Anaerobic ammonium oxidation (anammox) bacteria, which were discovered in wastewater sludge in the early 1990s, have the unique metabolic ability to combine ammonium and nitrite to form nitrogen gas (N₂). This discovery led to the realization that a substantial part of the enormous nitrogen losses that are observed in the marine, freshwater, and soil environment were due to the activity of these bacteria. However, its occurrence and role in constructed wetland (CW) treating swine wastewater (SW) have not been well evaluated as yet.

In rural areas of China, SW has been identified as a major source of diffuse nitrogen pollution, causing various environmental problems, such as eutrophication. To lessen such environmental impact, the wastewater must be treated prior to its discharge. The CWs has been increasingly applied in the last decades to remove nutrients from wastewater due to its cost-effectiveness and efficiency. This eco-engineering system uses natural wetland plants and soil as the substrate to remove pollutants from treated SW, preventing the effluent from overloading agricultural land with nutrients.

Researchers in the Institute of Subtropical Agriculture, Chinese Academy of Sciences (ISA) employ quantitative PCR assay, 15N-stable isotope tracing technique, and 16S rRNA clone library to document the distribution, diversity and roles of anammox bacteria in sediments of CWs treating high strength SW.

The team found anammox bacteria were abundant in the CW sediments and may played an important role in N-removal. "In our study, both molecular and isotope tracing experiments corroborated the occurrence of anammox in the collected sediment samples, suggesting a ubiquitous distribution of anammox bacteria in the CWs treating swine wastewater" said CHEN Liang, a post-doctoral researcher at ISA, "we found that the abundance of anammox bacteria ranged from 0.75-3.46*10⁶ gene copies g^-1 dry soil, which is generally higher than the value reported for paddy soils of southern China. Moreover, according to the anammox bacterial activity, we estimated that anammox account for the removal of 0.83-2.68 kg N yr^-1 from the CWs."

So far, there are five known genera of anammox bacteria: "Candidatus Scalindua" "Ca. Brocadia" "Ca. Kuenenia" "Ca. Anammoxoglobus" and "Ca. Jettenia". The team also investigated the anammox bacterial community composition in the CWs treating SW. Phylogenetic analysis revealed the presence of multiple co-occurring anammox genera, with the "Ca. Brocadia" being the dominant one. Besides, it is important to unravel the potential links between functional microbes and environmental factors in a wetland system. Results revealed that the activity and abundance of anammox bacteria was most strongly related to sediments NH₄⁺-N, NO₂^--N and pH in the examined CWs.

The present study contributes to the understanding of N-removal mechanisms and the factors influencing the presence of anammox bacteria in SW treatment CWs.This research was supported by funding from the China Postdoctoral Science Foundation Funded Project (2014M560648, 2015T80878), the National Natural Science Foundation of China (41601272), and the key CAS Programs (KZZD-EW-11).

The study entitled "Anaerobic ammonium oxidation in sediments of surface-flow constructed wetlands treating swine wastewater" has been published in the February issue of Applied Microbiology and Biotechnology, details could be found at http://link.springer.com/article/10.1007/s00253-016-7957-y.

Contact: CHEN Liang

E-mail: lchen52416@isa.ac.cn

Institute of Subtropical Agriculture, Chinese Academy of Sciences