Drinking water supply, inland fishing and as local recreation areas: Lakes are very important for all of these. An enrichment of ammonium would endanger these ecosystem services. At the same time, ammonium is an important component of agricultural fertilizers, which is why its concentrations in the environment have increased dramatically and the global nitrogen cycle as a whole is out of balance. The over-supply of nutrients (for example, nitrogen) in water bodies, for example, leads to an increase in algae growth, thus also to a lack of oxygen and subsequently to hostile conditions for plant and animal life.
Nutrient-poor lakes with large bodies of water — such as Lake Constance and many other pre-Alpine lakes — harbor large populations of archaea, a special group of microorganisms, in their depths. Until now, these archaea were only thought to be involved in converting ammonium to nitrate, which is further converted to harmless nitrogen (N2) — a major component of air — in sediments and other low-oxygen habitats.
A team of environmental microbiologists from the Technical University of Braunschweig, the Leibniz Institute DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH, the Max Planck Institute for Marine Microbiology and the University of Konstanz was able to demonstrate for the first time that these archaea are indeed involved in ammonium oxidation. They were able to quantify this activity in one of Europe’s largest lakes, Lake Constance.
How microbes regulate nitrogen levels in freshwater ecosystems
Much of our planet is covered with water, but only 2.5 percent of it is freshwater. About 80 percent of this freshwater is not available to humans because it is stored in glaciers and the polar ice caps. In the European Union, about 36 percent of drinking water comes from surface waters. It is therefore important to understand how this ecosystem service is maintained by environmental processes such as so-called microbial nitrification. Nitrification prevents ammonium from accumulating and converts it to nitrate via nitrite. Although nitrification does not directly alter the amount of inorganic nitrogen (N) in freshwater ecosystems, it provides a critical link between the mineralization of organic nitrogen or ammonium pollution and its eventual conversion to harmless nitrogen (N2) through anaerobic processes.
The results now published show that in Lake Constance, a single species of Archaea converts up to 1760 tons of N‑ammonium per year. This corresponds to eleven percent of the annual nitrogen biomass produced by algae. In the process, the newly discovered Archaea build up an enormous biomass at depth, equivalent to twelve percent of the organic carbon produced annually by plant plankton.
Novel archaea species responsible for ammonium conversion
Using state-of-the-art methods from environmental microbiology and biogeochemistry, the scientists* identified a novel archaea species, Candidatus Nitrosopumilus limneticus, that is responsible for the ecosystem service of ammonium oxidation in Lake Constance. This species forms huge populations of up to 39 percent of all microorganisms in the deep waters of this large lake, which covers an area of 536 square kilometers.
Using metagenomics and metatranscriptomics, the genome of this novel microorganism could be extracted from the environment and its activity tracked over the seasons. Stable isotope-based activity measurements revealed that this single species is responsible for ammonium conversion in the range of over 1000 tons. At present, it remains unclear how this newly discovered microorganism, which is widespread in large inland waters, will respond to changes caused by climate warming.
