Biomarkers are molecules that contain information on the presence of individual organisms in the environment. They span a variety of molecules with different chemical characteristics and are read using the “omic” approaches. DNA and RNA, for example, contain the genetic information of present and active organisms. This genomic information can be read and individual organisms can be identified. Both molecules, however, are very easily decomposed in the environment and consequently provide only snapshots of actual communities. In contrast, lipids are compounds used to make cell membranes and cuticular waxes that can persist for a long time in the environment, and can even be isolated from Archaean rocks. Lipidomics is used to develop profiles that can identify individuals or groups of organisms as well as lipid profiles characteristic for environmental conditions including salinity, anoxia, and desiccation. The key questions in molecular biogeochemistry are: Who is there, what are they doing, and why? Proteomic and metabolomic approach bridges from the presence of organisms to their function in the environment. However, in order to explore the function of individual processes and how the microbial fluxes link to the overall functioning of ecosystems, additional information is drawn from the isotopic information of biomarkers. Compound specific isotopes (13C, 14C, 15N, 18O and 2H) of biomarkers trace the flow of matter through the element cycles. The group of molecular biogeochemistry combines approaches using the natural abundance of stable isotopes, isotope labeling, and stable isotope probing (SIP) to quantify key processes in the environment.
Soil organic matter (SOM) remains the largest single unknown in the terrestrial carbon cycle. The group investigates in various projects how abiotic factors like organic matter input, parent material, humidity and temperature as well as biotic factors such as stand age, plant and microbial diversity influence SOM storage. The isotopic information of 13C,14C and 15N of biomarkers from individual compounds and fractions determines the molecular turnover of SOM and suggests high vulnerability of SOM stored in soils. We determine the molecular and isotopic composition of dissolved organic matter (DOM) in order to understand the role of DOM in the environment. We use the molecular fingerprints of DOM using ultrahigh resolution mass spectrometry to identify sources and evolution of organic matter. The isotopic content of DOC in soil depth profiles suggests that DOM from the surface is reactively transported in the soil and that DOM in deeper soil horizons is not related to the DOM in upper soil horizons.
Carbohydrates are the central molecules in plant metabolism. During the day, they transport energy and carbon fixed by photosynthesis to support respiration, storage, growth and defense. At night, they provide energy for the cellular metabolism using mitochondrial respiration. However, so far the regulation of carbohydrate metabolism and the role of different processes in plant metabolism is still not completely understood. The group develops and applies molecular techniques to use the isotopic information of plant metabolites to trace the flow of carbon in plants and to understand its regulation.
Plants react not only to abiotic factors like climate, but also to the presence of other plants and microorganisms in the soil. The interaction can be positive if, for example, resources are used complementarily but also negative if pathogens are infecting plants. At the community level, these interactions are difficult to investigate. Molecular tools can help to differentiate between the responses of individual species and communities. Our work is focused on the effect of grassland diversity on (1) the link between above- and below- ground diversity; and (2) the link between plant diversity, soil organic matter (SOM) dynamics and export of dissolved organic matter. In short term experiments we use isotopic labeling to trace the effect of diversity on how carbon is allocated from plants to soil microorganisms and SOM (Ecotron Experiment). In the long term we investigate if higher plant diversity gives the insurance for a long term success of the community, even if some individual species of the community may fail (Jena Experiment).
Understanding of the links between ecosystems and past and present climate will improve our prediction for future climates and how they may affect biodiversity and ecosystem function. Reconstructions of the Holocene climate are strongly linked to information from polar ice cores, while climate reconstructions for larger areas that permit separation of the effects of local climate effects from large-scale circulation patterns are still very sparse. The group explores the use of hydrogen isotopes of biomarkers as a proxy for palaeoclimate reconstructions. A major focus has been the construction of quantitative transfer functions that link hydrogen isotopes to the hydrologic cycle across humid, tropical, boreal and mountain ecosystems. These methods are now being applied to the large-scale climate reconstructions of monsoon variation in the past 10,000 years on the Tibetan plateau (TiP), Kyrgyzstan (CADY)and Sri Lanka.