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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 Libes, 1992 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) Non-bioavailable N2O 200 Tg N (+1) N2 2.2*107 Tg N (0) Marine Fixed N Budget Hypothetical Fixed N Evolution Codispoti et al. (2001) Fixed N (Tg) 8.E+05 Marine Reservoir: 6.3*105 Tg N 6.E+05 Sources: 287 Tg N/yr 4.E+05 2.E+05 Sinks: 482 Tg N/yr 0.E+00 -2.E+05 0 500 1000 1500 2000 2500 3000 3500 Time (years) Atmospheric deposition: 86 Tg N/yr N2O loss: 6 Tg N/yr N2 fixation: 125 Tg N/yr Water Column denitrification: 150 Tg N/yr River Input: 76 Tg N/yr Organic N export: 1 Tg N/yr Sedimentation: 25 Tg N/yr Benthic denitrification: 300 Tg N/yr Fixation N2 Nitrification NH4 NO3 Uptake Phytoplankton Grazing Chlorophyll Zooplankton Mortality Water column Mineralization Susp. particles Large detritus Nitrification N2 NH4 NO3 Denitrification Sediment Organic matter Aerobic mineralization Nitrogen Cycle http://www.petsforum.com/personal/trevor-jones/nitrogencycle.gif Organic Matter Oxidation Sequence Morel & Herring, 1993 Respiration 1 4 CH 2O O2 CO2 H 2O 1 4 1 4 1 4 Denitrification 1 1 1 1 1 1 4 CH 2O 5 NO3 5 H 4 CO2 10 N 2 2 H 2O MnO2 reduction 2 1 1 1 1 CH O MnO H CO Mn 34 H 2O 2 2 2 4 8 4 2 Fe oxide reduction 2 1 CH O Fe ( OH ) 2 H Fe 14 CO2 114 H 2O 2 3 4 Sulfate reduction 2 1 1 1 1 1 1 4 CH 2O 8 SO4 8 H 8 HS 4 CO2 4 H 2 O Methanogenesis 1 4 CH 2O 18 CH 4 18 CO2 ΔG° (kJ/mol) -119 -113 -96.9 -46.7 -20.5 -17.7 Alternative pathways to N2 Microbially mediated Nitrification Anammox NH 4 2O 2 NO3 H 2 O 2H Heterotrophic Denitrification 5CH 2 O 4NO 3 4H 2N 2 5CO 2 7H 2 O NH 4 NO 2 N 2 2H 2 O OLAND NH 4 34 O 2 12 N 2 32 H 2 O H Nitrogen Fixation N 2 5H 2 O 2NH 4 2OH 32 O 2 Chemical Reactions MnO2 Reduction 3MnO 2 2NH 4 4H 3Mn2 N 2 6H 2 O 4MnO 2 NH 4 6H 4Mn 2 NO3 5H 2 O Mn2+ Oxidation 5Mn2 2NO3 4H 2 O 5MnO 2(solid) N 2 8H Marine Fixed N Budget Unbalanced WHY?????????????????????? 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 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 Benthic primary production (BPP) 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 Isotope tracer experiments 15 15 A. Experiment 1 14N14N POM 1E 1A 15NH +, 4 14NO 3 ation NO 3 14,15 NO 3 denitrific 29,30 N 2 ion ation NH 4 nitrificat 15 NO 3 14,15 NO 3 denitrific 29,30 N 2 15NO 3 1B 14N15N, 15N15N 1C 1F 1D 15N15N 14N15N B. Experiment 2 15N15N 2E 15NO -, 3 14NH + 4 2A 14NO 3 POM 2D 14N14N 14N15N 14N15N, 14N14N 2C 2F 2B 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 Sampling Sampling Membrane Inlet Mass Spec. (MIMS) Results xp er im R en 4t1 Ex -C pe or ri m e1 R en 4Ex t1 -C pe or ri m e2 R en 4Ex t2 -C pe or ri m e3 en t2 R -C 4or C e4 on tro l-C R 4or C e5 on tro W 27 l-C -A or m e6 en de d C or W e 27 -C on tro l 4E R 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 mM N2 Produced 3 2.5 2 1.5 1 0.5 0 29N2 30N2 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) Future work (cont’d) NO3 + Hg Samples 400 400 350 350 300 250 NO3 200 NO2 150 NH4 100 Concentration (uM) Concentration (uM) NO3 Samples 300 250 NO3 200 NO2 150 NH4 100 50 50 0 0 0 10 20 30 40 0 50 10 20 40 50 Time (Days) Time (Days) NO3 Samples NO3 + Hg Samples 7 7 6 6 5 N29 4 N30 3 2 1 Concentration (uM-N) Concentration (uM-N) 30 5 N29 4 N30 3 2 1 0 0 0 10 20 30 Time (Days) 40 50 0 10 20 30 Time (Days) 40 50