N cycling in the world’s oceans

Nitrogen

N is an essential nutrient for all living organisms (nucleic acids and amino acids)

N has many oxidation states, which makes speciation and redox chemistry very interesting

NH4+ is preferred N nutrient

Marine N

Non-bioavailableN2O 200 Tg N (+1)

N2 2.2*107 Tg N (0)

Bioavailable/Fixed (oxidation state) NO3

- 5.7*105 Tg N (+5)

NO2- 500 Tg N (+3)

NH4+ 7.0*103 Tg N (-3)

Organic N 5.3*105 Tg N (-3)

Libes, 1992

Marine Reservoir: 6.3*105 Tg N

Sources: 287 Tg N/yr

Sinks: 482 Tg N/yr

Water Column denitrification:

150 Tg N/yr

Atmospheric deposition: 86 Tg N/yr

N2O loss: 6 Tg N/yr

N2 fixation: 125 Tg N/yr

Sedimentation: 25 Tg N/yr

Benthic denitrification: 300 Tg N/yr

Organic N export: 1 Tg N/yr

River Input: 76 Tg N/yr

Codispoti et al. (2001)Hypothetical Fixed N Evolution

-2.E+05

0.E+00

2.E+05

4.E+05

6.E+05

8.E+05

0 500 1000 1500 2000 2500 3000 3500

Time (years)

Fix

ed N

(T

g)

Marine Fixed N Budget

NO3

Chlorophyll

Largedetritus

Organic matter

N2 NH4 NO3

Water column

SedimentSediment

Phytoplankton

NH4

Mineralization

Uptake

Nitrification

Nitrification

Grazing

Mortality

Zooplankton

Susp.particles

Aerobic mineralizationAerobic mineralizationDenitrificationDenitrification

N2

Fixation

Nitrogen Cycle

http://www.petsforum.com/personal/trevor-jones/nitrogencycle.gif

Respiration ΔG° (kJ/mol)

-119

Denitrification-113

MnO2 reduction-96.9

Fe oxide reduction-46.7

Sulfate reduction-20.5

Methanogenesis-17.7

281

481

241 COCHOCH

OHCOOOCH 241

241

241

241

OHNCOHNOOCH 221

2101

241

51

351

241

OHCOFeHOHFeOCH 2411

2412

3241 2)(

OHCOHSHSOOCH 241

241

81

812

481

241

OHMnCOHMnOOCH 2432

21

241

281

241

Organic Matter Oxidation Sequence Morel & Herring,

1993

Alternative pathways to N2

OHCONHNOOCH 22232 752445

HOHNOONH 22 2324 OHNNONH 2224 2

HOHNONH 223

221

243

4

OHNMnHNHMnO 222

42 63423

Nitrification Anammox

OLAND

MnO2 Reduction

Microbially mediated

Chemical Reactions

Heterotrophic Denitrification

223

422 225 OOHNHOHN Nitrogen Fixation

OHNOMnHNHMnO 232

42 5464

HNMnOOHNOMn )solid( 85425 22232

Mn2+ Oxidation

N Fixation may have been underestimated

Limited data on Trichodesmium and other N fixers; variability in abundances and fixation rates of organisms

Recent estimates of N fixation rates have increased (Gruber and Sarmiento, 1997; Karl et al., 1997)

Denitrification may have been overestimated

Stoichiometric and model-based estimates used; limited data on direct denitrification measurements

Marine Fixed N Budget UnbalancedWHY??????????????????????

My research Denitrification describes the removal of fixed N, mostly

NO3-, resulting in the formation of non-biologically

available N, primarily N2 gas

Continental shelf sediments are responsible for up to 67% of marine denitrification estimates

Sandy sediments comprise 70% of continental shelves; global estimates of denitrification are mostly based on muddy sediments

Sands contain less organic matter and nutrients, and high oxygen concentrations in overlying water

Sandy sediments have low organic matter content, substrate for heterotrophic denitrification

BPP supplies reactive organic matter through remineralization

Organisms compete with microbes for nutrients such as NH4

+

Organisms also produce oxygen during photosynthesis

Role of BPP remains unclear

Benthic primary production (BPP)

