Nitrogen is an essential nutrient and often a limiting factor for all life on our planet. It is present in proteins and DNA. In the oceans, microbial processes regulate the concentrations and fluxes of biological relevant nitrogen compounds like ammonia, nitrite and nitrate, which have to be available for the marine life. The major sink through which nitrogen can escape from the marine food web into the atmosphere is as nitrogen gas, N2. The driving forces balancing this system are more complex than previously thought.
The Arabian Sea is part of the Indian Ocean and is bordered by India, Pakistan, Oman and Somalia.
Manfred Schloesser, Max Planck Institute for Marine Microbiology
N-losses as a result of the coupling of two reaction pathways. In the Arabian Sea off the coast of Oman, DNRA (blue) provides ammonia for the anammox reaction (yellow), thus producing nitrogen gas N2that can escape from the water column. Nitrate reduction and nitrification also take place and act as sources of nitrite, and also of ammonia by the former reaction. Meanwhile, there is little evidence for denitrification activity (red dashes).
modified from Lam et al., PNAS, 106:4752-4757).
Now scientists from the Max Planck Institute for Marine Microbiology and their colleagues have taken a very close look at the microbial processes in the Arabian Sea and published their results in two scientific papers.The marine food web stores huge amounts of organic carbon compounds. The carbon cycle is interacting with both the dissolved molecular oxygen (O2) and the nitrogen cycle. Global warming results in a diminished solubility of oxygen, and the influx of waste-water loaded with organic compounds from the human civilization further consumes oxygen. Consequently, the oxygen-deficient waters or oxygen minimum zones (OMZ), which originally constituted only
Their findings were surprising. The central-northeastern area of the Arabian Sea, which was thought to be the stronghold, was proven to be almost inactive in N-losses during their visit. The scientists now explain the high nitrite concentrations found there by a slow nitrate reduction and little oxidation of ammonia. Both reactions can run under low oxygen conditions and form nitrite as a final product. Satellite data from the past 10 years show that surface phytoplankton production in this region is not particularly high on average. Due to such likely missing organic matter, nitrite cannot be reduced further. Together with the sluggish water circulation, nitrite therefore accumulates in this region of the Arabian Sea.
On the contrary, in the northwestern part off the coast of Oman, which was previously assumed to be irrelevant regarding nitrogen balances, the researchers detected very high N-loss activity. As shown in their publications, two coupled reactions in the nitrogen cycle can do the trick: the anammox reaction (anaerobic oxidation of ammonia) and the dissimilatory nitrate reduction to ammonia (DNRA). Like in a detective story, the scientists found the telltale evidence of 15N-labeled compounds, as double-15N-labeled N2 was formed from labeled nitrite through a combination of anammox and DNRA. DNRA provides the important ammonia for the anammox reaction, which needs both ammonia and nitrite to form gaseous N2. Further proof came from gene expression studies showing which microbial genes were actively engaged in the pathways. This DNRA-anammox coupling, in addition to some anammox alone, explains the high N-loss in these waters.
Dr. Marcel Kuypers, Max Planck director, says: “Our findings fit very well with our previous results from other OMZs like the upwelling regions off the coasts of Peru, Chile and Namibia, where we also found anammox to be the most important N-loss pathway. The high nitrite concentrations in the central-northeastern Arabian Sea are presumably the last traces of earlier events which are now leveling off.”
Dr. Phyllis Lam from the Max Planck Institute adds: “In the future, the Arabian Sea should remain in our research focus, as reactions therein have strong impacts on the global nitrogen balance. It is unlikely that active nitrogen cycling remains the same throughout the year with respect to the seasonal monsoons, and it will continue to alter with the increasing amounts of nitrogen inputs from the atmosphere and land due to human activities. Unfortunately, pirate activities will not allow further research expeditions in the area any time soon.”
Manfred SchloesserFurther information
InstitutionsMax Planck Institute for Marine Microbiology, Bremen, Germany
Department of Microbiology, IWWR, Radbound University Nijmegen, The Netherlands
Dr. Manfred Schloesser | Max-Planck-Institut
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