Impact of nitrogen loading rate on anammox reactor community

A comparison of the reactor performance and microbial communities of two anammox reactors operated under two distinct nitrogen loading rates, R1 - low loading rate (0.8 kgN•m-3•L-1) and R2 - high loading rate (10.5 kgN•m-3•L-1), was undertaken. While both reactors showed high NH4+-N removal efficiencies (>90%), the total nitrogen removal efficiency (<60%) was much lower in R1 due to relatively higher nitrate production. Metagenomic analysis found the reactors had significant differences in the number of reads derived from critical genes for the nitrogen cycling. In particular, the genes associated with anammox were significantly higher in R2. Another notable trend in read occurrence was the relatively higher levels of reads from nxr in R1, which encodes nitrite oxidoreductase. Assembly of the metagenome yielded 27 high quality bins and, as expected, the percentage of reads attributed to the anammox bins was much higher in R2, 43.0%, than in R1, 22.5%. Interestingly, the proportion of reads mapping to bins of obligate nitrite oxidizers were much higher in R1 than in R2. An additional subset of bins shown to contain nxr showed the same trend. Taken together these results indicate that the anammox reactor operated under lower nitrogen loading rate conditions had both a decrease in anammox organisms and an increase in nxr in the microbial community. The increase in nxr likely accounts for the relatively higher concentration of nitrate in the R1 effluent.