Dual-functionality of Thauera sp. JM12B12: aerobic denitrification and bioflocculation for nitrogen and suspended particles removal at low carbon-to-nitrogen ratios

Denitrifying bacteria with flocculation capacity were dual-function microorganisms that can simultaneously remove nitrogen (N) and reduce suspended particles in wastewater, providing a sustainable bioremediation strategy. In this study, a novel denitrifying bacterium capable of producing bioflocculants, <i>Thauera</i> sp. JM12B12, was isolated and investigated. The results confirmed that this strain could completely remove NO₃ ^--N and NO₂ ^--N under microaerobic conditions with a low C/N ratio of 5, using lactate as the optimal carbon source. Notably, no other harmful inorganic N species were produced during denitrification, and total N removal efficiency consistently exceeded 93.0%. Optimal denitrification conditions include a pH range of 7-9, salinity of 0-1.5%, temperature of 25-40 °C, and static incubation. Remarkably, this strain synthesized extracellular bioflocculants during NO₃ ^--N removal, achieving 91.4% flocculation efficiency with cell-free supernatant. Genome analyses revealed a complete denitrification pathway (possessing <i>napA</i>, two <i>nirS</i>, <i>norB</i>, <i>nosZ</i>) and 80 bioflocculant-related genes (polysaccharide production and protein secretion), highlighting its dual capacity for N and suspended particle removal. PCR also confirmed key denitrification genes. Therefore, JM12B12 could be a multifunctional microbial agent for N removal and flocculation, offering a sustainable solution for low C/N wastewater treatment, particularly valuable in recirculating aquaculture systems.