Biogeochemical fluxes in South-Alpine Lakes: Linking nitrogen and methane dynamics in lacustrine redox-transition zones using a combined stable isotope and molecular approach

Bioavailable (“fixed”) nitrogen (N) from anthropogenic sources is an important driver of lacustrine eutrophication. However, N loading in lakes is partially mitigated by microbially mediated processes that take place in redox transition zones (RTZ) within the water column and in sediments. RTZ are also sites of methane (CH4) and nitrous oxide (N2O) production and consumption. As a result of anthropogenic activities, emissions of these greenhouse gases to the atmosphere have significantly increased over the past decades. The role of lakes (i) as a terrestrial sink for fixed N and (ii) in the global CH4 budget are still poorly constrained. Furthermore, modes of suboxic N2 production and CH4 consumption, other than denitrification and aerobic CH4 oxidation, respectively, have barely been investigated in lakes, and the microbial communities involved in CH4 and N transformations in lacustrine RTZ are mostly unknown.Lake Lugano is an excellent model system for an anthropogenically impacted lake that represents a hot-spot of quantitative redox-driven transformations involving the green house gases CH4 and N2O, as well as other N species acting as important macro nutrients. Previous studies have revealed that this lake represents an important sink for fixed N and that the anoxic hypolimnion and sediments contain high concentrations of CH4. We propose to address the following main research questions:1.)What are the different metabolic pathways of suboxic N2 production in the Lake Lugano water column and in sediments? What are the respective transformation rates and fluxes? Which organisms are responsible for observed N transformations?2.)What are the rates and metabolic pathways of CH4 oxidation in the lake’s RTZ? What is the identity and relative abundance of microorganisms involved?Employing novel tools, the proposed research efforts will aim to investigate the concentration, distribution and isotopic composition of inorganic nitrogenous compounds, CH4, and biomarkers in order to assess the processes that are responsible for the observed geochemical signatures, thereby linking community structures and biogeochemical activities. These data, in combination with basic 16S rDNA analyses, laboratory cultivations, and (radio-) tracer experiments, will help us (i) to understand the metabolic pathway involved in N and CH4 elimination in Lake Lugano, (ii) to assess the metabolic rates, at which the respective elimination processes take place, as well as their variability in time and space, (iii) to constrain the isotope effects associated with specific N and CH4 transformations, and (iv) to provide information about the microorganisms involved in these transformations. We will particularly focus on reactions that have essentially been neglected in lacustrine studies thus far, namely the anarobic oxidation of ammonium (anammox) and the anaerobic oxidation of methane (AOM). We will also investigate potential links between the N and CH4 cycles (i.e., the coupling of denitrification and AOM). We will specifically examine AOM and/or anammox in the benthic nepheloid layer in the South Basin of the lake, with the particular goal to elucidate the mechanisms behind its formation.The research proposed here will result in the first comprehensive characterization of early diagenetic reactions in Lake Lugano. Moreover, this research may help gain insights into novel modes of autotrophic life in lakes. Quantitative estimates of N and CH4 elimination in both the water column and sedimentary RTZ of Lake Lugano will be a prerequisite for ecosystem-scale N and C budgets. Moreover, estimates of isotope effects of specific C and N transformations in the modern lake will provide the basis for paleolimnological extrapolation. Thus, the proposed research will help us address biogeochemical processes that are important for the general understanding of a complex ecosystem both today and in the past. It will also provide useful information that is directly pertinent to the health of an important Swiss water body.