Copper Stimulates Microalgal–Fungal Consortia Metabolism and Enhances Nutrient Synergic Removal in Livestock Wastewater: Insights from Mathematical Modeling and Transcriptomic Evidence

Excessive residual Cu(II) substantially impedes the advanced treatment of livestock wastewater. The influence of Cu(II) on nutrient removal and microbial metabolism in microalgal–fungal consortia was investigated in this study via a combination of mathematical modeling and transcriptomic analysis. The increase of Cu(II) concentration from 0.1 to 0.5 mg/L promoted the maximum removal efficiency of ammonium nitrogen (NH4+-N), total phosphorus (TP), and chemical oxygen demand (COD) by 83.62, 92.94, and 86.51%, respectively. The novel kinetic model revealed that optimal pollutant removal was achieved under 0.28 mg/L Cu(II). Transcriptomic analysis revealed that the improvement in NH4+-N removal was primarily driven by the upregulation of key genes involved in assimilation, including the glutamine synthetase-glutamate synthase (GS-GOGAT) cycle and glutamate dehydrogenase (GDH) pathway. In addition, 0.2 mg/L Cu(II) enhanced microalgal photosynthesis while downregulating the activity of dehydrogenases involved in fungal carbohydrate metabolism, resulting in an increased polysaccharide content. However, Cu(II) downregulated amino acid biosynthesis, leading to an obvious decrease in protein content. Meanwhile, an increase in protein content was observed in dissolved organic matter (DOMs), as protein could complex with Cu(II), thus reducing its biotoxicity. This study comprehensively evaluated the impact of Cu(II) on nutrient removal by microalgal-fungal consortia, providing novel insights to advance livestock wastewater treatment strategies.