Lay summary

Previously, we generated a comprehensive data set that provides compelling biogeochemical evidence for an anaerobic mode of methane oxidation in the North Basin of Lake Lugano ("Biogeochemical fluxes in South-Alpine Lakes: Linking nitrogen and methane dynamics in lacustrine redox-transition zones using a combined stable isotope and molecular approach" (SNF 121861; Dec. 2008 - Nov. 2011). The dominance of monoenoic fatty acids in deep waters of the North Basin with very low δ13C-values, together with the finding of 14CH4 tracer turnover in incubations of water depth from 175m and below, indicate the incorporation of methane-derived carbon into bacterial biomass well below the redoxcline.

Thus far, no compounds typically associated with anaerobic methanotrophic archaea (e.g. archaeol) could be detected, suggesting potential bacterial anaerobic methanotrophy in the water column of Lake Lugano. Yet, the phylogenetic identity of the microbes that perform AOM, as well as the possible terminal electron acceptor for AOM still needs further evaluation. First sulphate reduction rate measurements suggest that sulphate reduction cannot account for the observed AOM.  In this project we will conduct sampling campaigns and laboratory incubation experiments, combining radio-label, stable isotopic, and molecular (16S rRNA gene diversity, qPCR, FISH) analyses, with the specific goals 1) to assess the role of oxidized metal species (FeIII; MnIV) or NOx involved as oxidant in methane oxidation in the water column of Lake Lugano, and 2) to further constrain the identity of microbes that perform methane oxidation by linking microbial identity and organism-level C-isotope signatures.

Quantitative estimates of CH4 elimination in the water column of Lake Lugano will be a prerequisite for ecosystem-scale C budgets. The analyses of bacterial and archaeal functional genes are expected to yield valuable insight into the ecological structure of both the permanent (North Basin) and transitional (South Basin) water column redox-transition zones in Lake Lugano. Moreover, estimates of isotope effects of specific C 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.