Heavy Metal-Resistant Biohybrid System Boosts Dissimilatory Nitrate Reduction to Ammonium for Agronomic Sustainability

Dissimilatory nitrate reduction to ammonium (DNRA) by functional microbes is crucial for optimizing nitrogen preservation and sustainable utilization in agricultural soils, particularly in paddy soils where nitrogen management is essential for crop yields, while the biocatalytic efficiency for this process is always limited, especially in soils polluted with heavy metals such as cadmium (Cd). In this study, we strategically designed a heavy metal-resistant nanogel (Bg) material with variable functional groups incorporating imidazole, carboxyl, and thiol groups and hybridized this material with DNRA functional strain, Shewanella oneidensis MR-1, resulting in a biohybrid system. Notably, the biohybrid system efficiently catalyzed DNRA processes, achieving up to around 2–4 times increase in nitrogen transformation rates in the absence/presence of Cd2+ compared to S. oneidensis MR-1 alone because the Bg material from the biohybrid system can promote DNRA by functional strain and mediate Cd immobilization processes through distinct functional groups. Within the biohybrid-mediated DNRA process, the thiol groups derived from Bg act as electron shuttles, significantly promoting the extent and rate of DNRA. For the Cd immobilization process, the imidazole and carboxyl groups within Asp-xxx-His sequences from Bg serve as primary Cd binding sites, as confirmed by dynamic force spectroscopy (DFS). This study provides valuable insights into the precise control of nitrogen cycling and sustainable utilization in agricultural soils, highlighting the significant role of the designed biohybrid materials in enhancing both DNRA efficiency and heavy metal immobilization.