Meta-azotomics of engineered wastewater treatment processes reveals differential contributions of established and novel models of N-cycling

The application of metagenomics and metatranscriptomics to field-scale engineered biological nitrogen removal (BNR) processes revealed a complex N-cycle network (the meta-azotome) therein in terms of microbial structure, potential and extant function. Autotrophic nitrification bore the imprint of well-documented Nitrosomonas and Nitrospira in most systems. However, in select BNR processes, complete ammonia oxidizing bacteria, comammox Nitrospira , unexpectedly contributed more substantially to ammonia oxidation than canonical ammonia oxidizing bacteria, based on metatranscriptomic profiling. Methylotrophic denitrification was distinctly active in methanol-fed reactors but not in glycerol-fed reactors. Interestingly, glycerol metabolism and N-reduction transcript signatures were uncoupled, possibly suggesting the role of other carbon sources in denitrification emanating from glycerol itself or from upstream process reactors. In sum, the meta-azotome of engineered BNR processes revealed both traditional and novel mechanisms of N-cycling. Similar interrogation approaches could potentially inform better design and optimization of wastewater treatment and engineered bioprocesses in general.