Lake nitrification under increasing temperature

• Applicant: Professor Dr. Michael Pester
• Subject Area: Microbial Ecology and Applied Microbiology
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 445467451

Climate change causes lake warming, resulting in longer thermal stratification, incomplete mixing, and higher ammonia availabilities. While C cycling is in the focus of climate change research, N cycling receives much less attention. Nitrification is a key process in the N cycle that converts ammonia to nitrate via nitrite. It prevents build-up of ammonia in freshwaters, which is critical for ecosystem functioning and drinking water supply. Deep oligotrophic lakes comprise >80% of global lake water. Therefore, deep oligotrophic Lake Constance is an ideal model habitat to study lake nitrification in relation to global change. The nitrifier community is well characterized with a single species of ammonia oxidizing archaea (AOA) dominating in the cold (5°C) hypolimnion. Its abundance is strongly correlated to nitrite-oxidizing Nitrospira species. However, our recent analyses show that much higher nitrification rates in warmer (ca. 10°C) waters below the thermocline cannot be explained by AOA alone. This goes along with occasional blooms of ammonia-oxidizing bacteria (AOB) in such waters. In addition, we were recently able to enrich nitrite-oxidizing Nitrotoga species at elevated substrate and temperature conditions from Lake Constance. Hypothesis I will test whether AOA/Nitrospira-driven nitrification switches to AOB/Nitrotoga-driven nitrification under higher lake temperatures and ammonia availabilities across spatial gradients in the water column. Hypothesis II will test whether these changes can occur rapidly in a time period of a few weeks. Both hypotheses will be tested in two separate work packages (WP). In WP1, two water bodies differing in in situ temperature and ammonia availability will be tested for ammonia and nitrite oxidation activities using 15N-labeling techniques. Bulk activities will be analyzed by GC-IRMS and compared to the nitrifier community using 16S rRNA gene amplicon sequencing and CARD-FISH analyses. This will be extended by single-cell analyses using nanoSIMS to estimate relative contributions and in situ growth rates of the individual nitrifier guilds. Subsequent 15N-labeling-based temperature and substrate range experiments will discern in situ temperature optima and ammonia half-saturation concentrations of the respective nitrifier communities. In WP2, a bioreactor setup will be used to study temporal responses to changes in temperature, ammonia concentrations, and substrate type. Nitrification rates will be monitored by standard analytical techniques targeting ammonia, nitrite, and nitrate. Changes in the nitrifier community will be analyzed using 16S rRNA gene amplicon sequencing and CARD-FISH analyses. This work program will be important to assess how a major ecosystem service of freshwater lakes responds to global change, both in terms of process performance and the underlying microbiota. The anticipated results will have the potential to be easily implemented in molecular screenings and to feed ecosystem models.

• DFG Programme: Research Grants