Role of lake-catchment-atmosphere linkages for bacteria

Alpine lakes are characterized by small catchment areas and dissolved organic carbon (DOC) concentrations generally below 1 mg L-1 . Consequently, their waters are very transparent to solar ultraviolet radiation (UVR). Although being remote these ecosystems are particularly sensitive to climate change and can be influenced by long-range transport of nutrients and microorganisms associated to aerosols. The increase in air temperatures as a consequence of climate warming seems to enhance the linkages between atmosphere, catchment, and high- mountain lakes. Because of their very low nutrient and DOC content, allochthonous dissolved organic matter (DOM) inputs might have particularly marked consequences for the bacterial assemblage of alpine lakes. The community structure and functioning of heterotrophic bacteria in high-mountain lake remains still poorly understood. Recent methodological developments have permitted to determine that the bacterial community from these ecosystems is often dominated by few bacterial groups, particularly Actinobacteria and the R-BT group of ß- Proteobacteria appear to be key actors in these systems, yet exhibiting a different strategy. Nevertheless, knowledge on the consequences of compositional changes for the functioning (production, respiration) of the heterotrophic bacterial assemblage is still scarce in these and other type of lakes. During the 3-year project, we aim to evaluate the importance of the atmospheric and terrestrial compartments as a source of organic nutrients and heterotrophic bacteria to two alpine lakes, as well as to assess how the autochthonous bacterial assemblage respond to these allochthonous inputs. In addition, we will assess to which extend temporal shifts in the bacterial community structure relate to the functioning of the lake and how UVR may affect the balance between bacterial production and respiration. To test the five hypotheses defined in this project, we will conduct both field and laboratory activities using a combination of methods to estimate the abundance (flow cytometry), activity (microautoradiography), growth efficiency (calculated from bacterial production and respiration measurements), and composition of bacterioplankton as tracked by the CARD- FISH method. Results obtained in this project will fundamentally contribute to understand the importance of linkages between the terrestrial, atmospheric, and aquatic compartment and their consequences for the heterotrophic bacterial assemblage, and thus for the biogeochemical cycle of carbon in alpine lakes.

Alpine lakes are characterized by small catchment areas and dissolved organic carbon (DOC) concentrations generally below 1 mg L-1 . Consequently, their waters are very transparent to solar ultraviolet radiation (UVR). Although being remote these ecosystems are particularly sensitive to climate change and can be influenced by long-range transport of nutrients and microorganisms associated to aerosols. The increase in air temperatures as a consequence of climate warming seems to enhance the linkages between atmosphere, catchment, and high- mountain lakes. Because of their very low nutrient and DOC content, allochthonous dissolved organic matter (DOM) inputs might have particularly marked consequences for the bacterial assemblage of alpine lakes. The community structure and functioning of heterotrophic bacteria in high-mountain lake remains still poorly understood. Recent methodological developments have permitted to determine that the bacterial community from these ecosystems is often dominated by few bacterial groups, particularly Actinobacteria and the R-BT group of ß- Proteobacteria appear to be key actors in these systems, yet exhibiting a different strategy. Nevertheless, knowledge on the consequences of compositional changes for the functioning (production, respiration) of the heterotrophic bacterial assemblage is still scarce in these and other type of lakes. During the 3-year project, we aim to evaluate the importance of the atmospheric and terrestrial compartments as a source of organic nutrients and heterotrophic bacteria to two alpine lakes, as well as to assess how the autochthonous bacterial assemblage respond to these allochthonous inputs. In addition, we will assess to which extend temporal shifts in the bacterial community structure relate to the functioning of the lake and how UVR may affect the balance between bacterial production and respiration. To test the five hypotheses defined in this project, we will conduct both field and laboratory activities using a combination of methods to estimate the abundance (flow cytometry), activity (microautoradiography), growth efficiency (calculated from bacterial production and respiration measurements), and composition of bacterioplankton as tracked by the CARD- FISH method. Results obtained in this project will fundamentally contribute to understand the importance of linkages between the terrestrial, atmospheric, and aquatic compartment and their consequences for the heterotrophic bacterial assemblage, and thus for the biogeochemical cycle of carbon in alpine lakes.