Kathrin Rousk

The research in my group focusses on cyanobacteria that can fix atmospheric nitrogen, thereby converting it into a plant available form. Nitrogen is essential for all life but is not easily available - it has to be fixed from the atmosphere by a unique enzyme, nitrogenase. Many cyanobacteria possess this enzyme and many of them happen to colonize a specific plant group, mosses.


Mosses are dominant ecosystem components in many habitats such as arctic tundra, boreal forests and tropical cloud forests. Here, they can cover most of the ground or stems and branches and together with their cyanobacterial associates, they contribute at least half to total ecosystem nitrogen input.


In my group, we are trying to find out under which conditions nitrogen fixation by cyanobacteria on mosses occurs, how it is affected by climate change, who is doing the job and what role the moss host plays in promoting colonization and activity of cyanobacteria. We employ lab and field based experiments that takes us across the globe from arctic via temperate and Mediterranean to tropical zones.

A few years ago, I have received a Sapere Aude Starting Grant from the Independent Research Fund Denmark that laid the foundation for the ERC Starting grant, which I received during the pandemic in 2020. Luckily, I could delay the start of the ERC Starting grant so that we could make sure we are more or less allowed back to Uni. In both grants, we aim to answer fundamental questions on moss-cyanobacteria associations using traditional (gas chromatography, microscopy, culturing) as well as novel tools (transcriptomics, nanoSIMS), aiming to link ecology with biogeochemistry.

Since the beginning of the ERC grant this spring, I have hired one PhD student complementing my growing group, and we have already been to one field trip to Northern Sweden. Here, we assessed how nitrogen fixation by cyanobacteria on mosses changes along an elevation gradient in the Subarctic. We measured nitrogen fixation in situ as well as in the laboratory in Copenhagen. At the moment, the samples are being processed to identify the bacterial community composition on the mosses along the gradient to be able to link function (nitrogen fixation) to the players, and to determine how the communities change along the gradient.


We also assessed how nutrients (nitrogen, phosphorus) affect nitrogen fixation by cyanobacteria along the gradient using a large scale nutrient addition field experiment established by collaborators from Lund University, Sweden.

The same collaborators provided also access to a unique climate warming experiment where entire field plots were heated up with café-style infrared heaters to realistic and extreme temperatures. Here, we also collected moss samples and they will be assessed for nitrogen fixation activity and more by two students who are doing their research projects (“PUK”) with us.


So, we have many samples from the field that are now being dealt with in my lab. We also work with moss samples from e.g. deserts in New Mexico (who is there and how much nitrogen is fixed here?) and other exciting places!

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My group will take to Costa Rica in November for fieldwork. Here, we will hike up the highest mountain in Costa Rica (Chirripo, 3800m), passing different ecosystem types from lowland forests through cloud forests to the alpine-like top.

We will assess nitrogen fixation in mosses along this steep and fascinating elevation gradient, and answer questions like who is there (which bacteria) and do they fix nitrogen? How much nitrogen is fixed by them? Which moss species are colonized?  How is climate change affecting the composition and activity of moss-associated cyanobacteria?


Besides the effects of abiotic factors (climate, nutrients) on cyanobacterial activity on mosses, were are also very much interested in the relationship between moss and colonizing cyanobacteria. Do they interact with each other or do they just sit on top of each other? Do they exchange nutrients with each other and thereby, benefitting from growing together?

These are some of the questions we aim to answer in the ERC Starting grant and we hope to use moss-cyanobacteria associations as model systems to unravel how plant-microbe interactions have been and are being, evolved.  

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