Carbon Mass Balance in Arthrospira platensis Culture with Medium Recycle and High CO2 Supply

Medium recycling combined with CO2 recovery helps sustainable use of the alkaline medium in Arthrospira culture. However, high CO2 supply may cause inorganic carbon accumulation and pH reduction, which could result in low CO2 recovery and reduced algal growth. This study aimed to elucidate the effect of medium recycling and high CO2 supply through carbon mass balance analysis in Arthrospira culture. In all CO2 supply conditions, carbon supply was higher than Arthrospira carbon assimilation, which accounted for 30–58% of supply. However, CO2 recovery of nearly 100% and 63% for lower (0.20 and 0.39 gC L−1 d−1) and higher (0.59 gC L−1 d−1) CO2 supply rates were achieved, respectively, because of the high concentration of the alkaline agent. The excess carbon accumulated in the medium and ultimately escaped from the system in a form of dissolved inorganic carbon (DIC). Dissolved organic carbon (DOC) contributed to 16–24% of the total photosynthetically assimilated carbon, and the final concentration reached 260–367 mgC L−1, but there was no significant growth reduction caused by DIC and DOC accumulation. This study demonstrated the stability of the medium-recycling process even at high CO2 supply rates although a balanced supply is recommended for longer operations.