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
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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
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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
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How is N cycled in the estuary?
NO3-
DNRA
NH4+
NO3-
N-recycled
DNRA = Dissimilatory nitrate reduction to ammonium
ANOXIC
Denitrification
N-removed
N2
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When does DNRA occur?
1. Reducing conditions:
- Carbon decomposition
- Sulphate Reduction
2. NO3- limited conditions
DNRA = Dissimilatory nitrate reduction to ammonium
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When does DNRA occur?
Hypothesised:
DNRA would dominate
under anoxic conditions
DNRA = Dissimilatory nitrate reduction to ammonium
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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
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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
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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
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Why does DNRA predominate
under oxic rather than
anoxic conditions?
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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
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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
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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
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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
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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
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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
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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?
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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
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19
2. Sediment
Profiles
electrodes
DET Gels
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2. Sediment
Profiles
electrodes
DET Gels
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2. Sediment
Profiles
ANOXIC
OXIC
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2. Sediment
Profiles
ANOXIC
15NH +
4
production
No S2-
Fe2+
OXIC
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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
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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
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Future Work
Slurry experiments:
 Inoculated controls
 Measure Fe3+ products eg Goethite
Bacterial studies look for Fe2+ oxidisers previously
linked to DNRA eg Geobacter sp.
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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]
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