Aarhus University Seal / Aarhus Universitets segl

Turnover of nitrogen and carbon in marine areas

Carbon turnover in Arctic waters

Our research is focused on how production and decomposition of plankton algae influence the exchange of carbon dioxide between the sea and the atmosphere and how this is regulated in Arctic waters. The regulating factors explored are, among others, nutrient availability, light conditions, content of organic matter, freshwater input, etc.


Accumulation of carbon dioxide (CO2) in the atmosphere is significantly counteracted by the marine uptake of CO2. This CO2 is bound in algal material forming part of the food chains. The major part of this is decomposed to CO2 near the surface and can be used for new production or is returned to the atmosphere. The remaining part is exported to the deep sea or is buried in the sea bed and thus no longer active as a greenhouse gas. The production of algal material requires both light and nutrients.

For some part of the year in the Arctic, sea ice functions as a lid that more or less hinders the penetration of light and thereby inhibits algal growth. In summer light is available, but algal growth is often limited by the availability of nutrients. The surface layer will be relatively fresh in consequence of melting of ice and snow, which creates stratification that prevents transfer of nutrients from below. The stratification vanishes in the autumn, but then formation of ice may prevent the light from reaching the algae. The global warming delays the formation of ice and prolongs the period of algal production. Whether this late production will have quantitative significance is unknown; its size will depend on whether the transfer of nutrients during autumn mixing will occur so early that light is sufficient for production to continue.

Nitrogen turnover in the sea

Today, two microbial processes, anammox and denitrification, are known to catalyse the removal of plant-available nitrogen from the marine environment. My research focuses on how these processes are regulated and the rate with which they remove nitrogen, comprising projects in Danish waters, the Baltic Sea and the Pacific Ocean.


Nitrogen is necessary for all living organisms, primarily for incorporation in proteins and genetic material. In the sea the production of algae, primary production, is mainly limited by the availability of nitrogen. Thus, input of nitrogen from land permits a higher primary production in coastal-near waters than in the oceans where the main source of nitrogen comes from fixation of nitrogen from the air (N2) by microorganisms.

Concurrently with the input of nitrogen, plant-available nitrogen is removed and converted to N2. This takes place in areas with very low oxygen concentrations, for instance in the sea bottom where oxygen is only available in the surface layer and in the oceans’ oxygen minimum zones. The latter are huge areas in the Pacific Ocean and the Arabian Sea that are virtually devoid of oxygen. It is assumed that up to half of the marine nitrogen removal takes place in the oxygen minimum zones and the remainder in the sea bottom.

Turnover of nitrogen and nitrate storage by microorganisms

Several widely different microorganisms are specialists in storing nitrate in their cells. This nitrate storage provides the organisms with a unique life strategy without oxygen, which impacts the availability of nitrogen in the marine environment.


Nitrogen storage is particularly well-known for the large white sulphur bacteria Beggiatoa and Thioploca forming visible white mats on the sea bottom in areas with low oxygen concentrations. But also other organisms, for instance foraminifers and gromiids, store large amounts of nitrate. The nitrate-storing life forms impact the nitrogen cycle and we are currently investigating the nitrate turnover by microorganisms. Sulphur bacteria typically convert nitrate to ammonium and the nitrogen remains present in the environment. In contrast, foraminifers convert the nitrate to free nitrogen which is not available to the marine algae.

Recent research is directed at gromiids. We do not know whether the gromiids’ own enzymes may get energy from the stored nitrate or if the gromiids enter into a symbiosis with intracellelur bacteria. The end product of the gromiid nitrate conversion is also unknown. In other projects focus is on microorganism production of nitrous oxide. Nitrous oxide is a potent greenhouse gas and microbial biofilm attached to algae and invertebrates may, under certain conditions, produce large amounts of nitrous oxide (N2O). The research is undertaken in Greenland and Denmark in cooperation with the Sections of Microbiology and Aquatic Biology, the Arctic Centre and the Greenland Institute of Natural Resources.