Microbial community characteristics and stable relationships centered on anammox bacteria revealed by global-scale analysis

Anaerobic ammonium-oxidizing bacteria (AnAOB) play important roles in both artificial wastewater treatment systems and natural ecosystems. To date, AnAOB pure cultures are not available and they tend to coexist with various microbial species. However, anammox community characteristics including the relationships between AnAOB and their companion bacteria at the global perspective and their impacts on anammox metabolism remain unclear. Here, we systematically analyzed the characteristics of anammox communities and the stable relationships concerning AnAOB using a global dataset containing 619 anammox-related amplicons. Different anammox systems showed significant differences in alpha and beta diversity, but shared some core taxa of interest. A total of 89 and 55 core genera and species were identified respectively across anammox communities worldwide, which formed the backbone of artificial anammox systems. Through the analysis of co-abundance networks derived from four distinct artificial anammox systems--biofilm, granular sludge, flocculent sludge, and planktonic cells--we identified 208 stable and 92 limited stable relationships associated with AnAOB. Functional analysis suggested that stable positively-correlated companion bacteria may provide essential cofactors (e.g., molybdenum cofactor, tetrahydrofolate, and coenzyme A) to AnAOB. The companion bacteria which showed limited stable positive correlations with AnAOB in the anammox attachment-growth systems, may mutualize with AnAOB via type pili. This study deepens the understanding of anammox communities, anammox core microbiome, and AnAOB symbiotic relationships. These (limited) stable companion bacteria and corresponding cofactors can potentially guide the development and application of bioaugmentation methods, synthetic anammox communities, and deterioration biomarkers for engineered anammox systems.