PHB-Producing Azospirillum brasilense Boosts Microalgal Performance During Biogas CO₂ Fixation

**New Research Uncovers How PHB-Producing Azospirillum brasilense Strengthens Microalgal CO₂ Fixation Under Biogas**

A recently published study has demonstrated that Azospirillum brasilense—a well-known plant growth-promoting bacterium—also functions as a powerful microalgal growth-promoting partner during biogas upgrading. The research reveals how the bacterium produces polyhydroxybutyrate (PHB) in response to biogas stress and how this trait enhances its interaction with the microalga Chlorella sorokiniana.

The study evaluated PHB production, gene expression changes, and metabolic impacts during co-cultivation under a biogas atmosphere containing 25% CO₂ and 75% CH₄. Results show that biogas exposure substantially increased PHB accumulation in A. brasilense, reaching 23.8% of its dry biomass. During co-culture with C. sorokiniana, this PHB content increased even further.

After three days of interaction under biogas, A. brasilense exhibited 189 differentially expressed genes, primarily associated with carbon and energy metabolism. This suggests that PHB accumulation—not just as a bacterial survival mechanism—plays a functional role in supporting stress adaptation during microalgal interaction.

While microalgal growth did not increase during co-culture, important metabolic enhancements were observed. Protein production and nitrogen consumption improved in C. sorokiniana when paired with PHB-producing A. brasilense. The effect became even more pronounced when the bacterium was pre-enriched with PHB before inoculation. In that condition, microalgae displayed a 216% increase in CO₂ fixation and a 16% rise in nitrogen consumption.

The findings indicate that PHB-rich A. brasilense cells provide stability and metabolic support to microalgae under harsh biogas conditions. This positions PHB as a key growth-promotion trait, with potential applications in biogas upgrading, biofuel production, and broader microbial-algal biotechnologies.

Reference

Contreras, C.A., Palacios, O.A., Garciglia-Mercado, C. et al. Polyhydroxybutyrates Production by Azospirillum brasilense as a Microalgal Growth Promotion Trait During CO₂ Fixation from Biogas. Curr Microbiol 82, 569 (2025). https://doi.org/10.1007/s00284-025-04534-1