Nitrogen Cycling in the Sea Matt Church (MSB 612 / 9568779/ mjchurch@hawaii.edu) Marine Microplankton Ecology / OCN 626 NH 4 N0 2 N0 2 NH 4
Outline Nitrogen species in marine watersdistributions and concentrations New, regenerated, and export production Microbial transformation of nitrogen N 2 fixation, nitrification, and denitrification/anammox
Summary Microbes control global nitrogen budgets The oxidationreduction potential of nitrogen facilitates the role of Ncontaining compounds as electron donors and acceptors. The principal transformation 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
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
Nitrogen assimilation Nitrogen is an essential nutrient found in amino acids, protein, and nucleic acids. Nitrogen is assimilated by both autotrophs and heterotrophs. Nitrogen in organic matter is reduced; however, most fixed nitrogen in the ocean is oxidized (nitrate) so energy is required to assimilate N into biomass. In large areas of the world s ocean, nitrogen limits primary production.
Distributions of nitrate in near surface waters Nitrate White contour lines are nitrate concentrations, colors are satellite ocean color derived Chl a
Stoichiometry of production and remineralization Organic matter production: 106 CO 2 16 HNO 3 H 3 PO 4 122 H 2 O (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 )138O 2 Consumes CO 2 Consumes nutrients Produces oxygen Nitrogen assimilation by photosynthesis (note that bacteria also compete for nitrate, nitrite, ammonium)
Redox states and chemical forms of nitrogen Oxidized N 5 stable oxidation states Reduced N (Sarmiento & Gruber, 2006)
0 Concentrations and distributions of ocean nitrogen 0 1000 1000 Depth (m) 2000 3000 Depth (m) 2000 3000 4000 4000 10 4 10 3 10 2 10 1 10 0 10 1 10 2 10 3 Concentration N (µm) 10 4 10 3 10 2 10 1 10 0 10 1 10 2 10 3 Concentration N (µm) N2 DON PN NH 4 NO 3 : concentrations range nanomolar to micromolar NO 2 : concentrations typically nanomolar N 2 O : concentrations typically subnanomolar NO 3 NO2 NO2 N 2 O N 2 : inert; concentrations ~600 µmol L 1 NH 4 : rapidly consumed in the photic zone, concentrations typically nanomolar DON : concentrations typically 46 µmol L 1
Energy and reducing power required for assimilation of various N compounds Substrate Enzyme Reaction Electrons ATP N 2 Nitrogenase N 2 NH 4 8 16 NO 3 Nitrate reductase NO 3 NO 2 2 0 NO 2 Nitrite reductase NO 2 NH 4 6 0 NH 4 Glutamine synthetase Glutamate synthetase NH 4 Glutamate 2 1
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
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 7 31 5.8 x 10 5 1570 370 NO 3 Nitrite 0.1 0.006 160 NO 2 Ammonium 0.3 0.01 340 7000 0.05 NH 4 Dissolved Organic Nitrogen 6 4 7.7 x 10 4 3400 20 DON Particulate Nitrogen 0.4 0.01 400 8580 0.05 PN Nitrous Oxide 0.01 0.04 750 6 125 N 2 O Dinitrogen gas 450 575 1 x 10 7 200 54,000 N 2
Aerobic remineralization of organic matter: (CH 2 O) 106 (NH 3 )16H 3 PO 4 138O 2 106CO 2 122H 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
Station ALOHA nutrient profiles DIC (µmol kg 1 ) 1900 2000 2100 2200 2300 2400 NO 3 NO 2 (µmol kg 1 ) 0 0 10 20 30 40 50 0 1000 1000 Depth (m) 2000 3000 Depth (m) 2000 3000 4000 4000 NO 3 NO 2 : PO 4 3 5000 0 1 2 3 4 5 PO 3 4 (µmol kg 1 ) 5000 0 5 10 15 N:P NO 3 NO 2 PO 4 3 DIC
NO 3 NO2 (µmol L 1 ) 50 40 30 20 10 0 TDN = 14.57(TDP) 1.5 TDN : TDP NO3 : PO4 3 Nearly identical slopes, but different intercepts 0.0 0.5 1.0 1.5 2.0 2.5 3.0 PO 4 3 (µmol L 1 ) NO 3 NO 2 = 14.62(TDP) 1.08
New production and the fratio Dugdale and Goering (1967) identified 2 forms of primary 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. Over short time scales, input of new N supports higher trophic levels and maintains carbon export. Why do these generalizations apply to the open sea but not near shore environments?
