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Home , Nitrogen cycle

Lecture 23: Marine Cycle

Karen Casciotti Overview

• Why study the ?

• Nitrogen pools, fluxes, and distributions

• Biogeochemical transformations

• Open questions

• Human impacts on the nitrogen cycle Life Needs Nitrogen

Overall % Wet Weight C:N = 6.6 , small molecules (4%) N:P= 16:1 Bacterial Cell * Phospholipids (2%) DNA (1%) * C39H53O24N15P4 30% chemical RNA (6%) C:N = 2.6

* C61H97O20N16 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 % • (N2O) • (NO)

• “Fixed” Nitrogen • Inorganic nitrogen: - • (NO3 ) 2.5 % - • (NO2 ) + • (NH4 )

• Organic nitrogen: • Detritus and Living biomass 2.3% • Dissolved • Proteins/Amino acids • Urea N2 Inorganic Organic • Nucleic acids Surface Chlorophylla Dugdale and Goering, 1967: the New Production paradigm

N2 fixation

Grazing New Production Phytoplankton Zooplankton

Regenerated + Production NH4 DON 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 “”

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 )

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] 2.0 Remineralization of generalized organic matter: 1.5 (CH2O)106(NH3)16(H3PO4) + 138 O2 Phosphate [ µ mol kg 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 NO

Assimilation Org NH - - 3 NO2 NO3 Nitrite oxidation

Nitrification NH2OH

Oxidation 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] 2.0 Remineralization of generalized organic matter: 1.5 (CH2O)106(NH3)16(H3PO4) + 138 O2 Phosphate [ µ mol kg 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 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 1050 15 20 25 30 35 40 45 Higher δ N in 0 10 bulk sediment diatom-bound 20

30 organic matter 18 40 δ 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 1050 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) δ 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 µm 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 δ 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 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. Yoshinari, 1976 Nitrification

Ammonia-oxidizing nitrifiers: , Nitrosospira, Nitrosococcus

- + NH3 + 3/2 O2 NO2 + H2O + 2 H

Nitrite-oxidizing nitrifiers: , , Nitrospina - - NO2 + 1/2 O2 NO3

- + NH3 + 2 O2 NO3 + H2O + 2 H Overall Rates and Distributions

Chart removed due to copyright restrictions. Dore, J. E., B. N. Popp, D. M. Karl, and F. J. Sansone. "A Large Source of Atmospheric Nitrous Oxide from Subtropical North Pacific Surface Waters." Nature 396, 63-66. Denitrification and N2O

Chart removed due to copyright restrictions. Yoshinari, T., et al. "Nitrogen and Oxygen Isotopic Composition of N2O from Suboxic Waters of the Eastern Tropical North Pacific and the Arabian Sea—Measurement by Continuous-Flow Isotope-ratio Monitoring." Marine Chemistry 56 (1997): 253-264. 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? Nitrate deficits

* - 3- “N ” based on Redfield relationship of NO3 and PO4 : (CH2O)106(NH3)16(H3PO4) + 138 O2 106 CO2 + 16 HNO3 + H3PO4 + 119 H2O

* - 3- N = [NO3 ] - 16 [PO4 ] + constant

- 3- Denitrification: N* ↓ (lower [NO3 ], unchanged [PO4 ]) Also, higher N2/Ar because of N2 production from nitrate production/accumulation of N2 yields “excess N2”

But, there’s more N2 than expected from nitrate deficits!! this phenomenon has been termed “extra excess N2” “Extra Excess N ”

What is it? • Discrepancy between N deficit based on N:P ratios and

N2 excess from N2/Ar ratios

How could it be explained? • Remineralization of organic matter with high N:P ratio • Lateral mixing of N2 from sedimentary denitrification • Anammox Anammox

+ - NH4 + NO2 N2 + 2 H2O

Who: Bacteria in the order Planctomycetales + - What: anaerobically combine NH4 and NO2 to form N2 Where: anoxic sediments and watercolumns; Black Sea; Gulfo Dulce, Chile; Benguela Upwelling System When: ?? Why: ?? How: Anammoxosome; enzymology known incompletely, but genome sequencing is providing targets for biochemical analysis. Measurement of Anammox

Anammox Isotopic tracers: 15 + 14 - 15 14 Organic geochemistry: NH4 + NO2 N N unique ladderane lipids

Molecular : Detection of anammox-type 16S rRNA genes

Kuypers et al., Nature 2003 Isotopic Tracers of Anammox

Masses 14-14 14-15 15-15 For N2

15NH + 14NO - 14 + 15 - 4 3 NH4 NO3 Isotopic Tracers for Anammox

15N-NH4 addition

1.2

1

0.8 15N-NO3 addition

0.9 anammox1 0.6 denitrif1 0.8

0.4 0.7

0.6 0.2 0.5 anammox2 0 0.4 denitrif2 1 2 3 0.3 14-14 14-15 15-15 0.2 0.1

0 1 2 3 14-14 14-15 15-15 Overview

Why study the nitrogen cycle?

Nitrogen pools, fluxes, and distributions

Biogeochemical transformations

Open questions

• Human impacts on the nitrogen cycle Human perturbation of the N cycle

Nitrogen fixation by 250 humans is now equivalent

Anthropogenic to natural terrestrial 200 nitrogen fixation (~140 Tg N/yr). 150

Legume Crops and Synthetic N Green Manures The amount of human- 100

Background produced N entering the

Global (Tg/y) oceans is not well known, "Natural" biological N fixation 50 but is on the order of 20-40 Tg N/yr

1920 1940 1960 1980 Year

Figure by MIT OCW. and Anoxia

Mississippi River nitrate loads and Sea Surface Chlorophyll a