Intertwining of the C-N-S cycle in passive and aerated constructed wetlands

The microbial processes occurring in constructed wetlands (CWs) are difficult to understand owing to the complex interactions occurring between a variety of substrates, microorganisms, and plants under the given physicochemical conditions. This frequently leads to very large unexplained nitrogen losses in these systems. In continuation of our findings on Anammox contributions, our research on full-scale field CWs has suggested the significant involvement of the sulfur cycle in the conventional C-N cycle occurring in wetlands, which might closely explain the nitrogen losses in these systems. This paper explored the possibility of the sulfur-driven autotrophic denitrification (SDAD) pathway in different types of CWs, shallow and deep and passive and aerated systems, by analyzing the metagenomic bacterial communities present within these CWs. The results indicate a higher abundance of SDAD bacteria (Paracoccus and Arcobacter) in deep passive systems compared to shallow systems and presence of a large number of SDAD genera (Paracoccus, Thiobacillus, Beggiatoa, Sulfurimonas, Arcobacter, and Sulfuricurvum) in aerated CWs. The bacteria belonging to the functional category of dark oxidation of sulfur compounds were found to be enriched in deep and aerated CWs hinting at the possible role of the SDAD pathway in total nitrogen removal in these systems. As a case study, the percentage nitrogen removal through SDAD pathway was calculated to be 15-20% in aerated wetlands. The presence of autotrophic pathways for nitrogen removal can prove highly beneficial in terms of reducing sludge generation and hence reducing clogging, making aerated CWs a sustainable wastewater treatment solution.