Nitrogen Cycling in the Sea

Transcription

1 Nitrogen Cycling in the Sea NH 4 + N0 2 N0 2 NH 4 +

2 Outline Nitrogen species in marine watersdistributions and concentrations New, regenerated, and export production The processes: Assimilation, N 2 fixation, Nitrification, and Denitrification / Anammox

3 Summary Microbes control global nitrogen budgets The oxidationreduction potential of nitrogen facilitates the role of Ncontaining compounds as electron donors and acceptors. The principal transformations of N in the ocean include: assimilation, regeneration, nitrification, denitrification, and nitrogen fixation Total amount of fixed nitrogen depends on the balance between N 2 fixation and denitrification Ocean nitrogen budgets remain highly uncertain

4 Pools and pathways of nitrogen in the sea Nitrogen as a nutrient (nitrogen assimilation): NO 3 NO 2 NH 4 + DON N 2 only selected groups of prokaryotes use N 2 Nitrogen as an electron donor: NH 4+ : ammonium oxidation/annamox NO 2 : nitrite oxidation DON : heterotrophic catabolism Nitrogen as an electron acceptor: NO 3 : denitirification NO 2 : denitrification, annamox NO: denitrification N 2 O: denitrification

5 Redox states and chemical forms of nitrogen Oxidized N 5 stable oxidation states Reduced N (Sarmiento & Gruber, 2006)

6 Nitrogen assimilation Nitrogen is an essential nutrient found in amino acids, protein, and nucleic acids. Nitrogen is assimilated by both autotrophs and heterotrophs. In large areas of the world s ocean, nitrogen limits primary production. Nitrogen in organic matter is reduced; however, most fixed nitrogen in the ocean is oxidized (nitrate) and thus requires reductant for assimilation into biomass.

7 Energy and reducing power required for assimilation of various N compounds Substrate Enzyme Reaction Electrons ATP N 2 Nitrogenase N 2 NH NO 3 Nitrate NO 3 NO reductase NO 2 Nitrite reductase NO 2 NH NH 3 Glutamine synthetase Glutamate synthetase NH 3 Glutamate 2 1

8 The relationships between concentrations and planktonic uptake of reduced and oxidized N Substrate (moles L 1 ) NH 4 + NO 3 Time (hours) Patterns of NO 3 and NH 4 + disappearance due to preferential uptake of NH 4 + total V V (time 1 ) NH 4 + NO 3 NH 4 + Concentration Simultaneous rates of NO 3 and NH 4 + uptake as a function of NH 4 + concentration

9 0 Concentrations and distributions of ocean nitrogen Depth (m) Depth (m) Concentration N (µm) N2 DON PN NH Concentration N (µm) NO 3 + NO2 NO2 N 2 O NO 3 : concentrations range nanomolar to micromolar NO 2 : concentrations typically nanomolar N 2 O : concentrations typically nanomolar N 2 : biologically inert (mostly); concentrations ~600 µmol L 1 NH 4+ : rapidly consumed in the photic zone, concentrations typically nanomolar DON : concentrations typically 46 µmol L 1

10 Oceanic concentrations, inventories and turnover of nitrogen Species Mean Euphotic zone (µmol L 1 ) Mean aphotic zone (µmol L 1 ) Oceanic inventory (Tg N) Turnover rate (Tg N yr 1 ) Turnover time (years) Nitrate NO 3 Nitrite NO 2 Ammonium NH 4 + Dissolved Organic Nitrogen DON Particulate Nitrogen PN Nitrous Oxide N 2 O x x Dinitrogen gas x ,000 N 2

11 Distributions of nitrate in near surface waters Nitrate White contour lines are nitrate concentrations, colors are satellite ocean color derived Chl a

12 New and regenerated production 1) new production supported by external input of N (e.g. NO 3 and N 2 ), 2) recycled or regenerated production, sustained by in situ recycling of N. Assumes steady state: Input of new N is balanced by export of N.

13 The fratio f = VNO 3 / VNO 3 + N R Fratio describes relative contribution of new production to total production. N R includes regenerated N uptake (historically thought to include urea and NH 4+ ) N 2 regenerated NH + 4 NO 3 new new Biological production export In steady state N inputs are balanced by export/grazing loss. NO 3 NO 2 NH 4 + N export

14 Eppley and Peterson (1979) examined the contribution of nitrate and ammonium to total primary production (as determined by 14 Cbicarbonate assimilation) to provide a quantitative evaluation of the amount of production able to support fisheries and sink to the deep sea Peru upwelling New production (NO 3 based) ~550% of total production across ocean basins; lower in oligotrophic systems, greater in upwelling systems E. tropical Pacific S. Cal. Bight Oligotrophic Med. Sea Oligotrophic N. Pacific Coast Rica Dome Eppley and Peterson (1979)

15 Redfield stoichiometry of production and remineralization Organic matter production: 106 CO HNO 3 + H 3 PO H 2 O (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 )+138O 2 Consumes CO 2 Consumes nutrients Produces oxygen This equation describes nitrate assimilation by photosynthesis Aerobic remineralization of organic matter: (CH 2 O) 106 (NH 3 )16H 3 PO O 2 106CO H 2 O +16HNO 3 + H 3 PO 4 Consumes O 2 Produces CO 2 Produces nutrients

16 Station ALOHA nutrient profiles DIC (µmol kg 1 ) NO 3 + NO 2 (µmol kg 1 ) Depth (m) Depth (m) NO 3 + NO 2 : PO PO 4 3 (µmol kg 1 ) N:P NO 3 + NO 2 PO 4 3 DIC

