Effects of multiple temperature variations on nitrogen removal and microbial community structure in tidal flow...

Tidal-flow constructed wetlands (TFCWs) provide distinct advantages for nitrogen removal by enhancing microbial activity through dynamic water level fluctuations. However, effects of temperature on nitrogen transformation processes and microbial community dynamics in TFCWs remain unclear. We analyzed the effects of TFCWs on nitrogen transformation and microbial community structure under different temperature conditions (23, 16, 12, and 8 °C) through 140 days of temperature-controlled experiments. The nitrogen removal efficiency was considerably enhanced at 23 °C, with transformation rates for ammonia nitrogen (NH₄⁺-N) and total nitrogen (TN) reaching 9.28 ± 0.06 g/m³/day and 8.35 ± 0.08 g/m³/day, respectively. Conversely, at 8 °C, the nitrogen removal efficiency declined, with NH₄⁺-N and TN transformation rates decreasing to 7.38 ± 0.05 g/m³/day and 6.78 ± 0.05 g/m³/day, respectively. Temperature markedly influenced the microbial diversity and community structure, as evidenced by the considerably higher Shannon diversity indices for bacterial communities at 23 °C (5.12 ± 0.21) compared with those at 8 °C (4.52 ± 0.40). Positive microbial interactions were more prevalent at lower temperatures (12 and 8 °C), leading to stronger symbiotic relationships, although the network complexity diminished. The microbial community composition of taxa such as Firmicutes, Proteobacteria, and Thaumarchaeota exhibited greater resilience at lower temperatures. Changes in dissolved oxygen levels also drove changes in bacterial and archaeal communities. These findings underscore the pivotal role of temperature in regulating ecological function and nitrogen removal efficiency of TFCWs and highlight the importance of accounting for temperature variations in the design and management of wastewater treatment systems.