Why does DNRA* predominate under oxic rather than anoxic conditions in the Yarra River estuary? Keryn Roberts Co-authors: Dr. Perran Cook, Assoc Prof. Mike Grace and Adam Kessler DNRA = Dissimilatory nitrate reduction to ammonium 1 Why the Yarra River Estuary? The Yarra River Estuary drains into Port Phillip Bay a nitrogen limited system Salt wedge estuary prone to hypoxia in the bottom waters N-limited 2 Why the Yarra River Estuary? The Yarra River Estuary drains into Port Phillip Bay a nitrogen limited system Salt wedge estuary prone to hypoxia in the bottom waters 3 How is N cycled in the estuary? NO3- OXIC NH4+ nitrification DNRA NH4+ NO3- NO3- N-recycled DNRA = Dissimilatory nitrate reduction to ammonium Denitrification N-removed N2 4 How is N cycled in the estuary? NO3- DNRA NH4+ NO3- N-recycled DNRA = Dissimilatory nitrate reduction to ammonium ANOXIC Denitrification N-removed N2 5 When does DNRA occur? 1. Reducing conditions: - Carbon decomposition - Sulphate Reduction 2. NO3- limited conditions DNRA = Dissimilatory nitrate reduction to ammonium 6 When does DNRA occur? Hypothesised: DNRA would dominate under anoxic conditions DNRA = Dissimilatory nitrate reduction to ammonium 7 Denitrification Denitrification (µmol m-2 hr-1) DNRA DNRA (µmol m-2 hr-1) Background – observational data* 0 50 100 150 O2 concentration (µmol L-1) 200 *Roberts. K. L, Eate. V. M, Eyre. B. D, Holland. D. P and Cook. P. L. M (In press) ‘Hypoxic events stimulate nitrogen recycling in a shallow salt wedge estuary: The Yarra River Estuary, Australia,’ Journal of Limnology and Oceanography 250 8 Background – observational data* Hypoxic Oxic Denitrification : DNRA 250 200 150 100 50 0 0 50 100 150 O2 (µmol L-1) 200 250 DNRA - competitive under oxic conditions 9 Background – experimental data Oxic Hypoxic Denitrification : DNRA p<0.05 150 100 50 0 0 5 10 15 20 25 Day DNRA - competitive under oxic conditions 10 Why does DNRA predominate under oxic rather than anoxic conditions? 11 11 Why does DNRA increase with O2? Possible Cause Tested* Carbon? X No relationship with dissolved inorganic carbon flux Temperature? X No relationship with temperature Microbial Type? X 15N-DNRA potential is not related to O2 this suggests Salinity? Nitrate concentration? microbial population consistent at all O2 concentrations X Salinity is not an independent variable X Denitrification and DNRA both have similar kinetics *see: Roberts. K. L, Eate. V. M, Eyre. B. D, Holland. D. P and Cook. P. L. M (Submitted) ‘Hypoxic events stimulate nitrogen recycling in a shallow 12 salt wedge estuary: The Yarra River Estuary, Australia,’ Journal of Limnology and Oceanography Hypothesis DNRA coupled to Fe2+ oxidation http://school.discoveryeducation.com/clipart/clip/thinkingcapwhoa.html 13 13 NO3 reduction and 2+ Fe oxidation Denitrification has been linked to Fe2+ oxidation Abiotic Fe2+ oxidation has been previously linked to NH4+ production Weber et al (2006) first identified enzymatic Fe2+ oxidation coupled to DNRA in cultures http://school.discoveryeducation.com/clipart/clip/thinkingcapwhoa.html 14 14 The Investigation… 1. Slurry Incubations Control Treatment: 1mM 15NO3Iron Treatment: 1mM 15NO3- + ~5mM Fe2+ Sampled over 8 days: 15NH +, NO -, NO -, 15N and Fe2+ 4 3 2 2 15 15 1. Slurry Results 2700 600 60 2600 50 2500 40 2400 30 2300 500 400 300 200 15N-N 20 100 10 0 0 2200 2 15NH + 4 2+ Fe 0 50 100 Time (h) 150 Fe2+ (µmol L-1) 70 (µmol L-1) 700 15NH + 4 15N-N 2 (µmol L-1) Control Treatment 2100 2000 200 16 1. Slurry Results 4600 600 60 4400 500 400 300 4200 50 4000 40 3800 30 3600 20 15N-N 2 3400 100 10 3200 0 15NH + 4 2+ Fe 0 200 Fe2+ (µmol L-1) 70 (µmol L-1) 700 15NH + 4 15N-N 2 (µmol L-1) Iron Treatment 3000 0 50 100 Time (h) 150 200 17 1. Slurry Results 15NH + 4 (µmol L-1 h-1) Fe2+ (µmol L-1 h-1) Control 1.15 4.97 1 : 4.3 ±2.3 Fe Treatment 1.24 12.66 1 : 10.3 ±5.5 15NH + 4 : Fe2+ Uncertainty 15NH + : Fe2+ 4 The expected stoichiometry 1:8 8Fe2+ + NO3- +10H+ 8Fe3+ + NH4+ + 3H2O Linked 15NH4+ production to Fe2+ in slurries is this true for intact cores? 18 The Investigation… 2. Sediment Profiles Microelectrode Profiles: O2, H2S, pH electrodes DET Gels DET Gels: 15NH +, 15N , NO , NH + , Fe 4 2 X 4 tot Water Column Conditions: Anoxic, Oxic 19 19 2. Sediment Profiles electrodes DET Gels 20 2. Sediment Profiles electrodes DET Gels 21 2. Sediment Profiles ANOXIC OXIC 22 2. Sediment Profiles ANOXIC 15NH + 4 production No S2- Fe2+ OXIC 23 2. Sediment Profiles ANOXIC OXIC No 15NH4+ production No Fe2+ S224 DNRA coupled to Fe2+ oxidation OXIC ANOXIC NO3- NO3NH4+ NH4 + NO3- NO3 - Oxic N2 Fe3+ Fe2+ Anoxic NH4+ N2 NO3FeS Anoxic 25 25 Ecological Implications Estuaries with high inputs of colloidal Fe and subsequent pore water Fe2+ Fe driven DNRA may be an important process Leading to more NH4+ recycled than N2 removed 26 Future Work Slurry experiments: Inoculated controls Measure Fe3+ products eg Goethite Bacterial studies look for Fe2+ oxidisers previously linked to DNRA eg Geobacter sp. 27 Acknowledgements Water Studies Centre, Monash University Post Graduates, Staff and Analytical Laboratory Prof. Bo Thamdrup and Elizabeth Robertson University of Southern Denmark, The Department of Biology Research Partners Contact: [email protected] 28 28
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