In 2002, a novel microbial N cycle pathway - anaerobic ammonium oxidation (anammox) - was found in marine sediments. One year later, anammox was also detected in water column of Black Sea and Golf Dulce Costa Rica. In freshwater system, anammox was primarily observed in 2006 in Lake Tanganyika. The discovery of anammox challenged our view that denitrification was the only pathway for microbial nitrogen loss.
Since 2008, we have been studying anammox. As revealed by the investigations in the last decade, anammox is ubiquitous in freshwater ecosystem, including paddy fields, lakes, river, marshes, etc. Furthermore, it is evidenced that riparian interfaces sediments are the anammox hotspots with effective reduction of nitrous oxide (N2O) emission. However, less than 10% of the land surface area is covered by freshwater and wetlands. Apparently, the anammox in terrestrial ecosystems cannot be such influential as in marine. Considering this, we raised the hypothesis that larger-scale distributions of anammox exist in global aquifer system.
To manifest this hypothesis, we took samples from Africa, Europe, Asia and Oceania, and spent six years researching them. Finally, hard work pays off. We’re so honored that our results can be published on the ISME Journal. This article chiefly tells that anammox is turned on and off between saturated and unsaturated soil, contributing 36.8–79.5% to N loss in saturated soil. Although anammox activity was undetectable in the unsaturated soil, it could potentially be activated by ascending groundwater. As indicated by the isotopic pairing experiments, coupling anammox with ammonium oxidation and respiratory ammonification enabled a revised N cycle to form in aquifer systems, in which respiratory ammonification acted as an important coordinator. Therefore, anammox may contribute substantially to aquifer N cycling and its role in remediation of N-oxides contaminated aquifers may be of global importance.
Scientifically speaking, this article is conductive to deepening our understanding of microbial nitrogen cycle in terrestrial subsurface. For engineering applications, as nitrate pollution becomes more and more serious in groundwater globally, anammox with no consumption of carbon and nitrogen sources should be a practical treatment method.
Firstly, we want to express our sincere thanks to the reviewers who has provided us with professional and pertinent constructive suggestions during the revisions. Under his help, we not only improve our shortage, but also increase persuasion of results. Meanwhile, great appreciations and congratulations should be given to our first author, Dr. Shanyun Wang, who starts this revision no more than half a month after a big surgery. Fortunately, she has got return of her efforts.
Although lots of works have been done, there are still some problems that need further discussion. Firstly, the functions of water are unclear. As demonstrated by the previous studies, water flooding can reactivate anammox bac¬¬teria in unsaturated layers. Therefore, it is essential to further investigate whether water can be directly used by anammox bacteria, or just acts as reaction medium promoting substrates, or whether it induces suitable anoxic−anaerobic environment. Secondly, it’s still uncertain to the driving forces during the formation of anammox hotpots and the activation of anammox bacteria. If we can achieve this only by recharging of groundwater and elevation of groundwater table, using this method to remove nitrates of groundwater will have great potential. Thirdly, since anammox occurs in groundwater of global scale, it is very significant for global nitrogen cycle, especially for constructing calculation models for N2O conversion and nitrogen oxides concentration in groundwater. However, eco-environmental benefits of anammox hotpots remain to be determined.
Hopefully, these views can be discussed with scholars worldwide. Although our work is just a small step on the path of science, we have faith to cope with these unsolved scientific problems together with all of you.
The link of the paper: https://www.nature.com/articles/s41396-019-0513-x