Novel Anammox Bacteria Discovered in the Untapped Subsurface Aquifers

Anaerobic ammonium-oxidation (anammox) reaction is a crucial microbial nitrogen (N) transformation process contributing to the global N cycling. The currently known anammox bacteria are affiliated with two families, the marine Ca . Scalinduaceae and the freshwater Ca . Brocadiaceae, in the Planctomycetes . Here we report a discovery of new groups of anammox bacteria consisting of two new orders, two new families, and six new genera within the class Candidatus Brocadiia, all from geochemically distinct aquifer systems. Up to 25 high-quality metagenome-assembled genomes (MAGs) equipped with the core anammox metabolisms were recovered from 13 metagenomic datasets of aquifers and dominated the anammox bacterial communities in distinct aquifers unexpectedly. The comparatively smaller genome size (< 2.6 Mb) and higher coding density (> 85%) likely confer the survival advantage of them by reducing the energy consumption in cell replication and maintenance to increase the nutrient acquisition efficiency in the typically oligotrophic aquifers. Overall, the highly diversity of novel anammox bacterial lineages in aquifer systems largely expands our knowledge of the taxonomic diversity of anammox bacteria and highlights their global importance in aquifer N cycling. ### Competing Interest Statement The authors have declared no competing interest.