Auxin-dependent alleviation of oxidative stress and growth promotion of Scenedesmus obliquus C1S by Azospirillum brasilense

There is currently an increasing interest in the use of microalgae for wastewater treatment and the use of its biomass as a feedstock for biofuels. Both of these applications are often performed more efficiently by microalgal-bacteria consortia. However, the mechanisms that account for the stability and robustness of this kind of interactions are poorly understood. In this study, we confirmed the growth promotion activity of the plant growth-promoting bacterium Azospirillum brasilense Sp245 on the microalgae Scenedesmus obliquus C1S. We show that this activity is critically dependent on bacterial indole-3 acetic acid (IAA) production, which results in a decrease in algal reactive oxygen species (ROS) levels, higher cell densities and ameliorates algal cells bleaching after nitrogen deprivation. We also show a close inter-species interaction between both partners and an active expression of the bacterial ipdC gene involved in production of IAA when co-cultivated. This study extends the current knowledge of the mechanisms underlying bacteria-microalgae consortia to improve their technological applications and to better understand ecological relationships in the environment.