Page path:

02.03.2009 The Mar­ine Ni­tro­gen Cycle - Re­vis­ited

Predicting the ocean’s role for global climate requires a profound understanding of the cycling of matter in the sea.
Regarding nitrogen losses, scientists were apparently off the mark. A novel study reveals the complex interactions in the nitrogen cycle of the Eastern Tropical South Pacific oxgen minimum zone.
 
In­duced by global warm­ing, re­gions of oxy­gen-poor wa­ter – so-called ox­gen min­imum zones – are ex­pand­ing in the world’s oceans. That has sig­ni­fic­ant con­sequences on the mar­ine hab­itat and fish­er­ies, as higher or­gan­isms avoid these re­gions. The global ele­mental cycles of car­bon and ni­tro­gen are closely linked to ox­gen min­imum zones. There­fore, de­tailed know­ledge of these cycles is es­sen­tial for pre­dict­ing the ef­fects of cli­mate change on the oceans as well as pos­sible feed­back mech­an­isms. A study by an in­ter­na­tional group of sci­ent­ists around Phyl­lis Lam from the Max-Planck-In­sti­tute for Mar­ine Mi­cro­bi­o­lgy in Bre­men, Ger­many, pub­lished in the journal “PNAS”, brings us a big step closer to this un­der­stand­ing.
The sci­ent­ists con­cen­trated on the ni­tro­gen cycle of the Per­uvian ox­gen min­imum zone in the east­ern Trop­ical South Pa­cific. This re­gion is one out of three regoins in the world’s oceans where ni­tro­gen es­capes from sea­wa­ter. „For a long time, this loss was at­trib­uted to de­ni­tri­fic­a­tion, which trans­forms ni­trate to gaseous ni­tro­gen, which can then es­cape to the at­mo­sphere“, Lam ex­plains. „This pic­ture is chan­ging: Ap­par­ently, the so-called anam­mox-bac­teria are re­spons­ible for the ma­jor part of the lost ni­tro­gen. However, up to now it has been un­clear where the anam­mox-bac­teria ob­tain their re­sources for this trans­form­a­tion.“ Moreover, the lack of de­ni­tri­fic­a­tion strongly ques­tions our un­der­stand­ing of the closely-linked car­bon cycle – if not by de­ni­tri­fic­a­tion, how else is or­ganic mat­ter de­graded in these oxy­gen-de­pleted wa­ters?
Lam’s res­ults shake the pre­vi­ous as­sump­tions about the ni­tro­gen cycle in the Per­uvian oxy­gen min­imum zone. Ex­per­i­ments as well as mo­lecu­lar ana­lyses show that sev­eral pro­cesses (present­ing the lay­man with quite some tech­nical terms) are in­volved (Fig. 1): The ma­jor pro­por­tion of ni­tro­gen is in­deed lost through Anam­mox. It is dir­ectly coupled to ni­trate re­duc­tion and aer­obic am­mo­nia ox­id­a­tion (the first step of ni­tri­fic­a­tion) for sources of NO2-. The NH4+ re­quired by anam­mox ori­gin­ates from dis­sim­il­at­ory ni­trate re­duc­tion (DNRA) and re­min­er­al­iz­a­tion of or­ganic mat­ter via ni­trate re­duc­tion and likely mi­croaer­obic res­pir­a­tion. The im­port­ance of the single pro­cesses var­ies between shelf and open ocean set­tings as well as the depth lay­ers of the OMZ. Be­sides, the find­ing of DNRA it­self is also sur­pris­ing, be­cause up till now, it has gen­er­ally been con­sidered in­sig­ni­fic­ant in the open ocean.
Figure 1: Anammox (yellow) has been found to be the predominant nitrogen loss pathway and was directly coupled to nitrate reduction (red) and aerobic ammonia oxidation (green) for sources of NO2-. The NH4+ required by anammox originated from DNRA (blue) and remineralization of organic matter via nitrate reduction and likely microaerobic respiration. Microaerobic conditions at least in the upper part of the OMZ were implicated by the occurrence of nitrification, which diminishes in importance from shelf to open ocean as well as deeper into the lower OMZ. In contrast, NH4+ production due to nitrate reduction and DNRA became increasingly important in the lower OMZ and offshore. ‘Assim.’ and ‘Remin.’ stand for assimilation (grey) and remineralization (brown) respectively. Nitrogen fixation (grey dashed) might be spatially coupled to nitrogen loss near the OMZ but has not been assessed in this study. (Source: Phyllis Lam, MPI; published in PNAS)
There­with, Lam and her col­leagues chal­lenge the pre­vail­ing opin­ion that ni­trate from the deep sea is re­spons­ible for all the ni­tro­gen losses from the Ocean. Its frac­tion sums up to only about 50 per­cent, while the re­main­ing losses were at­trib­uted to re­min­er­al­ized ni­tro­gen (ori­gin­at­ing from or­ganic ma­ter­ial).
Hitherto ex­ist­ing cal­cu­la­tions of ni­tro­gen losses, re­ly­ing only on meas­ure­ments of the ni­trate de­fi­cit, may there­fore have sub­stan­tially un­der­es­tim­ated the ef­fect­ive losses from the Ocean – par­tic­u­larly if the same ap­plies to the other OMZs in the world. „Es­pe­cially the role of re­min­er­al­ized ni­tro­gen needs to be re­con­sidered“. Lam points out, „That is the only way to en­able re­li­able pre­dic­tions about the fu­ture role of the oceans for global cli­mate.“

