Seasonal population changes of ammonia-oxidizing organisms and their relationship to water quality in a constructed wetland

Nitrogen removal in wastewater treatment systems such as constructed wetlands mainly relies on autotrophic nitrification followed by heterotrophic denitrification. Two major microbial groups are now believed to be involved in ammonia oxidation during the first step of nitrification: ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). Here we determined the abundance and composition of ammonia-oxidizing organisms and their seasonal population changes in a constructed wetland to determine the potential links between the constructed wetland water quality and nitrifying microbial population. By using specific primers targeting the amoA genes of AOA and AOB and real-time quantitative polymerase chain reaction (q-PCR), AOA were found to be generally higher than AOB in wetland soils and water in both summer and winter over a 2-year period of sampling. The AOB were not detected at the outlet of the first wetland treatment unit in the winter seasons indicating their more sensitivity to low temperature than AOA. Terminal restriction fragment length polymerization (T-RFLP) analysis confirmed low peak intensities for the AOB species in the winter. The removal efficiencies of NH4+-N in wetland water in the summer and winter were 96% and 93%, respectively, while the wetland effluent NO3−-N concentrations increased from 4 mg/L in the winter to 11 mg/L in the summer. The more complete nitrification was correlated with higher abundance of ammonia-oxidizing organisms in summer than in winter. Furthermore, nitrification was correlated with a substantial increase of AOB population while the number of AOA in soils remained unchanged. These results suggest that nitrification is mainly driven by AOB in the constructed wetlands treating ammonia-laden wastewater.