The coupling of primary piggery wastewater as a culture medium with elevated CO2 aeration is thought to be an economically feasible option for the cultivation of microalgae. However, little information is available regarding the photosynthetic characteristics of microalgae and nutrient removal from wastewater at different CO2 concentrations. It was found that elevated CO2 aeration provided sustained growth at CO2 concentrations ranging from 5% to 15% and performed best with 5% CO2 aeration in primary piggery wastewater for Chlamydomonas reinhardtii growth. Photosynthesis, respiration, and nutrient uptake (total nitrogen and total phosphorus) were stimulated in response to CO2 enrichment, thus increasing nutrient uptake in primary piggery wastewater, particularly total nitrogen and total phosphorus. A study of carbon-concentrating mechanism-related gene expression revealed that the levels of mRNAs, such as CAH1, LCIB and HLA3, were significantly downregulated. This represents a possible method for the reconciliation of CO2-stimulated growth with mixotrophic cultivation of C. reinhardtii in diluted primary piggery wastewater.

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