Objective
Reactions at oxic/anoxic interfaces belong quantitatively to the most important conversion steps in the global cycles of carbon and sulfur. Over geological time scales they were of central importance during the evolution of the biosphere. In the present day earth system the microbial communities at redox boundaries control major fluxes in the global transfer of carbon and sulfur between atmosphere, hydrosphere and lithosphere. Meromictic lakes with permanent stratification and sharp boundaries between the oxic and anoxic "world" represent ideal model systems for quantitative studies of the relevant microbial reactions. Different microbial communities are spatially separated and sometimes dramatically enriched near steep redox transitions. These properties facilitate the quantitative analysis of these microbial ecosystems under in-situ conditions. The aims of the proposed research are to analyse and model microbial processes at the oxic/anoxic interfaces of different meromictic lakes and to characterize the microbial communities involved. A multidisciplinary approach will be used including microbiology, molecular ecology, isotope geochemistry, aquatic chemistry and numerical modelling. The established cooperation will allow a complementary approach based on new and well-proven methods.
The following research questions will be addressed by this project:
- What is the quantitative distribution of microorganisms belonging to different physiological groups at the oxic/anoxic interface of meromictic lakes?
- How large are the relative rates of the relevant processes contributing to carbon fixation and sulfur turnover in the meromictic zone?
- Which are the consequences of this microbial activity on nutrient cycling in the water column?
Two expeditions to meromictic lakes in Southern Siberia study (lakes Shira and Belio) and a lake in Azerbaijan (Lake Geck Gel) will be organized and carried out. A multidisciplinary aquatic systems analysis will include profiling campaigns in the water column for hydrochemical and isotopic analyses, determination of microbial rates and water sampling for detailed microbiological studies. The chemical profiles will include the major biogenic compounds and dissolved gases. Mass spectrometry will yield the carbon isotopic composition of dissolved inorganic carbon, particulate organic carbon and methane. Sulfur isotope ratios of hydrogen sulfide and sulfate will also be analyzed. Incubations with radiotracers will yield the depth distribution of microbial transformation rates such as methanogenesis and CH4 oxidation, sulfate reduction and sulfide oxidation, dark CO2 fixation and photosynthesis. The bacterial population will be estimated using most probable number techniques and fluorescent in-situ hybridisation.
Those samples for which the largest transformation rates have been detected in chemical profiles and rate measurements will be used for enrichment cultures, isolation and genetic characterization using sequencing techniques. A one-dimensional lake model will be adapted in order to determine continuous depth profiles of microbial transformation rates. The model will integrate concentration profiles and radiotracer incubations to estimate the optimum values of kinetic parameters and determine the sensitivity and uncertainty of model estimates. It will be used to determine the pathways in microbial CO2 fixation, to generalize the findings based on published values from other meromictic lakes and to quantify the nutrient cycling at the redox boundary.
The study will yield insights into the accuracy and precision of different methods for quantifying microbial reactions under field conditions. The microbial pathways which will be elucidated in two different field sites and cross-checked with other published studies will also form a solid background for ongoing studies in marine systems such as the Black Sea and other major anoxic basins of the world oceans.
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Coordinator
8600 Duebendorf
Switzerland
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