Inhibition Effects of Free Ammonia (FA) on the Rates of Growth, Photosynthesis and Respiration of Chlorella vulgaris

The potential use of microalgae in advanced biological wastewater treatment processes has been actively studied due to their excellent nitrogen and phosphorus removal capabilities. This study investigated the inhibitory effects of free ammonia (FA) on microalgae under autotrophic, heterotrophic, and mixotrophic growth conditions, since the inhibitory effects in heterotrophic and mixotrophic modes has not been reported yet. Chlorella vulgaris were exposed to 3–20 mg-N L −1 of FA in the autotrophic, heterotrophic, and mixotrophic modes and the effects on growth, photosynthesis, and respiration were studied. The microalgal growth rate was 2–3 times greater in the mixotrophic mode than the other modes. Microalgal growth in the autotrophic mode was most vulnerable to FA and the growth inhibition was observed at FA concentration of ≥ 9 mg-N L −1 . Similarly, the photosynthesis and respiration of C. vulgaris were inhibited at FA concentrations higher than 12 mg-N LT −1 . The removal of carbon and ammonia was greater in the mixotrophic mode than the other modes. The ammonia removal by microalgae increased in proportion to the FA concentration unlike the carbon removal. This was attributed to the ammonium accumulation in microalgae due to the proton imbalance and differences in intracellular and external pH conditions, which were caused by the presence of excess FA in water. Overall, the results show that the mixotrophic microalgae have greater tolerance to FA, and hence, greater potential for use in treatment of wastewaters having high FA concentrations.