PhD Defence: Lisa Johanna Winberg Von Friesen

Thesis title: Pelagic nitrogen fixation and diazotrophs in the Eurasian Arctic Ocean - A missing piece of the Arctic nitrogen puzzle

Supervisor: Prof. Lasse Riemann, Department of Biology, University of Copenhagen

Assessment Committee:

Prof. Per Juel Hansen, Department of Biology, University of Copenhagen
Prof. Claire Mahaffey, University of Liverpool, United Kingdom
Prof. Pia Moisander, University of Massachusetts Dartmouth, USA

Popular scientific abstract

Nitrogen-fixing bacteria fertilise the Arctic Ocean

Just like plants on land need nutrients from the soil to grow, ocean plants need nutrients from the surrounding seawater. Nitrogen is one of those nutrients because it is an important element in cells as a building block in for example proteins and DNA. The atmosphere consists of ~78% nitrogen gas which is a form of nitrogen not possible for most organisms to use. The artificial fertilisers used in agriculture come from a chemical process called Haber-Bosch, where nitrogen gas from the atmosphere is industrially converted to ammonia – a type of nitrogen available to organisms. There is, however, a group of microorganisms (bacteria and archaea) that naturally have the capability of converting nitrogen gas into ammonia inside the cell. This process is called nitrogen fixation and the microorganisms are called diazotrophs. In the oceans, diazotrophs can sometimes be the main providers of nitrogen to photosynthesizing organisms (i.e. those who transform carbon dioxide into organic matter with energy from sunlight). That means that when nitrogen fixation is active in the ocean, more algae can grow and transform carbon dioxide into biomass (primary production). In this way, the availability of nitrogen in the ocean can influence how much carbon dioxide there is in the atmosphere – of importance because of its impact on the greenhouse effect and climate change.

Marine nitrogen fixation was originally thought to take place mainly in tropical oceans but has now been detected even in parts of our northernmost ocean – the Arctic Ocean. Nitrogen is the main nutrient limiting the growth of photo-synthesising organisms in the Arctic Ocean. This means that if nitrogen fixation is active in a certain location, the nitrogen it adds can enable more primary production. Climate change is rapidly transforming the Arctic Ocean, where the decreasing sea ice results in more light entering this previously darker ocean. The combination of light and nutrients like nitrogen will influence what role the Arctic Ocean plays in the global climate system now and in the future.

In this PhD thesis, I investigate how much nitrogen fixation takes place in sea-water, sea ice and sediment and study which diazotrophs are present in Atlantic-influenced parts of the Arctic Ocean. Different diazotrophs have different metabolisms where some require light and can photosynthesize (cyano-bacterial diazotrophs) and others instead need organic matter (non-cyanobacterial diazotrophs). This PhD thesis shows that nitrogen fixation is an active process in the Atlantic-influenced Arctic where it was previously only known from the coastal Pacific-influenced parts of the Arctic. The main diazotrophs likely responsible for nitrogen fixation are different from those in the Pacific Arctic and other oceans by being dominated by non-cyanobacterial groups. Which diazotrophs who are present sharply changes when moving from temperate to polar waters and are, once inside the Arctic Ocean, influenced by glacial meltwater and season. Three key groups of non-cyanobacterial diazotrophs are identified in this study, and the nitrogen fixation activity is mainly linked to (i.e. being controlled by) organic carbon and the availability of nutrients.

Altogether, it is in this PhD thesis shown that nitrogen fixation is an overlooked source of nitrogen in the Atlantic-influenced Arctic Ocean, being driven by different diazotrophs than elsewhere. The distribution of diazotrophs and their nitrogen-fixing activity will likely change and regionally increase with climate change-induced alterations of the Arctic system. The information in this PhD thesis contributes to understanding how the Arctic ecosystem functions in present-day conditions and proposes hypotheses of how nitrogen availability may change in the future.

Zoom link: https://ucph-ku.zoom.us/j/69530126125?pwd=bGV1WDFsVUtqR0Q0RXk5dFRPMXI1UT09

Meeting ID: 695 3012 6125
Passcode: 652824