Low-carbon nature-based system for industrial wastewater treatment: harnessing H2/CO2 to drive robust algae-bacteria symbiosis.

Aeration-free algae-bacteria symbiosis systems (ABSS) represent a promising low-carbon alternative for wastewater treatment, but stability is often compromised when treating toxic industrial effluents with a low carbon/nitrogen (C/N) ratio. This study proposes engineering the system with an oxygen-deprived H2/CO2 headspace to enhance its resilience and elucidates the underlying synergistic mechanisms. The headspace composition was optimized and the performance was evaluated under increasing quinoline stress. The optimized H2/CO2 atmosphere created a dynamically buffered reaction environment, achieving > 90 % total inorganic nitrogen removal. Under stringent oxygen-deprived conditions, the system demonstrated remarkable stability, removing 84 % total nitrogen at 100 mg/L quinoline. Mechanistic analysis elucidated a "dual-engine" denitrification enabled by H2, stress-induced algal-bacterial aggregation for physical protection, and an adaptive community shift dominated by the ecosystem engineer Zoogloea. This work revealed that aeration-free ABSS overcomes key metabolic bottlenecks in nature-based systems, offering a robust, low-carbon paradigm for toxic industrial wastewater treatment.