11. March 2022 | Press releases:

Microorganisms in Lake Constance convert tonnes of ammonium Microbiologists show how important ammonium-oxidising microorganisms are for Germany's largest lake

Press release of Technische Universität Braunschweig and the Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures GmbH

A new globally distributed species of archaea converts tonnes of ammonium in one of Europe’s largest lakes. The microorganisms thus contribute to the safety of the drinking water supply for over five million people. Scientists from Braunschweig, Bremen and Konstanz were able to prove this. They have now published their results in the journal “The ISME Journal” of the Nature Publishing Group.

Lakes are important for drinking water supplies, inland fishing and as recreational areas. An enrichment of ammonium would endanger these ecosystem services. At the same time, ammonium is an important component of agricultural fertilisers, 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 leads, for example, to an increase in algae growth, thus also to a lack of oxygen and consequently 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 – harbour large populations of archaea, a special group of microorganisms, in their depths. Until now, it was only assumed that these archaea are involved in the conversion of 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 Technische Universität Braunschweig, the Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures GmbH, the Max Planck Institute for Marine Microbiology and the University of Konstanz was able to prove for the first time that these archaea are actually involved in ammonium oxidation. They were able to quantify this activity in one of the largest lakes in Europe, Lake Constance.

How microbes regulate nitrogen levels in freshwater ecosystems

Much of our planet is covered with water, but only 2.5 per cent of it is freshwater. Around 80 per cent 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 per cent 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 crucial link between the mineralisation of organic nitrogen or ammonium pollution and its eventual conversion to harmless nitrogen (N2) through anaerobic processes.

The now published results show that in Lake Constance, a single species of archaea converts up to 1760 tonnes 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 in the depths, corresponding 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, which 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 water of this large lake with an area of 536 square kilometres.

Using metagenomics and metatranscriptomics, the genome of this novel microorganism could be extracted from the environment and its activity followed over the seasons. Stable isotope-based activity measurements revealed that this single species is responsible for the conversion of ammonium in the range of over 1000 tonnes. At present, it remains unclear how this newly discovered microorganism, which is widespread in large inland waters, will react to changes caused by global warming.