Lecture 23: Marine Nitrogen Cycle Karen Casciotti Overview • Why study the nitrogen cycle? • Nitrogen pools, fluxes, and distributions • Biogeochemical transformations • Open questions • Human impacts on the nitrogen cycle Life Needs Nitrogen Overall Phytoplankton % Wet Weight C:N = 6.6 Ions, small molecules (4%) Bacterial Cell N:P= 16:1 * Phospholipids (2%) DNA (1%) * C39H53O24N15P4 30% chemical RNA (6%) C:N = 2.6 C H O N * 61 97 20 16 Proteins (15%) Macromolecules 70% H2O C:N = 3.8 Polysaccharides (2%) Figure by MIT OCW. Nitrogen transformations Chemical Oxidation species state + Norg, NH4 -III Reduced NH2OH -I N2 0 N2O I NO II - NO2 III - NO3 V Oxidized Marine Nitrogen Pools • Nitrogen gas (N2) 95.2 % Nitrous oxide (N2O) Nitric oxide (NO) • “Fixed” Nitrogen Inorganic nitrogen: - . Nitrate (NO3 ) 2.5 % - . Nitrite (NO2 ) + . Ammonium (NH4 ) Organic nitrogen: . Detritus and Living biomass 2.3% . Dissolved organic matter • Proteins/Amino acids • Urea N2 Inorganic Organic • Nucleic acids Sea Surface Chlorophylla Dugdale and Goering, 1967: the New Production paradigm N2 fixation Grazing New Production Phytoplankton Zooplankton Regenerated + Production NH4 DON Bacteria Euphotic zone Export production Aphotic zone - + NO3 NH4 Dead organic matter Nitrification Ammonification Dugdale and Goering, 1967: the New Production paradigm • Introduced the concept of balanced new and export production • Introduced the use of 15N-labeled compounds to measure rates of new and regenerated production. • “Ammonium is an important nitrogen source… but nitrate and nitrogen fixation are the most important parameters with respect to nitrogen limitation of primary productivity.” Eppley and Peterson, 1979: Export Production and the “Biological Pump” CO2 Atmospheric N2 fixation deposition New Production Phytoplankton Zooplankton Regenerated Production + NH4 DON, DOC + CO2 Bacteria Euphotic zone Export production Aphotic zone - + NO3 NH4 Dead organic matter + CO2 + CO2 Eppley and Peterson, 1979 • “Only the sinking flux due to new production associated with N2 fixation and atmospheric sources of N can be identified as… transport of atmospheric CO2 to the deep ocean.” • Introduced “f ratio” as ratio of new/total production Sea Surface Nitrate Map Sea Surface Nitrate Map ‘HNLC’ regions (high nutrient, low chlorophyll) These regions are typically limited by other factors: • Light • Temperature • Iron, micronutrients Sea Surface Nitrate Map Distribution of Nitrate in Atlantic Ocean SOUTH NORTH Nitrate section through the ETP Redfield Ratios and Remineralization “Redfield Ratio”: C106:N16:P1 applies to both the average composition of phytoplankton biomass and the ratio of 3.5 nitrate and phosphate generated from organic 3.0 matter remineralization under oxic conditions ] -1 2.5 [µmol kg-1] mol kg 2.0 Remineralization of generalized organic µ matter: 1.5 (CH2O)106(NH3)16(H3PO4) + 138 O2 Phosphate [ 1.0 0.5 106 CO2 + 16 HNO3 + H3PO4 + 122 H2O 0.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 Nitrate [µmol kg-1] (106 O2) (32 O2) Figure by MIT OCW. Global Thermohaline Circulation INDIAN JAVA From Pacific SOUTHERN OCEAN MW & LOIW AUSTR. SA INDONESIA SLW PACIFIC NIIW ACCS BIW & RSW Antarctica IODW SAMW NORTH UPIW CDW NADW SAMW SAMW & LOIW LOIW LOIW CDW NPDW ATLANTIC UPIW NORTH NADW NADW CDW NADW AABW SOUTH NORTH SLW Surface layer water NADW North atlantic deep water _ _ SAMW Subantarctic mode water UPIW Upper intermediate water, 26.8 < σθ < 27.2 _ _ RSW Red sea water LOIW Lower intermediate water, 27.2 < σθ < 27.5 AABW Antarctic bottom water IODW Indian ocean deep water NPDW North pacific deep water BIW Banda intermediate water ACCS Antarctic circumpolar current system NIIW Northwest Indian intermediate water CDW Circumpolar deep water Figure by MIT OCW. Distribution of Nitrate in Atlantic Ocean AAIW NADW AABW SOUTH NORTH Overview Why study the nitrogen cycle? Nitrogen pools, fluxes, and distributions • Biogeochemical transformations • Open questions • Human impacts on the nitrogen cycle Microbial Nitrogen Cycle Nitrogen fixation N O Denitrification N2 2 Anammox NO Assimilation Org NH - - 3 NO2 NO3 Nitrite oxidation Nitrification NH2OH Oxidation Ammonia oxidation state (-III) (-I) (0) (I) (II) (III) (V) Overview Why study the nitrogen cycle? Nitrogen pools, fluxes, and distributions • Biogeochemical transformations • Open questions • Human impacts on the nitrogen cycle Redfield Ratios and Remineralization “Redfield Ratio”: C106:N16:P1 applies to both the average composition of phytoplankton biomass and the ratio of 3.5 nitrate and phosphate generated from organic 3.0 matter remineralization under oxic conditions ] -1 2.5 [µmol kg-1] mol kg 2.0 Remineralization of generalized organic µ matter: 1.5 (CH2O)106(NH3)16(H3PO4) + 138 O2 Phosphate [ 1.0 0.5 106 CO2 + 16 HNO3 + H3PO4 + 122 H2O 0.