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

Abstract

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 μg ml^-1 ) when cultured under biogas composed of 25% CO₂ + 75% methane (CH₄ ) 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 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.

Keywords: Azospirillum brasilense; biogas; indole-3-acetic acid; ipdC gene expression; microalgae; tryptophan.