Active indole-3-acetic acid biosynthesis by the bacterium Azospirillum brasilense cultured under a biogas atmosphere enables its beneficial association with microalgae

Aims This study assessed, at the physiological and molecular levels, the effect of biogas on indole-3-acetic acid (IAA) biosynthesis by Azospirillum brasilense as well as the impact of this bacterium during CO2 fixation from biogas by Chlorella vulgaris and Scenedesmus obliquus. Methods and Results IpdC gene expression, IAA production and the growth of A. brasilense cultured under air (control) and biogas (treatment) were evaluated. The results demonstrated that A. brasilense had a better growth capacity and IAA production (105.7 +/- 10.3 mu g ml(-1)) when cultured under biogas composed of 25% CO2 + 75% methane (CH4) with respect to the control (72.4 +/- 7.9 mu g ml(-1)), although the ipdC gene expression level was low under the stressful condition generated by biogas. Moreover, this bacterium was able to induce a higher cell density and CO2 fixation rate from biogas by C. vulgaris (0.27 +/- 0.08 g l(-1) d(-1)) and S. obliquus (0.22 +/- 0.08 g l(-1) d(-1)). Conclusions This study demonstrated that A. brasilense has the capacity to grow and actively maintain its main microalgal growth-promoting mechanism when cultured under biogas and positively influence CO2 fixation from the biogas of C. vulgaris and S. obliquus. Significance and Impact of the Study These findings broaden research in the field of Azospirillum-microalga interactions and the prevalence of Azospirillum in environmental and ecological topics in addition to supporting the uses of plant growth-promoting bacteria to enhance biotechnological strategies for biogas upgrading.