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 Nitrate [µmol kg-1] (106 O2) (32 O2) Figure by MIT OCW. Sea Surface Nitrate Map HOTS: N2 fixation may account for 30-50% of export production Evidence for N2 Fixation • Occurrence of nitrogen fixing species, such as Trichodesmium spp. • Low δ15N of sinking organic matter suggestive of significant N2 fixation 15 • Acetylene reduction or N2 incorporation rate estimates Oceanic Diazotroph Diversity Zehr, 2000 Trends in Microbiology Image removed due to copyright restrictions. Water Column Denitrification Zones Eastern Tropical North Pacific Arabian Sea Eastern Tropical South Pacific Overview Why study the nitrogen cycle? Nitrogen pools, fluxes, and distributions Biogeochemical transformations • Open questions • Human impacts on the nitrogen cycle Major Questions in Marine N Cycling • Is the nitrogen cycle in balance? • How does the N cycle vary on glacial/interglacial timescales? • How is N2O produced in the ocean? • By what mechanism is ‘extra excess N2’ formed? Nitrogen Inputs to the Ocean Nitrogen Atmospheric Continental Fixation Deposition Runoff Fluxes in Tg N/yr Codispoti and 25 24 25 Christensen [1985] Gruber and Sarmiento 125 ± 50 15 ± 5 41 ± 20 [1997] (preindustrial) Gruber and Sarmiento 125 ± 50 30 ± 5 76 ± 20 [1997] (postindustrial) Codispoti et al [2001] Same as G&S 86 Same as G&S (includes DON) Nitrogen Exports from the Ocean Organic Sedimentary Water column Burial Denitrification Denitrification Anammox Fluxes in Tg N/yr Codispoti and Christensen [1985] 21 60 60 ? Gruber and Sarmiento [1997] 15 ± 5 85 ± 20 80 ± 20 ? (preindustrial) Gruber and Sarmiento [1997] 25 ± 10 95 ± 20 80 ± 20 ? (postindustrial) Codispoti et al [2001] 25 ± 10 300 150 ? Nitrogen Budgets Codispoti and Gruber and Codispoti Christensen Sarmiento [1997] et al [2001] [1985] Total sources 74 181 ±44 231 ±44 287 Total sinks 142 184 ±29 204 ±30 481 Residence time of N 5,000 years 3,500 years 1,500 years in the ocean Is the N cycle in balance? Maybe not! Is it a moving target? What are the consequences? Major Questions in Marine N Cycling • Is the nitrogen cycle in balance? • How does the N cycle vary on glacial/interglacial timescales? • How is N2O produced in the ocean? • By what mechanism is ‘extra excess N2’ formed? Southern ocean nitrate utilization changes 15 -1 δ O (% vs. air) N content (µmol N g sediment) 15 0 1 2 3 4 5 6 7 8 9 10 0 5 10 15 20 25 30 35 40 45 Higher δ N in 0 10 bulk sediment diatom-bound 20 30 organic matter 18 40 d o 50 suggests higher depth (cm) treated 60 >63µm traction 70 degree of nitrate treated diatoms 80 utilization in the 90 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 5 10 15 20 25 30 35 40 45 18 Antarctic zone δ O (% vs. PDB) o o 13 Depth profiles in gravity core All 107-22 (Antarctic Zone, Atlantic Sector, 55 S, 3 W, 2768 mi of (a) d N and (b) N content of bulk N (open circles), diatom N (solid circles), and the perchloric acid-treated >63 µm fraction (crosses). which is composed of large diatoms, during glacial radiolaria and some detrital gains, Replicate analyses are shown for the cleaned diatom and >63 mm fraction with solid line connecting the mean value of diatoms N analysis. The N content of the builk sediment is in some cases lower than that of diatorn fraction because 13 of the presence of dense detrital grains in the bulk sediment the planktonic focaminiferal d stratigraphy [from keigwin and Boyle, 1989] times. suggested that sediment from the last Glacial maximum is at 65 cm depth. Figure by MIT OCW. Sigman et al., 1999 Southern ocean nitrate utilization changes Figure by MIT OCW. Sigman and Boyle, 2000 Changes in Denitrification Sedimentary δ15N changes from the ETNP Chart removed due to copyright restrictions. Ganeshram, R., et al. "Glacial-interglacial Variability in Denitrification in the World’s Oceans: Causes and Consequences." Paleoceanography 15, no. 4 (2000): 361–376. Changes in Denitrification Present LGM PN PN N - N - 2 NO3 2 NO3 High δ15N Lower δ15N High δ15N-PN Lower δ15N-PN Major Questions in Marine N Cycling • Is the nitrogen cycle in balance? • How does the N cycle vary on glacial/interglacial timescales? • How is N2O produced in the ocean? • By what mechanism is ‘extra excess N2’ formed? N2O vs. AOU Anticorrelation of N O N2O µg/l O2 ml/l 2 0 0.2 0.4 0.6 0.8 1.0 3 4 5 6 7 and O concentrations 200 2 400 600 suggests N2O is produced 800 1000 during organic matter DEPTH (m) 2000 remineralization 3000 October 1971 (nitrification) 4000 July 1972 5000 N2O µg/l O2 ml/l 0 0.2 0.4 0.6 0.8 1.0 3 4 5 6 7 200 400 600 800 1000 DEPTH (m) 2000 3000 October 1971 May 1972 4000 July 1972 5000 Vertical NO2 and O2 profiles at three different atations in the western North Atlantic, a, slope water of f o o o o o o Nova Scotia (42 18' N, 61 24' W ); b, Gulf Stream (39 07' N, 62 21' W); c, Sargasso Sea (35 52' N, 63 44' W) Figure by MIT OCW.