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26.07.2011 Ni­tro­gen losses off the Coast of Oman

In­tens­ive Ni­tro­gen losses off the coast of Oman
Caused by Coup­ling of two mi­cro­bial pro­cesses
 
Bremen, 26 July 2011

Intensive Nitrogen losses off the coast of Oman
Caused by Coupling of two microbial processes

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, N<sub>2</​sub>. The driv­ing forces bal­an­cing this sys­tem are more com­plex than pre­vi­ously thought. 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 (O<sub>2</​sub>) 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 <<1% global ocean volume yet re­spons­ible for 30-50% of the global mar­ine N-losses, have been spread­ing world­wide in the last few dec­ades. More N-losses will thus be ex­pec­ted.

The Ar­a­bian Sea har­bors one of the three largest OMZs in the world and about 10-20 % of all global mar­ine N-losses is thought to hap­pen there. Thus far, it has been re­garded as a fact that a bac­terial pro­cess called de­ni­tri­fic­a­tion was the ma­jor path­way res­ult­ing in N-losses from the Ar­a­bian Sea, via the step­wise re­duc­tion of ni­trate to ni­trite, then to nitric ox­ide, ni­trous ox­ide and even­tu­ally gaseous ni­tro­gen N<sub>2</​sub>. Earlier stud­ies from other au­thors claimed low oxy­gen and sim­ul­tan­eously high ni­trite con­cen­tra­tions to be an in­dic­ator for de­ni­tri­fic­a­tion and sub­sequent N-losses, but ac­tual activ­ity meas­ure­ments have been rare. To solve this puzzle, re­search­ers from the Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy in­clud­ing Phyl­lis Lam, Mar­lene Jensen and Mar­cel Kuypers, teamed up with sci­ent­ists from Kiel, Olden­burg, Ham­burg, Aar­hus (Den­mark), Nijme­gen, (the Neth­er­lands) and Prin­ceton (USA), to trace the in­di­vidual re­ac­tion steps in the ni­tro­gen cycle by fol­low­ing the fate of com­pounds labeled with the stable iso­tope of <sup>15</​sup>N. Ad­di­tion­ally, they iden­ti­fied the re­spons­ible mi­croor­gan­isms and act­ive ex­pres­sion of the cor­res­pond­ing bio­marker genes.
Left: On board of the German research vessel METOR scientists tracked the fate of the nitrogen in the Arabian Sea. After intensive analyses and interpretation they can explain the underlying processes. (Source: Leitstelle Meteor) Right: The water sampler collected samples at different depths.
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 N<sub>2</sub>that 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). (Source: modified from Lam et al., PNAS, 106:4752-4757).
Labeling the nitrite with the stable isotope <sup>15</sup>N produced a double labeled <sup>15</sup>N-<sup>15</sup>N gas which could easiliy be detected by mass spectrometry. Previously thought to be a signature of denitrification, combined results from various <sup>15</sup>N-labeling experiments, stoichiometric calculations and gene expression analyses show that this is more likely a result of DNRA-anammox coupling. DNRA produces the <sup>15</sup>N-ammonia which will subsequently be combined with <sup>15</sup>N nitrite in the anammox reaction to yield the double labeled nitrogen gas.
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 <sup>15</​sup>N-labeled com­pounds, as double-<sup>15</​sup>N-labeled N<sub>2</​sub> 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 N<sub>2</​sub>. 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 Schloesser

Fur­ther in­form­a­tion
Dr. Phyl­lis Lam
Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy
Phone +49 (0)421 2028 644; plam@mpi-bre­men.de

Dr. Mar­cel Kuypers
Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy
Phone +49 (0)421 2028 602; mkuypers@mpi-bre­men.de

Dr. Mar­lene Mark Jensen
Tech­nical Uni­versity of Den­mark
Phone +45 45251437; mmaj@env.dtu.dk

Press of­ficer
Dr. Man­fred Schloesser, phone +49 (0)421 2028 704;
mschloes@mpi-bre­men.de


1. Original article
Ori­gin and fate of the sec­ond­ary ni­trite max­imum in the Ar­a­bian Sea. P. Lam, M. M. Jensen , A. Kock , K. A. Lettmann, Y. Plancherel, G. Lavik, H. W. Bange , and M. M. M. Kuypers. Biogeosciences, 8, 1565–1577, 2011
doi:10.5194/​bg-8-1565-2011

In­sti­tu­tions
Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy, Bre­men, Ger­many
IFM-GEO­MAR, Kiel, Ger­many
In­sti­tut für Chemie und Bio­lo­gie des Meeres, Uni­versität Olden­burg, Ger­many
De­part­ment of Geosciences, Prin­ceton Uni­versity, USA

2. Original article
In­tens­ive ni­tro­gen loss over the Omani Shelf due to anam­mox coupled with dis­sim­il­at­ory ni­trite re­duc­tion to am­monium. Mar­lene M Jensen, Phyl­lis Lam, Niels Peter Revs­bech, Birgit Na­gel, Birgit Gaye, Mike SM Jetten and Mar­cel MM Kuypers. The ISME Journal (2011) 1-11. doi:10.1038/​is­mej.2011.44

In­sti­tu­tions

Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy, Bre­men, Ger­many
In­sti­tute of Bio­lo­gical Sci­ences, Uni­versity of Aar­hus, Aar­hus, Den­mark
In­sti­tute of Coastal Re­search, GKSS Re­search Cen­ter, Geesthacht, Ger­many
In­sti­tute of Biogeo­chem­istry and Mar­ine Chem­istry, Uni­versity of Ham­burg, Ger­many
De­part­ment of Mi­cro­bi­o­logy, IWWR, Rad­bound Uni­versity Nijme­gen, The Neth­er­lands
The Arabian Sea is part of the Indian Ocean and is bordered by India, Pakistan, Oman and Somalia. (Source: Max Planck Institute for Marine Microbiology).
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