Coupling heterotrophic and hydrogenotrophic partial denitrification via gel-based bio-carriers: microbial mechanisms...

Partial denitrification (PD) has emerged as a pivotal technology for addressing the limited nitrite (NO₂^-) supply that hinders the widespread application of anammox, as it efficiently provides NO₂^-. However, its reliance on organic carbon sources restricts its broad implementation. In this study, a system of heterotrophic coupled with hydrogen-autotrophic PD was established using polyvinyl alcohol gel bio-carriers. Operated under a low COD/NO₃^--N ratio of 2.00 for 90 days, the system achieved remarkable performances, with a NO₂^- transformation ratio (NTR) of 85.50 ± 3.10% and a nitrate (NO₃^-) removal rate (NRR) of 84.70 ± 5.00%. Metagenomic analysis revealed the effective enrichment ofHydrogenophaga(23.90%) as a key hydrogen-autotrophic denitrifier, which formed a functionally complementary consortium with heterotrophic denitrifiers (e.g.,Dokdonella). The abundance ratio of NO₂^- reduction genes in autotrophic to heterotrophic bacteria was 1.3:1. Furthermore, a putative metabolic model was constructed, which posits a potential cross-feeding interaction characterized by "hydrogen production by heterotrophs and consumption by autotrophs." The hydrogenase (EC:1.12.99.6) was proposed as a potential key gene facilitating this synergy between heterotrophic and autotrophic bacteria. The increased abundance ratio of nitrate reductase to nitrite reductase genes to 2.07 was identified as the key factor promoting the high accumulation of NO₂^-. Material characterization confirmed that the gel carriers possessed a hierarchical porous structure, with a mesopore-dominated pore size distribution conducive to hydrogen diffusion and the aggregation of functional microbial communities, thereby providing a stable micro-environment. This study offers a novel technological pathway for stable NO₂^- supply in the treatment of low-carbon wastewater.