Single and simultaneous effects of naphthalene and salinity on anaerobic digestion: Response surface methodology, microbial community analysis and potential functions prediction

Polycyclic aromatic hydrocarbons (PAHs) are a persistent and prevalent class of pollutants in petroleum-contaminated saline environment, which pose potential harm to organisms. Researches on anaerobic biodegradation of PAHs are gradually emerging, but the response of anaerobic microorganisms to salinity changes and the co-effects of salinity and PAHs in anaerobic digestion (AD) system have seldom been reported. Thus, we investigated the variations of AD system performance and anaerobic microbial community caused by different concentrations of naphthalene (Nap) or/and NaCl based on Box-Behnken Design (0 mg/L ≤ Nap ≤150 mg/L, 0 g/L ≤ NaCl ≤25 g/L). The promoted efficiencies of acidogenesis and methanogenesis were found in presence of moderate NaCl or Nap, but high salinity (NaCl >4.4 g/L) weakened AD performance. Moreover, the high salinity (NaCl >4.4 g/L) and Nap resulted in reduced microbial Ca²⁺ Mg²⁺- ATPase activity, poor EPS secretion and the highest difference of the microbial operational taxonomic units (OTUs), and synergistically inhibited AD process. Microbiological analysis revealed that the relative abundance of Clostridium and acetoclastic Methanosaeta was increased by 2.01 times and 2.17 times in single Nap treated group compared to control. With the simultaneous addition of NaCl and Nap, Proteiniphilum and hydrogenotrophic methanogens (Methanobacterium, Methanofollis, and Methanolinea) occupied the major abundance. Potential functions prediction indicated that high salinity could disrupt the co-metabolism between carbohydrate metabolism and Nap degradation. This study provides basis for anaerobic bioremediation of PAHs-polluted saline environment.