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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 CO 2 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 μg ml −1 ) when cultured under biogas composed of 25% CO 2 + 75% methane (CH 4 ) with respect to the control (72.4 ± 7.9 μg 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 CO 2 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 CO 2 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 growthpromoting bacteria to enhance biotechnological strategies for biogas upgrading.