Using carbon dioxide to maintain an elevated oleaginous microalga concentration in mixed-culture photo-bioreactors

• Mixed cultures seeded with the oleaginous, CO2-tolerant S. dimorphus were grown.
• Photobioreactors were sparged with ambient air or 20% v/v CO2 for 7 growth cycles.
• qPCR and rDNA sequencing tracked S. dimorphus abundance and microbial ecology.
• 20% CO2 condition maintained higher abundances of S. dimorphus than ambient air.
• Microalgae bioreactor parameters can be designed to control microbial ecology.

Microbial contamination of growth reactors is a major concern for microalgal biofuel production. In this study, the oleaginous, CO2-tolerant microalga Scenedesmus dimorphus was combined with a wastewater-derived microbial community and grown in replicated sequencing batch photobioreactors. The reactors were sparged with either ambient air or 20% v/v CO2. In the initial growth cycles, air and the 20% CO2 reactors were similar in terms of growth and microbial community structure. Beyond the fourth growth cycle, however, the ambient air reactors had larger decreases in cell density and growth rate, and increases in species richness and non-algal microorganisms compared to the 20% CO2 reactors. Both qPCR and rDNA sequence analyses demonstrated a greater loss in S. dimorphus enrichment in the ambient-air reactors compared to the 20% CO2 reactors. These results demonstrate that environmental parameters can be used to delay the adverse impacts of microbial contamination in open, mixed-culture microalgae bioreactors.