Anaerobic phenanthrene biodegradation by a new salt-tolerant/halophilic and nitrate-reducing Virgibacillus halodenitrificans strain PheN4 and metabolic processes exploration

Abstract

The biodegradation of polycyclic aromatic hydrocarbons (PAHs) under hypersaline environments has received increasing attention, whereas the study of anaerobic PAH biodegradation under hypersaline environments is still lacking. Here, we found a pure culture designated PheN4, which was affiliated with Virgibacillus halodenitrificans and could degrade phenanthrene with nitrate as the terminal electron acceptor and a wide range of salinities (from 0.3% to 20%) under anaerobic environments. The optimal salinity for biodegradation of phenanthrene by PheN4 was 5%, which could degrade 93.5% of 0.62 ± 0.04 mM phenanthrene within 10 days with the initial inoculum of 0.01 gVSS/L. Meanwhile, an increased microbial amount could efficiently promote the phenanthrene biodegradation rate. The metabolic processes of anaerobic phenanthrene biodegradation under hypersaline conditions by PheN4 were proposed based on intermediates and genome analyses. Phenanthrene was initially activated via methylation to form 2-methylphenanthrene. Next, fumarate addition and β-oxidation or direct oxidation of the methyl group, ring reduction and ring cleavage were identified as the midstream and downstream steps. In addition, PheN4 could utilize benzene, naphthalene, and anthracene as carbon sources, but Benz[a]anthracene, pyrene, and Benzo[a]pyrene could not be biodegraded by PheN4. This study could provide some guidance for the bioremediation of PAH pollutants in anaerobic and hypersaline zones.