Fanni Aspetsberger
For fur­ther in­form­a­tion please con­tact:
Dr. Phyl­lis Lam Tel. +49 (0)421 2028 644; plam@mpi-bre­men.de

or the MPI press of­ficers:

Dr. Man­fred Schlösser Tel. +49 (0)421 2028 704; mschloes@mpi-bre­men.de
Dr. Fanni As­pets­ber­ger Tel. +49 (0)421 2028 704; fas­petsb@mpi-bre­men.de

Ori­ginal art­icle: Re­vis­ing the Ni­tro­gen Cycle in the Per­uvian Oxy­gen Min­imum Zone. Phyl­lis Lam, Gaute Lavik, Mar­lene M. Jensen, Jack van de Vossen­berg, Markus Schmid, Dag­mar Woeb­ken, Di­mitri Gutiér­rez, Rudolf Amann, Mike S. M. Jetten and Mar­cel M. M. Kuypers. Pro­ceed­ings of the Na­tional Academy of Sci­ences. DOI: 10.1073/​pnas.0812444106

Par­ti­cip­at­ing in­sti­tu­tions:
Max Planck In­sti­tute for Mar­ine Mi­cro­bi­o­logy, Celsi­usstrasse 1, D-28359 Bre­men, Ger­many.
Mi­cro­bi­o­logy, IWWR, Rad­boud Uni­versity Nijme­gen, Nijme­gen, the Neth­er­lands.
Dir­ec­ción de In­vest­iga­ciones Ocean­o­gráficas, In­sti­tuto del Mar del Perú, Es­quina Gamarra y Gen­eral Valle S/​N Chucuito Cal­lao, Peru.
Lam-PNAS-eng.pdf (72.5 kB)
Press re­lease as pdf
For fur­ther in­form­a­tion please con­tact:
Dr. Phyl­lis Lam Tel. +49 (0)421 2028 644; plam@mpi-bre­men.de

or the MPI press of­ficers:

Dr. Man­fred Schlösser Tel. +49 (0)421 2028 704; mschloes@mpi-bre­men.de
Dr. Fanni As­pets­ber­ger Tel. +49 (0)421 2028 704; fas­petsb@mpi-bre­men.de

Ori­ginal art­icle: Re­vis­ing the Ni­tro­gen Cycle in the Per­uvian Oxy­gen Min­imum Zone. Phyl­lis Lam, Gaute Lavik, Mar­lene M. Jensen, Jack van de Vossen­berg, Markus Schmid, Dag­mar Woeb­ken, Di­mitri Gutiér­rez, Rudolf Amann, Mike S. M. Jetten and Mar­cel M. M. Kuypers. Pro­ceed­ings of the Na­tional Academy of Sci­ences. DOI: 10.1073/​pnas.0812444106
Figure 3: Major oceanic oxygen minimum zones (OMZs) in the World - Arabian Sea, Eastern Tropical North Pacific (ETNP) and Eastern Tropical South Pacific (ETSP). A map showing the annual mean dissolved oxygen levels at 200 m below surface. The bounding box indicates the Peruvian Oxygen Minimum Zone, as part of the ETSP and the region of interest in this study. (Source: http://iridl.ldeo.columbia.edu/, IRI/LDEO Climate Data Library, Columbia University; original raw data from World Ocean Atlas 2005, NODC, NOAA)
Figure 2: The research vessel R/V José Olaya Balandra from the Instituto del Mar del Perú (IMARPE) was used to retrieve samples from the Peruvian oxygen minimum zone. (Source: www.imarpe.pe)
Back to Top