Free ammonia is the primary stress factor rather than total ammonium to Chlorella sorokiniana in simulated sludge fermentation liquor

Abstract

Microalgal growth was inhibited in high-strength ammonium wastewater, but the actual stress factor and their physiological responses remain to be elucidated. In this study, compared to either the total ammonium or pH alone, free ammonia (FA) concentration clearly exhibited negative correlation with the growth of Chlorella sorokiniana cultivated in the simulated sludge fermentation liquor (SFL), which could be well described by an exponential decay model, indicating of the primary stress factor. Subsequently, six concentrations of FA were selected for further investigation and samples were monitored for: 1) growth and ammonium removal; 2) changes in photosynthetic system; 3) changes in reactive oxygen species (ROS) and soluble extracellular polymeric substances (S-EPS); as well as 4) biomass biochemical composition. Both the growth performance and photosynthetic parameters Fv/Fm and YII negatively correlated with FA concentrations. The highest biomass production was observed at the lowest initial FA concentration (2.06 mg/L) and almost no growth was observed in the highest FA concentration (110.97 mg/L).Ammonium nitrogen was removed by algae uptake and evaporation. Cell stress was indicated by significantly increased production of ROS (2.53–12.92 FI/1010cells) and secretion of S-EPS (144.13–160.86 mg/g DW). High FA concentrations induced the accumulation of cellular carbohydrates and palmitic acid. The stress responses to FA observed provide a guide in strategies for microalgal adaptation and growth in high-strength ammonium wastewater.