Abstract
Although the anaerobic ammonium oxidation (anammox) process has attracted attention regarding its application in ammonia wastewater treatment based on its efficiency, the physiological characteristics of anammox bacteria remain unclear because of the lack of pure-culture representatives. The coexistence of heterotrophic bacteria has often been observed in anammox reactors, even in those fed with synthetic inorganic nutrient medium. In this study, we recovered 37 draft genome bins from a long-term-operated anammox column reactor and predicted the metabolic pathway of coexisting bacteria, especially Patescibacteria (also known as Candidate phyla radiation). Genes related to the nitrogen cycle were not detected in Patescibacterial bins, whereas nitrite, nitrate, and nitrous oxide-related genes were identified in most of the other bacteria. The pathway predicted for Patescibacteria suggests the lack of nitrogen marker genes and its ability to utilize poly-N-acetylglucosamine produced by dominant anammox bacteria. Coexisting Patescibacteria may play an ecological role in providing lactate and formate to other coexisting bacteria, supporting growth in the anammox reactor. Patescibacteria-centric coexisting bacteria, which produce anammox substrates and scavenge organic compounds produced within the anammox reactor, might be essential for the anammox ecosystem.
Highlights
Anaerobic ammonium oxidation is a microbial process in which, under anoxic conditions, ammonia is directly oxidized to nitrogen gas with nitrite as the electron acceptor
To understand the contribution of bacteria to the nitrogen cycle in the anammox reactor, we focused on the following reconstructed nitrogen maker genes: ammonia oxidation, hydroxylamine oxidation, nitrite oxidation, dissimilatory nitrate reduction, dissimilatory nitrate reduction to ammonium (DNRA; nirBD and/or nrfAH), nitrite reduction, nitric oxide reduction, nitrous oxide reduction, nitrogen fixation, hydrazine dehydrogenase, and hydrazine synthase
The metabolic capacities prepredicted for the six Patescibacterial bins with high completeness suggest that Patescibacdicted for the six Patescibacterial bins with high completeness suggest that Patescibacteria teria can utilize chitin-related compounds and produce fermentation by-products of laccan utilize chitin-related compounds and produce fermentation by-products of lactate and tate and formate in the anammox reactor
Summary
Anaerobic ammonium oxidation (anammox) is a microbial process in which, under anoxic conditions, ammonia is directly oxidized to nitrogen gas with nitrite as the electron acceptor. The anammox process is mediated by a member of the phylum Planctomycetes [1]. Six anammox bacteria candidate genera have been proposed: Candidatus Brocadia, Candidatus Kuenenia, Candidatus Anammoxoglobus, Candidatus Jettenia, Candidatus Scalindua, and Candidatus Anammoximicrobium [2,3]. The physiological characteristics of several genera have been investigated [4,5,6,7], detailed physiologies remain unknown due to the lack of pure cultures [5]. Most of the candidate phyla were renamed, and superphyla predicted by single-cell genomics [8] and metagenomics [9,10] were proposed.
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