In the largest ocean area where the oxygen level is so low that it is not measurable, nitrogen escapes to the atmosphere. Located in the South Pacific, this region contributes significantly to the loss of nitrogen from the global ocean. In a study published in Nature , scientists reveal the main mechanisms involved.
The human disturbance is particularly important. Anthropogenic flows, related to industry and chemical fertilizers, are as important as natural flows. Industrial activities emit 90 million tonnes of oxides (NOx) and (NH 3 ) per year into the . The spreading 80 million tons of nitrogen compounds in the soil each year.
Understanding is a key factor in predicting the ocean’s response to , as it is one of the limiting for life in the oceans. Nitrogenxa exchanges are dominated by microbial transformations, with assimilating it only in the form of minerals (NO3– and NH4+). In the ocean, there are ( ) where are able to use various sources of dissolved nitrogen. They produce such as nitrous oxide, or (N 2 O), 300 times more powerful than CO 2 , which are then released into the atmosphere.
These OMZs contribute 20 to 40% of the total loss of nitrogen and would tend to spread. The largest OMZ in the world is located east of the South Equatorial Pacific. Scientists at the Max Institute for Marine Microbiology have studied this area closely. They sought to identify and interpret nitrogen loss regulation mechanisms. “This research is fundamental to improving current biogeochemical models because so far our models can not faithfully reproduce the nitrogen loss trends that are measured,” says Andreas Oschlies of Geomar of Kiel, Germany.
Bacteria by billions in areas without oxygen
Recently published in the journal, their results show that the loss of nitrogen in the OMZ of the southern equatorial Pacific is related to the burial of organic in coastal waters with low . Indeed, this region is home to billions of . They fix the nitrogen and convert it into gas by two processes, denitrification and ammonization (ie the anaerobic of the ammonium , NH4+ , with a nitrite ion NO2– ) . The NH4+ ion comes from the mineralization of organic matter.
The factors that regulate nitrogen loss by anammox remained previously unknown. In this study, the researchers estimated the total nitrogen balance of OMZ by measuring nutrient concentrations, nitrogen conversion rates, and from a that calculates the export rate of nitrogen. ‘nitrogen. They found that nitrogen loss rates peak in highly productive , where large amounts of organic matter are falling from the ocean surface.
The fall of organic matter as a trigger
Anammox was therefore the dominant mode of nitrogen loss at the time of . Overall, nitrogen transformation rates, including anammox, were strongly correlated with the export of . The drop of organic matter (and thus the release of ammonium in the water column) as well as the release of ammonium fuels the loss of nitrogen from the zones of minimum .
These results will make it possible to more realistically estimate the impacts of ocean deoxygenation. They will also help to determine precisely how bacterial productivity changes the nitrogen cycle in OMZs, as well as the rest of the ocean. This is essential to estimate the amount of that can be absorbed by the ocean in the future.