Carbon dioxide-assisted enhancement of microalgae growth and pollutant removal in piggery wastewater by newly-isolated ammonia-tolerant microalgae Chlorella sorokinfana

Livestock and poultry breeding produces a large amount of carbon dioxide (CO₂) and wastewater with high concentrations of ammonia‑nitrogen (NH₃−N), such as piggery wastewater (PW). Whether CO₂ can promote microalgae growth and pollutant removal is promising in the green and sustainable treatment of PW. Thus, this study isolated an ammonia-tolerance microalgae species from the PW, which was used to find optimal CO₂ aeration concentration in the microalgae-based PW treatment. The isolated species was identified as Chlorella sorokinfana (genetic similarity of 100 %). The optimal 20 % CO₂ addition could provide carbon sources and balance pH, when compared with the control group within nine days, the growth rate and chlorophyll a of the isolated microalgae reached 6.00 × 10⁷ cells/mL/d and 8.79 mg/L, which considerably increased by 113 % and 79 %, respectively (p < 0.05); the removal efficiencies of chemical oxygen demand, dissolved organics, total nitrogen, NH₃-N, and total phosphorous were significantly increased from 42 %, 80 %, 23 %, 28 %, and 34 % to 70 %, 88 %, 66 %, 72 %, and 99 %, respectively (p < 0.05). Differential expression genes were also the highest in the 20 % CO₂ vs the control groups. Based on the enrichment analysis, 20 % CO₂ aeration upregulated ribosome biogenesis and nitrogen metabolism, thus promoting microalgae growth and protein synthesis.