Gene expression and biomarker discovery of anammox bacteria in different reactors

The multiplicity and heterogeneity of anaerobic ammonia oxidation (anammox) bacteria make their absolute quantification difficult. To reveal the quantitative relationships among the 16S rRNA gene and typical functional genes (nir, hzo and hzs genes) in the anammox process and their quantitative association with nitrogen removal (ammonium and nitrite), the genes were extracted from four reactors and quantified by real-time quantitative PCR (q-PCR). The Pearson correlation coefficients among the associated genes exceeded 0.96 (P < 0.01), and the q-PCR results indicated that the relative copy numbers (compared with the level of the hzo gene, which was set as 1) of the hzs gene, nir gene and 16S rRNA gene per cell were 8.00, 25.22 and 157.67, respectively. Nitrogen removal efficiency was directly proportional to the logarithm of the gene concentration, with most of the R2 values being greater than 0.9. The functional genes, especially hzo and hzs, are more suitable to assess anammox activity with the R2 values of fitted equations being greater than 0.96. The mRNA quantity did not correlate with the system nitrogen removal performance directly because of different enzyme activities. This study provided valuable reference data to further reveal the molecular mechanism of anammox metabolism.