Study On Abundance And Community Structure Of Ammonia-oxidizing Microorganisms In Wastewater Treatment Systems

Nitrification,aerobic oxidation of ammonia to nitrate by way of nitrite,has been long thought to be catalyzed by ammonia-oxidizing and nitrite-oxidizing bacteria,respectively.The discovery of complete ammonia oxidizers(comammox)capable of performing the two-step nitrification process on their own have fundamentally upended our traditional perception towards biogeochemical nitrogen cycle.Their environmental distribution and functional importance in driving ammonia oxidation are still urgently awaited to be assessed.Here,we provide the first insight into the diversity and abundance of comammox bacteria in wastewater treatment systems taking advantage of newly developed primer sets specifically targeting the distinct Nitrospira-like amoA gene of comammox(comamoA),and compared it with traditional ammonia-oxidizing bacteria(AOB)and ammonia-oxidizing archaea(AOA).Quantitative polymerase chain reaction(qPCR)demonstrated a remarkably high abundance of comamoA gene ranging from 2.38×107 to 3.25×109 copies per g dry sludge in the fourteen activated sludge samples,which were up to 135-fold more abundant than their well-known bacterial counterparts.Besides,the abundant comamoA gene also actively expressed revealed by reverse transcription-qPCR(1.5X 105 to 3.0 X 109 copies per g dry sludge),suggesting they probably played significant role in ammonia oxidation.Furthermore,together with those recovered from NCBI database,the novel comamoA sequences formed a special phylogenetic cluster,which were clearly distinct from the canonical betaproteobacterial amoA gene.And phylogenetic analysis showed Nitrospira nitrosa and Niirospira nitrificans clusters were supposed to be the predominant comammox species in the man-made ecosystems.The phylogenetic analysis of AOA and AOB revealed that Nitrosospaera sister cluster and Nitrosospaera cluster dominant AOA in wastewater treatment plants and participate in the expression activity in ammonia oxidation process.Nitrosomonas cluster affiliated with AOB which widely distributed and played an important role in the nitrification process in wastewater treatment plants.The results clearly indicated that comammox bacteria might be the most abundant and active microorganism driving ammonia oxidation in WWTPs,which offered important new insights into their ecological features and provide fundamental basis for future researches.