The fratio Assumptions: 1) N 2 fixation is low 2) Steady state system 3) Euphotic zone nitrification is low f = VNO 3 / VNO 3 N R Note N R includes regenerated forms of N uptake (historically thought to include urea and NH 4 ) Mathematical description linking new production and organic matter export. At steady state, nitrogen input is balanced by nitrogen export. N 2 regenerated NH 4 NO 3 new new Biological production export Under steady state (i.e. nitrate input balanced by export/grazing loss), if export is less than input, biomass accumulates. This biomass must eventually be exported to keep the system in steady state. NO 3 NO 2 NH 4 N export
How is new production measured? Isotopes evaluate plankton assimilation of 15 N labeled NO 3 and NH 4 Measure particle flux Evaluate seasonal changes in [DIC] : [NO 3 ] : [PO 4 3 ] More recently, the eratio has been used to describe the fraction of carbon exported relative to production: eratio = carbon export / carbon production
Eppley and Peterson (1979) determined the contribution of nitrate and ammonium to total primary production (as determined by 14 Cbicarbonate assimilation). This provided a quantitative evaluation of the amount of production (in carbon units) able to support fisheries and sink to the deep sea Peru upwelling New production (NO 3 based) ~1445% of total production across ocean basins E. tropical Pacific S. Cal. Bight Coast Rica Dome Oligotrophic Med. Sea Oligotrophic N. Pacific Eppley and Peterson (1979)
Based on Eppley and Peterson, we might assume that we could calculate export production as: NO 3 uptake x Redfield ratio (6.6, C:N) = new carbon production Redfield Sambrotto et al. (1993) In this example, greater drawdown in inorganic carbon per unit nitrate than would occur if stoichiometry followed the Redfield ratio. What might cause deviations from Redfield in C:N ratios of inorganic C and N pools?
25 1000 60 DOC:DON (mol:mol) 20 15 10 5 DOC:DOP (mol:mol) 800 600 400 200 DON:DOP (mol:mol) 50 40 30 20 10 0 0 0 C:N C:P N:P Production and consumption of DOM generally does not follow Redfield stoichiometry
Redox states and chemical forms of nitrogen Oxidized N Energy to be gained in oxidation Reduced N (Sarmiento & Gruber, 2006)
N 2 fixation is the primary mode of nitrogen introduction to marine and terrestrial ecosystems. N 2 fixation converts N 2 to NH 4 by prokaryotic microorganisms Energy expensive to break triple bond in N 2 N input by N 2 Losses of N by fixation denitrification N 2 Atmosphere N 2 N 2 O Photosynthesis Organic Bacterial matter degradation NH 4 Nitrification NO 2 NO 3 Aerobic Suboxic Denitirifcation N 2 O N 2 O Detritus Bacterial degradation NH 4 N 2
At Station ALOHA, N 2 fixation can contribute ~3084% of new production
Global estimate of N 2 fixation based on NDIC drawdown in NO 3 depleted warm waters is equivalent to 0.8 ±0.3 Pg C yr 1
Let s look at dissimilatory nitrogen transformations Oxidized N Energy to be gained in oxidation Reduced N (Sarmiento & Gruber, 2006)
Processes of dissimilatory nitrogen transformation Denitrification: NO 3 NO 2 NO N 2 O N 2 Nitrification: NH 4 NO 2 NO 3 Anammox: NO 2 NH 4 N 2 2H 2 O
Aerobic regeneration of nitrogen Complete decomposition of organic matter (CH 2 O) 106 (NH 3 )16H 3 PO 4 138O 2 106CO 2 122H 2 O 16HNO 3 H 3 PO 4 However, the first step is ammonification, breakdown of amino acids to NH 4 ; this process is mediated by heterotrophic microorganisms 2NH 4 3O 2 2NO 2 4H 2H 2 O 2NO 2 O 2 2NO 3 These reactions yield energy (but not much ) Nitrification: predominately mediated by chemoautotrophic microbes (best studied are Nitrosomonas and Nitrobacter)
Recent isolation and cultivation of an abundant archaeal ammonium oxidizer
NO 2 forms an important intermediate in many Ncycling reactions
Denitrification The reduction of NO 3 and NO 2 to N 2 during heterotrophic respiration of organic matter. Occurs predominately in anaerobic or suboxic environments. C 106 H 175 O 42 N 16 P 104 NO 3 106CO 2 60N 2 H 3 PO 4 138 H 2 O NO 3 and NO 2 are used as terminal electron acceptors during heterotrophic respiration.
Oxygen concentrations along the 26.9 kg m 3 isopycnal surface (~500 m in the N. Pacific) Chlorophyll distributions
High productivity in surface water due to upwelling of nutrients. High organic matter flux depletes O 2 concentrations below the euphotic zone.
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
Global Nitrogen Budget Process Sources Pelagic N 2 fixation Benthic N 2 fixation River input (DON) River input (PON) Atmospheric deposition Total Sources Nitrogen Flux (TgN yr 1 ) 120 ± 50 15 ± 10 35 ± 10 45 ± 10 50 ± 20 265 ± 55 1 Tg = 10 12 g Sinks Organic N export Benthic denitrification Water column denitrification Sediment burial N 2 O loss to atmosphere Total Sinks 1 180 ± 50 65 ± 20 25 ± 10 4 ± 2 275 ± 55
Couplings among the cycles of nitrogen, carbon and phosphorus More on this on Tuesday.