Biogeochemical research in the Lehmann Lab - General scope and approach:

The Aquatic and Isotope Biogeochemistry Group studies biogeochemical cycles of biologically important elements and their isotopic composition in freshwater and marine ecosystems. Of particular interest is the microbial generation and consumption of green house gases, such as methane (CH4) and nitrous oxide (N2O). Given the integrative nature of biogeochemical research, we often combine different analytical tools, including stable isotope mass spectrometry (natural abundance and isotope label), biomarker analysis, radio-tracer rate measurements, and DNA/RNA-based approaches. One of the main goals of our research is to understand the environmental factors that modulate rates of organotrophic or lithotrophic microbial processes (e.g., methane oxidation, denitrification, nitrification, anammox), and the isotope signatures they leave behind in the natural environment. Towards this goal, we often study the distribution, chemical speciation and stable isotope composition of various elements and compounds, providing information as to the mechanisms and microbial community structures that underlie the observed patterns. Observations in the field are often calibrated by studying the relevant biogeochemical processes in the laboratory using incubation experiments, and put into a quantitative framework using numerical models.

Investigated ecosystems include lakes, estuaries, groundwater aquifers and saturated soils, the open ocean and coastal marine environments, as well as hydrothermal vent and cold seep systems in the deep ocean. Our record shows a long history of work in Lake Lugano in Southern Switzerland. Lake Lugano is an excellent model system to study biogeochemical processes, as well as the environmental and biological controls on C and N budgets and the production and consumption of climate-relevant trace gases in lacustrine environments.  Most recently, we also started to investigate N cycling reactions in engineered systems (e.g., sewage treatment bioreactors).

Current research themes:

One of the foci of recent research activities is to understand the isotope effects of individual biological N transformations (and associated microbiological controls). Specifically, we are interested in linking the N and O isotope distributions in dissolved inorganic nitrogen species (i.e., NOx and N2O) and N2O dynamics in aquatic  environments to the rates of key microbial nitrogen transformations such as ammonia oxidation, nitrite oxidation, denitrification, and anammox.

A second important research theme within our group is the Biogeochemistry of CH4. We study aerobic and anaerobic methane oxidation in various lacustrine and marine environments, investigating environmental controls on CH4 turnover and on the microbial community structure, and developing methane budgets on different spatial scales. Integrating Biomarker geochemistry with isotope analyses, the identification and isotopic characterization of specific lipids in environmental samples provides information on the community structure and thus biogeochemical processes, both in modern and paleo environments.

Another important research theme is the Cycling and bacterial degradation of organic matter in aquatic environments, its role in the generation of permanently or temporarily anoxic conditions, as well as its impact on solute exchange and Redox reactions within sediment pore waters of marine and lacustrine environments. We study the fate of sedimentary organic matter not only in the context of compositional changes during microbial remineralization and their relevance for elemental budgets, but also with regards to the susceptibility of different organic compounds, their biomarker signatures, as well as their isotopic composition to post-burial alteration.

Our work in modern ecosystems, and our experimental work validating the preservation potential of primary organic-geochemical and isotopic signatures, provides the basis for using isotope and biomarker proxies in Paleoenvironmental studies, for example the reconstruction of paleo-temperature or paleo-productivity in Swiss lakes through the analysis of specific bacterial membrane biomarkers and the isotopic composition of lacustrine organic material in sediment cores.