Isotope tracer experiments

23029

31514

315 NNONO ,ationdenitrific,

23029

31514

315

415 NNONONH ,ationdenitrific,ionnitrificat

15NH4+,

14NO3-

1A 15NO3-

14N15N, 15N15N

1B

15N15N

1C

14N15N

1D

14N14N 1E

POM1F

A. Experiment 1

15NO3-,

14NH4+

14NO3-

2A14N15N, 14N14N

2B

14N14N

2C

14N15N

2D

15N15N2E

POM

B. Experiment 2

2F

Possible outcomes of amendment experiments. 1A = Aerobic nitrification of 15NH4+; 1B = Heterotrophic denitrification with 14NO3- and/or 15NO3-; 1C = OLAND with 15NH4+ or partial nitrate reduction to nitrite followed by anammox with 15NH4+; 1D = Same as 1C except with standard nitrate; 1E = Heterotrophic denitrification with standard nitrate; 1F = Assimilation. 2A = Aerobic nitrification of standard ammonium; 2B = Heterotrophic denitrification with 14NO3- and/or 15NO3-; 2C = OLAND with standard ammonium or partial nitrate reduction to nitrite followed by anammox with standard ammonium; 2D = Same as 2C except with 15NO3-; 2E = Heterotrophic denitrification of 15NO3-; 1F = Assimilation

SamplingSampling

Membrane Inlet Mass Spec. (MIMS)

00.5

11.5

22.5

3

R4-Exp

erim

ent 1

-Core

1

R4-Exp

erim

ent1

-Cor

e2

R4-Exp

erim

ent2

-Cor

e3

R4-Exp

erim

ent2

-Cor

e4

R4-Con

trol-C

ore5

R4-Con

trol-C

ore6

W27-

Amend

ed Cor

e

W27-

Contro

l

m M N

2 P

rod

uce

d

29N2 30N2

W27 and Experiment 2 results suggest the presence of denitrification

Experiment 1 results suggest that within the 48-hr timescale of the experiment, no alternative pathway to N2 exists in these sediments

Results

Denitrification Rates

W27 Experiment provided a rate of 21.6 µmole N m-2 d-1

R4-Experiment 2 provided rates of 22.8 & 23.2 µmole N m-2 d-1

Rates obtained from other continental shelf studies of denitrification yielded 700-3200 µmole N m-2 d-1

Other continental shelf sites studied contain higher organic matter content than Georgia sediments

Georgia continental shelf sediments are oxic to at least 1-cm depth, thus inhibiting higher rates of denitrification

Discussion of results

Sandy, continental shelf sediments are potentially important sites of denitrification that may have been overlooked

These environments may have similar rates to current study site and if so, similar techniques can be used to measure such low rates of denitrification

Denitrification was not completely inhibited by low organic matter content or benthic primary production

BPP varies seasonally and spatially, yet denitrification rates were very close between two different stations during different seasons

Future work

Impact of BPP can be explored further by monitoring nutrient and dissolved O2 concentrations and benthic primary production rates (monitored by SABSOON)

Compare rates to Gulf of Mexico shelf denitrification rates (Nov. – Dec. 2004)

Further explore the presence of alternative pathways in salt marsh sediments by using isotope tracers, 15N isotopic analyses, and HgCl2 (Oct. – Nov. 2004)

NO3 Samples

0

50

100

150

200

250

300

350

400

0 10 20 30 40 50

Time (Days)

Co

nce

ntr

atio

n (

uM

)

NO3

NO2

NH4

NO3 + Hg Samples

0

50

100

150

200

250

300

350

400

0 10 20 30 40 50

Time (Days)

Co

nce

ntr

atio

n (

uM

)

NO3

NO2

NH4

NO3 Samples

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Time (Days)

Co

nce

ntr

atio

n (

uM

-N)

N29

N30

NO3 + Hg Samples

0

1

2

3

4

5

6

7

0 10 20 30 40 50

Time (Days)

Co

nce

ntr

atio

n (

uM

-N)

N29

N30

Future work (cont’d)