Scientists identify key factor controlling nitrogen availability in the ocean

During an expedition to the South Pacific Ocean, scientists from the Max Planck Institute for Marine Microbiology, along with their colleagues from the GEOMAR Helmholtz Centre for Ocean Research in Kiel discovered that organic matter derived from decaying algae regulates nitrogen loss from the ocean's oxygen minimum zones.

One of the central aims of today's marine research is to better predict the response of our ocean to global warming and human activity in general. Understanding of the oceanic nitrogen cycle is of key importance in this effort as nitrogen is the limiting nutrient for life in the ocean.

Its bio-available form (so-called fixed nitrogen, such as ammonium) is produced biologically from nitrogen gas by bacteria or is transported to the ocean as dust or river run-off.

However, due to the activity of billions of marine microorganisms, this fixed nitrogen is rapidly converted back to nitrogen gas, which escapes from the ocean to the atmosphere. There are two processes, which are mainly responsible for this nitrogen loss: denitrification and anammox (anaerobic oxidation of ammonium with nitrite), both performed by anaerobic bacteria.

Up to 40 per cent of global oceanic nitrogen loss occurs in oxygen minimum zones (OMZ), which are areas with low to non-measurable oxygen concentrations. "The eastern tropical South Pacific OMZ is one of the largest oxygen minimum zones in the world," explains Tim Kalvelage from the Max Planck Institute for Marine Microbiology, the first author of this study.

"We assumed that if we could identify and constrain the parameters that regulate N loss from this OMZ, we could better predict the nitrogen loss from all OMZs, and possibly from the Ocean, as well." Andreas Oschlies from GEOMAR in Kiel adds, "This research is fundamental for the current biogeochemical models to work because so far our models cannot reliably reproduce the patterns of nitrogen loss that we measure."