An intriguing discovery made by scientists based at Plymouth Marine Laboratory and the National Oceanography Centre, Southampton, could overturn present thinking about the role of shelf seas such as the English Channel and North Sea in global nitrogen budgets.
Nitrogen is an element essential for life, but in the form of a gas (N2), its value as a nutrient source is restricted to a few microorganisms. In a process called nitrogen fixation, such microbes convert N2 into key nutrient compounds such as ammonia and nitrate, thus providing fuel to marine organisms in otherwise low nutrient environments of the world’s ocean. Nitrogen fixation is also important because it replaces nitrogen lost from ecosystems by denitrification - a bacterial process that converts nitrate into N2 gas which escapes back to the atmosphere. Hence, nitrogen fixation ‘closes’ the global marine nitrogen budget.
Using stable isotopes of nitrogen, the researchers found high levels of nitrogen fixation in the western English Channel, a region characterised by high nutrient concentrations. “Until now, nitrogen fixation in the marine environment was largely thought to be restricted to regions in the ocean with low nutrient concentrations” says Dr Boris Kelly-Gerreyn of the National Oceanography Centre and one of the authors of the work.
This is the first evidence of nitrogen fixation in a shelf sea environment. Areas like the English Channel and North Sea were assumed to be ‘sinks’ of nitrogen from the marine environment, because of removal by denitrification. “But that was before our finding...which is tantalizing because it is based on a few measurements”, says Dr Boris Kelly-Gerreyn; “Our data show that rates of nitrogen fixation are similar to known rates of denitrification.” Furthermore, molecular biology techniques allowed the researchers to identify diverse variants of one particular nitrogen fixation gene, demonstrating the unexpected presence of many different kinds of nitrogen-fixing bacteria.
If confirmed by more extensive sampling, the finding that nitrogen fixation more or less counteracts denitrification would overturn present thinking about these seas being globally important nitrogen sinks.
This work contributes to the OCEANS 2025 core science programme of the Plymouth Marine Laboratory and National Oceanography Centre funded by the Natural Environment Research Council.
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