Abstract
Nitrogen holds a key role in the biosphere. As macronutrient it is often in short supply and limits biological production. Nitrogen fixation is a key process transforming the abundant, but for most biota, inaccessible molecular nitrogen to bioavailable forms. In freshwaters, nitrogen fixation has historically been attributed to bloom-forming cyanobacteria that use solar energy to carry out the energetically costly nitrogen fixation. However, some studies point to a significant role of other bacteria in ecosystem-scale nitrogen fixation. The aim of this project is to study the mechanisms and ecosystem features that control nitrogen fixation in stratified freshwater lakes and determine the significance of various energy acquisition strategies in driving this ecosystem process. We will test whether redox gradients in stratified lakes are sites of enhanced nitrogen fixation activity and study the respective roles of mixotroph and methanotroph bacteria in this process. The role of non-cyanobacterial nitrogen fixers will be compared between lakes and also in mesocosms with contrasting redoxcline stability and humic substance inputs. A better understanding of this process will help us understand the role of lakes in global and regional nitrogen budgets, predict the change in nitrogen fixation in response to environmental change and develop watershed management strategies to avoid adverse environmental effects locally and downstream in the coastal biome.
Sweden