Enrichment and identification of a moderately acidophilic nitrite-oxidizing bacterium.

This study enriched a novel nitrite-oxidizing bacterium (NOB, ' Nitrobacter acidophilus') in a laboratory reactor operating at pH 4.5 for treating low-strength ammonia wastewater. Batch experiments showed that '. N. acidophilus' oxidized nitrite to nitrate at a rate of 20.7 ± 2.3 μM/h with optimal growth at pH 5, distinguishing it from most previously known NOB strains. Phylogenetic analysis showed that this strain clustered with other strains obtained from acidic environments but was divergent from each other with an average nucleotide identity (ANI) below 85 %. Genomic characteristics revealed that ' N. acidophilus' possesses versatile transporter systems. They are different from previously reported strains and indicate acid adaptation mechanisms. Interestingly, the mutualistic interaction with acidophilic ammonia-oxidizing archaea (AOA) markedly increased the archaeal A gene expression by 149 times and enhanced ammonia oxidation rates by 5 times, highlighting the NOB's role in alleviating nitrite inhibition on the acidophilic AOA. These findings expand our understanding of bacterial nitrite oxidation and provide valuable insights into an important partnership between acidophilic AOA and NOB in acidic environments.