A Hydrogen-Based Bacterial–Algal Biofilm for Winter Nutrient Mitigation in a Paddy Soil Mesocosm: A Proof-of-Concept Study

The limited nitrogen and phosphorus concentration reduction performance of wetlands during cold weather is a global concern. To address the challenge of low nutrient removal efficiency in winter, this study introduces a hollow fiber membrane module integrated into paddy soil, enabling the in situ formation of a hydrogen-based bacterial–algal biofilm. Over a 60-day experiment at water temperatures of 6.2–15.0 °C, the biofilm-enhanced system (HS) reduced aqueous ${\mathrm{NH}}_4^{+}$-N from 25 mg/L to 0.2 mg/L by day 30 and maintained this low level through day 60, whereas the S system still had 5.5 mg/L ${\mathrm{NH}}_4^{+}$-N. Moreover, the HS system consistently maintained lower ${\mathrm{NO}}_3^{-}$-N and total phosphorus (TP) concentrations than the S system. Microbial communities were characterized using third-generation 16S full-length sequencing and 18S rRNA V4 amplicon sequencing. The biofilm was dominated by Geminocystis_PCC_6308, Rikenellaceae_RC9_gut_group, and Rhodobacter, while eukaryotic green algae, mainly Botryococcus sp., were also abundant. In the paddy soil, Terrimonas and unclassified_AKYH767 were identified as potential key taxa associated with nitrogen and phosphorus concentration reduction. Overall, the hydrogen-based bacterial–algal biofilm effectively improves the nitrogen and phosphorus concentration reduction of paddy soil wetlands, offering a promising strategy for cold-weather eutrophic water remediation.