Freshwater bacterial groups and phosphorus in mountain lakes

Alpine lakes, located by definition above the treeline, are very oligotrophic systems characterized by low dissolved organic carbon and nutrient concentrations. Inorganic phosphorus is particularly scarce, but also total dissolved phosphorus concentrations are generally < 0.1 M. Bacterial production in these systems has been found to be limited by phosphorus and the ubiquitous R-BT subgroup of Betaproteobacteria, to dominate the bacterial community when the phosphorus supply increases. Nonetheless, knowledge on how different freshwater bacterial groups/lineages cope with phosphorus (scarcity) in these ecosystems is very limited. In this 3 years project, we want to investigate the strategies used by different heterotrophic bacterial groups to deal with the low phosphorus concentrations present. The project will cover four different, but closely interrelated, objectives. The first one aims to determine which bacterial groups are responsible for inorganic versus organic phosphorus uptake using ATP as a model phosphoester. The second one is the characterization of bacterial groups utilizing different dissolved organic phosphorus compounds, which represent different transport and uptake mechanisms (direct versus enzymatic breakdown). Our third objective aims to assess which enzymes are expressed by bacterial groups in situ, as well as after the addition of model phosphorus sources differing in their stoichiometry (e. g., wet deposition). The last objective is to understand how the elemental composition of dissolved organic matter (DOM) sources affects the stoichiometry of bacteria in mountain lakes situated both below and above the treeline. To fulfill these objectives, we will conduct field and laboratory research employing a variety of single-cell and molecular methods as microautoradiography (MAR), fluorescent in situ hybridization with catalyzed reporter deposition (CARD-FISH), flow cytometry/cell sorting and metatranscriptomics in order to tighten the link between community composition and function. Significant progress will be made in characterizing which bacterial groups/clades are key for the cycling of inorganic phosphorus and diverse organic phosphorus compounds in alpine lakes, the underlying transport and enzymatic mechanisms, as well as how their stoichiometry is affected by phosphorus sources. Results from this project will fundamentally contribute to a better understanding of the biochemical cycle of phosphorus in mountain lakes and freshwaters in general.

The purpose of this project was to determine which heterotrophic freshwater bacteria were responsible for inorganic and organic phosphorus uptake and which strategies individual bacterial taxa use to cope with the low phosphorus concentrations found in mountain lakes. Further, we tried to identify the potential impact of climate-induced shifts in vegetation (treeline) and concomitant changes in soil composition on lake bacterial community composition and function. Phosphorus is an essential nutrient for life needed in numerous metabolic pathways such as in energy generation, nucleic acid (e.g., DNA) and phospholipid synthesis (e.g. cell wall). The generally low phosphorus concentrations in mountain lakes limit both algal and heterotrophic bacterial growth. Heterotrophic bacteria play a central role in the biogeochemical cycle of elements in aquatic ecosystems. They (re-) cycle dead organic material to biomass and thus, provide energy and nutrients to higher organisms (e.g. zooplankton, fish). Despite the important role of phosphorus in aquatic ecosystems and the common limitation of bacterial growth by this nutrient, very little is known about the identity of freshwater bacteria involved in phosphorus uptake. By applying cultivation-independent approaches, we gained first insights in the phosphorus dynamics of mountain lakes and identified different strategies used by bacterial members to cope with low phosphorus concentrations. For instance, we found that at low ambient phosphorus concentrations the uptake of dissolved organic phosphorus compounds is a widespread strategy among all freshwater bacterial taxa examined. Furthermore, we found that different bacterial taxa have variable phosphorus requirements, which means that bacterial taxa differ in their needs for phosphorus to maintain growth. It seems that slow-growing bacteria (here: AcI Actinobacteria) have lower phosphorus requirements than other freshwater bacteria, favouring their prevalence in phosphorus-limited lakes. By contrast, the phosphorus requirements of some fast-growing bacterial lineages such as the R-BT cluster of Betaproteobacteria were either proportional or higher than their growth activity. Phosphorus is present in the ribosomes that are essential organelles for (fast) growth. The overrepresentation of the RBT cluster in phosphorus uptake could explain why they are able to rapidly overgrow lake bacterial communities at high loadings of organic matter and/or at high phosphorus concentrations.This information is particular relevant because climate-induced induced changes in soil runoff composition (carbon and phosphorus availability) could affect bacterial growth and thus, lake ecosystem status. In experiments done in a high altitude and in a high latitude lake, we assessed whether climate-induced changes in plant cover (treeline) and soil chemical composition alter lake bacterial community composition and function. The results obtained in this study highlight that climate-induced changes in lake catchments affecting the availability of phosphorus and carbon in their soils, have a strong impact on bacterial community composition and biomass production rates and thus, on the flux of carbon and phosphorus to higher organisms.