Research Projects

Large amounts of dissolved inorganic carbon, fixed inorganic nitrogen (NH4+, NO2-, NO3-), and phosphate in ocean-surface waters are consumed by algae and cyanobacteria during photosynthesis. The N/C and P/C ratios of the organic products are similar throughout the marine realm. On average, the C:N:P ratio of this phytoplanktonic biomass is 106:16:1. Carbon dioxide, ammonium and phosphate are released in the same ratio, the so-called Redfield ratio, when this phytoplankton-derived organic matter is remineralized. Under oxic conditions, nitrifying organisms (mainly chemoautotrophic bacteria), will consume O2 and oxidize ammonium via nitrite to nitrate. The nitrate can be reutilized to produce new phytoplanktonic biomass. Alternatively, in oxygen-deficient marine environments, predominately facultative anaerobic prokaryotes can use nitrate as an electron acceptor while oxidizing organic matter. During this process (i.e. heterotrophic denitrification) nitrate is reduced to N2. Some ammonium is also released. In the absence of oxygen, it is expected to remain as ammonium.
For some time, however, oceanographers have known that far less ammonium accumulates in anoxic fjords and basins than would be expected from the stoichiometry of heterotrophic denitrification. To explain this shortfall, it was suggested that microorganisms can combine ammonium and nitrate to yield N2. The first direct evidence for the anaerobic oxidation of ammonium was found in wastewater bioreactors, where so-called ‘anammox’ bacteria belonging to the Order Planctomycetales directly oxidize ammonium to N2 with nitrite as the electron acceptor. Nitrite is an intermediate in both heterotrophic denitrification and aerobic ammonium oxidation.
The anaerobic oxidation of ammonium with nitrite to N2 (anammox), was recently recognized as a major sink for fixed inorganic nitrogen in coastal sediments and in anoxic waters of basins isolated from oxygenated deep circulation (e.g. Kuypers et al., Nature, 2003). The discovery of anammox as a distinct novel pathway of N2 production in marine environments calls for a quantification of its global significance, and an identification of the main factors that regulate this process. Nutrient measurements indicate that 30-50% of the total nitrogen loss in the ocean occurs in oxygen minimum zones. This pelagic nitrogen removal takes place within only ~0.1% of the ocean volume, and hence moderate variations in the extent of oxygen minimum zones may have a large impact on the global nitrogen cycle (see Fig 1). This nitrogen loss has been fully attributed to nitrate reduction to N2 by heterotrophic bacteria (denitrification) because until recently there was no other process known that could transform fixed inorganic nitrogen into N2. In fact, to the best of our knowledge there is so far no published evidence from 15N-labeling experiments that nitrate is directly converted to N2 by heterotrophic denitrifiers in the OMZ waters of the ocean. Recently we showed that anammox bacteria are responsible for massive loss of fixed nitrogen as gaseous N2 from the Benguela OMZ water (Kuypers et al., PNAS, 2005). The possibility that anammox is also a dominant process for nitrogen removal in other OMZ waters of the ocean will now be explored.