'Protector' DNRA bacteria, shielding anammox systems from perfluorooctanoic acid by mitigating nitrite accumulation

The anaerobic ammonium oxidation (anammox) process is integral to nitrogen removal in wastewater treatment, but its effectiveness can be compromised by contaminants such as perfluorooctanoic acid (PFOA). This study revealed the protective mechanism of dissimilatory nitrate reduction to ammonium (DNRA) in protecting anammox bacteria from PFOA-induced toxicity in a lab-scale anaerobic membrane bioreactor (anammox-AnMBR), which operated for over 100 days. The system was exposed to stepwise PFOA increases (0.25, 0.5, and 0.75 mg/L). Nitrogen removal efficiency and extracellular polymeric substance (EPS) production were monitored. Microbial community dynamics were analyzed by metagenomic sequencing. Notably, increasing PFOA concentrations corresponded to stable NO₂^- levels and fluctuating NH₄⁺, suggesting a biphasic response to PFOA toxicity by the DNRA and anammox communities. Results indicate that DNRA bacteria were more tolerant to PFOA, even at high concentrations, than anammox bacteria. Metagenomic analysis further demonstrated the acute toxicity of high PFOA concentrations to anammox. The abundances of anammox functional genes (hzs, encoding hydrazine synthase; hdh, encoding hydrazine dehydrogenase) showed initial increases followed by declines at elevated PFOA concentrations. Concurrently, the increase in EPS (polysaccharides: 33.3 to 111.1 mg/g VSS; proteins: 3.0 to 247.6 mg/g VSS) indicated defense responses.