may have been important given significant consumption of algal photosynthate by bacteria. B-vitamin exchange is another widespread mechanism of algal-bacterial symbiosis. The riboflavin metabolite, lumichrome, was detected in low concentrations in co-cultures of A. brasilense (0.4-0.6 ng/mL) and E. coli (5.5-13 ng/mL). These concentrations of lumichrome were found to provide small but statistically significant growth benefits for C. sorokiniana UTEX 2805 and Auxenochlorella protothecoides.