Polyhydroxybutyrates Production by Azospirillum brasilense as a Microalgal Growth Promotion Trait During CO₂ Fixation...

Polyhydroxybutyrate (PHB) is a compound produced by the Azospirillum genus to ensure its adaptation and survival in stressful conditions. Particularly, the PHB production by Azospirillum brasilense is related to improving colonization and promoting plant growth during its interaction with plants. Nowadays, this bacterium is considered a suitable partner to improve microalgal performance during the biogas upgrade process-an effluent with a stressful composition (25% CO₂-75% CH₄) for this bacterium. Thus, the present study evaluates changes in gene expression and PHB production by A. brasilense associated with Chlorella sorokiniana microalga, as well as the A. brasilense inoculum pre-enriched PHB effect on C. sorokiniana physiological performances during CO₂ biogas fixation. The results demonstrate that A. brasilense recorded 189 differentially expressed genes (DEGs) associated with this microalga at day 3 of interaction, when genes related to carbon and energy metabolism were mostly expressed. Furthermore, biogas increased PHB production in this bacterium (23.8 ± 0.23% dry weight; DW). While the A. brasilense-C. sorokiniana interaction stimulated an increment in the PHB content of this bacterium (89.5 ± 5.8 DW), microalgal growth was not enhanced but only protein production and nitrogen consumption improved. Similarly, the pre-enriched PHB A. brasilense cells increased the CO₂ fixation rate (216%) and the N consumption rate (16%) in C. sorokiniana. Overall, the present study demonstrates that PHB accumulation is constant in A. brasilense during its interaction with this microalga strain and could be considered an important microalga growth promotion trait during the biogas upgrading process.