Genome-centric metagenomics reveals the effect of organic carbon source on one-stage partial denitrification-anammox in biofilm reactors

Nitrogen removal from wastewater with anammox saves energy and resources. Partial denitrification-anammox (PDA) is a promising process alternative for municipal wastewater treatment, given that the understanding about how to control the microbiome and its activity reach sufficient level. Here, two moving bed biofilm reactors were fed with either acetate or propionate to study the role of organic carbon type for microbiome composition and nitrogen turnover during development of PDA. With acetate, 87 % of the removed nitrogen was converted via anammox during stable operation at a rate of 0.52 g N/(m²·d). With propionate, the anammox contribution was considerably lower (41 %), as was the rate of nitrogen removal (0.27 g N/(m²·d)). The microbiome composition in the acetate- and propionate-fed reactors was however similar, with an enrichment of metagenome assembled genomes (MAGs) having genes for nitrate reduction (narG, napA). A large fraction of these MAGs had the potential to accumulate nitrite since they lacked genes for nitrite reduction (nirS, nirK, nrfA). Genes for acetate utilization were common among these MAGs, but the necessary genes for propionate conversion were rare, suggesting that the genetic make-up of the individual denitrifiers had major influence on the nitrogen turnover. One anammox MAG (Ca. Brocadia sapporoensis), harboring genes for organic carbon utilization, prevailed in the PDA reactors. Another three anammox MAGs (Ca. B. fulgida, Ca. B. pituitae and a potentially new species within Ca. Brocadia), lacking genes for organic carbon utilization, decreased in abundance in the reactors, indicating the importance of metabolic versatility for anammox bacteria in PDA.