Indole-3-acetic acid from Azosprillum brasilense promotes growth in green algae at the expense of energy storage products

Plant growth-promoting bacteria, such as Azospirillum brasilense, have the potential to significantly increase algal growth rates through a variety of mechanisms including the production of indole-3-acetic acid, an auxin hormone. A. brasilense promotion of growth in Chlorella sorokiniana is well-established for co-cultures suspended in alginate bead structures, however, its impacts on other types of green algae grown in suspended cultures is not well-understood. The objective of this research was to determine the impact of A. brasilense and indole-3-acetic acid on growth promotion and energy storage product accumulation in suspended cultures of C. sorokiniana and Auxenochlorella protothecoides. Suspended cultures were grown in lab-scale photobioreactors under the following conditions: algae grown on chemical medium (control), in co-culture with A. brasilense, chemical medium supplemented with exogenous indole-3-acetic acid, and algae grown on spent medium from A. brasilense. The results showed that co-cultures and exogenous indole-3-acetic acid stimulated growth in both algae types but the effect was stronger in C. sorokiniana. These same treatments also suppressed neutral lipids (particularly triacylglycerol) and starch during exponential growth of C. sorokiniana. Indole-3-acetic acid and co-cultures suppressed starch in A. protothecoides. Spent medium resulted in slight growth promotion in C. sorokiniana but significant growth suppression in A. protothecoides. It also led to significantly different compositional changes compared to using live A. brasilense, indicating that bioactive constituents in A. brasilense secretions are transient or that physical cell attachment is important for ensuring adequate mass transfer of these constituents. Overall, the findings suggest that indole-3-acetic acid and live A. brasilense mobilize cellular energy resources for growth. © 2020 Elsevier B.V.