17 Inorganic Nutrient Concentrations and Stoichiometry at Station ALOHA Depth (m) DIC (µmol L 1 ) NO 3 + NO 2 (µmol L 1 ) PO 4 3 (µmol L1) DIC: NO 3 (mol: mol) DIC: PO 4 3 (mol: mol) NO 3 : PO 4 3 (mol: mol) Redfield : 106C : 16 N : 1 P (mol:mol:mol)

18 For net carbon removal by biology, need to export material with greater C:N ratio than nutrient input C:N ratio A parcel of water brought to the surface from 150 m: DIC: NO 3 = ( ) / (1.1) = 48 A parcel of water brought to the surface from 300 m: DIC: NO 3 = ( ) / (9.8) = 9.7 A parcel of water brought to the surface from 750 m: DIC: NO 3 = ( ) / (41) = 8.5

19 Not all new nutrients are introduced to the euphotic zone from below Atmospheric deposition (both dry and wet) can form an important source of nutrients. Advection: lateral input of nutrients N 2 fixation

20 N 2 fixation is the primary mode of introducing fixed nitrogen to the biosphere. N 2 fixation converts N 2 to NH 3 ; process is exclusively mediated by prokaryotes Energy expensive to break triple bond in N 2 N 2 + 8H + + 8e + 16 ATP 2NH 3 + H ADP + 16 PO 4 3

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22 At Station ALOHA, N 2 fixation appears to contribute ~3084% of new productionnote that this is <5% of total production Dore et al. (2002) δ 15 N particulate N export ( ) NO 3 N Year

23 Global estimate of N 2 fixation based on NDIC drawdown in NO 3 depleted waters averages 0.8 ±0.3 Pg C yr 1

24 N 2 fixation may also play an important role in controlling nutrient stoichiometry in the sea NO 3 + NO2 (µmol L 1 ) TDN = 14.57(TDP) TDN : TDP NO3 : PO4 3 Nearly identical slopes, but different intercepts PO 4 3 (µmol L 1 ) NO 3 = (PO 4 3 ) 1.08

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26 Aerobic remineralization of organic matter: (CH 2 O) 106 (NH 3 )16H 3 PO O 2 106CO H 2 O +16HNO 3 + H 3 PO 4 Consumes O 2 Produces CO 2 Produces nutrients This reaction does not show the complex series of reactions required to transform organic nitrogen to nitrate (ammonification, ammonium oxidation, nitrite oxidation) DON NH + 4 NH 4+ NO 2 NO 2 NO 3

27 Dissimilatory nitrogen transformations NH 2 OH NO N 2 O Nitrification: NH 4 + NO 2 NO 3 Denitrification: NO 3 NO 2 NO N 2 O N 2 Anammox: NO 2 + NH 4+ N 2 + 2H 2 O

28 Nitrification Biological oxidation of NH 3 to NO 3 using oxygen as terminal electron acceptor. Two step process; ammonia oxidation followed by nitrite oxidation; both reactions yield energy. NO 2 serves as an important intermediate; incomplete nitrification also yields N 2 O.

29 Redox states and chemical forms of nitrogen Oxidized N Energy to be gained in oxidation Reduced N (Sarmiento & Gruber, 2006)

30 NH 2 OH NO N 2 O Nitrification: NH 4 + NO 2 NO 3 Degradation of organic N to ammonium occurs during heterotrophic metabolism. Nitrification is a 2 step process that is mediated by different groups of microbes. The first step (termed ammonium oxidation) oxidizes NH 4+ to NO 2, and the second step converts NO 2 to NO 3.

31 Aerobic regeneration of nitrogen Complete decomposition of organic matter (CH 2 O) 106 (NH 3 )16H 3 PO O 2 106CO H 2 O +16HNO 3 + H 3 PO 4 2NH O 2 2NO 2 + 4H + + 2H 2 O 2NO 2 + O 2 2NO 3 These reactions yield energy (but not much ) Most nitrifying microbes are chemoautotrophic (best studied are Nitrosomonas and Nitrobacter)

32 Recent isolation and cultivation of an abundant archaeal ammonium oxidizer

33 Denitrification The reduction of NO 3 and NO 2 to N 2 during heterotrophic respiration of organic matter. Denitrification implies N is lost from the ecosystem as a gaseous biproduct. Occurs predominately in anaerobic or suboxic environments. C 106 H 175 O 42 N 16 P NO 3 106CO N 2 + H 3 PO H 2 O NO 3 and NO 2 serve as terminal electron acceptors during heterotrophic respiration. NO 2 serves as an important intermediate, as do N 2 O and NO. Serves as the major sink for fixed nitrogen from aquatic environments.

34 NO 2 forms an important intermediate in many Ncycling reactions

35 Anaerobic ammonium oxidation (anammox) NH 4+ + NO 2 2N 2 + 2H 2 O Anaerobic ammonium oxidation Major source of N 2 gas (along with denitrification) Anoxic sediments, marine water column, and sewage wastewater Mediated by Planctomyces

36 Oxygen concentrations along the 26.9 kg m 3 isopycnal surface (~500 m in the N. Pacific) Chlorophyll distributions

37 High productivity in surface water due to upwelling of nutrients. High organic matter flux depletes O 2 concentrations below the euphotic zone.

38 Coupling between N 2 fixation and denitrification

39 Global Nitrogen Budget Process Nitrogen Flux (TgN yr 1 ) Sources 120 ± 50 Pelagic N 2 fixation Benthic N 2 fixation 15 ± 10 River input (DON) 35 ± 10 River input (PON) 45 ± 10 Atmospheric deposition 50 ± 20 Total Sources 265 ± 55 Sinks 1 1 Tg = g Organic N export Benthic denitrification 180 ± 50 Water column denitrification 65 ± 20 Sediment burial 25 ± 10 N 2 O loss to atmosphere 4 ± 2 Total Sinks 275 ± 55