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Microbially mediated redox cycling at the oxic-anoxic boundary in sediments: Comparisons of disparate habitats: 

Microbially mediated redox cycling at the oxic-anoxic boundary in sediments: Comparisons of disparate habitats Mark E. Hines Department of Biological Sciences University of Massachusetts Lowell

Redox Gradients:Where the Action Is!: 

Redox Gradients: Where the Action Is!

Perturbations of Redox Boundaries: 

Perturbations of Redox Boundaries Seasonal temperature changes Light-dark cycles Surface roughness and protrusions Bioturbation Plant roots and rhizomes Hydrologic changes

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Seasonal Temperature Changes Jørgensen 1977

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Light-Dark Cycles After turning off light

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Surface Effects

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Bioturbation Irrigation and sediment reworking Aller

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Oxidation Burial Reduction Ingestion FeS  FeOOH + SO42- FeOOH  Fe2+ SO42-  S2- Fe2+  FeS Conveyor Without Irrigation Heteromastus filiformis (10,000+ m-2) Turns over upper ~10 cm of sediment every 14 days “Turned on” at 12°C

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0 100 200 300 0 50 100 12 10 8 6 4 2 0 0 100 200 300 400 500 Depth (cm) Fe (µM) Sulfate Reduction (nmol m l-1 d -1 ) Bioturbated FeS-rich pellets; FeS + O2  FeOOH + SO42- FeOOH  Fe2+ (bacterial and/or Fe3+ + ∑H2S  Fe2+ + S°) Sediment Ingestion Burial Defecation No detectable ∑H2S Fe2+ + ∑H2S  FeS Profoundly Affects Biogeochemistry

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0 50 100 150 200 250 300 350 400 Mar Apr May Jun Jul Feb 0 50 100 Strong Influence on Temporal Biogeochemistry SO42- Reduction (nmol ml-1 day-1) Dissolved Fe (µM)

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Strong Influence on Trace Metal Biogeochemistry Dissolved Cu and Mo (µg l-1) 5 10 15 20 0 Mo Cu The Commencement of Bioturbation Affects Other Trace Elements

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Strong Influence on Nutrient Biogeochemistry Nutrients are removed from sediments rapidly during bioturbation (flux chambers) Residence time (days) of nutrients in porewaters

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Bioturbation and Hypoxia Italy Adriatic Sea

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Depth (cm) Carbon Oxidation (nmol ml-1 day-1) Degradation Path Changes During Hypoxia Sulfate Reduction Fe Reduction Fe Reduction Sulfate Reduction Oxic Hypoxic Upper 3 cm % Fe Reduction: 73% 14%

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Plants Also Perturb Redox Gradients

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Release of O2 from root leads to steep gradients

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Spartina Salt Marsh Spartina alterniflora can grow over 2 m high Extensive rhizopshere delivers DOC and O2 O2 enters mostly via arenchyma at low tide due to strong transpiration Aboveground elongation ceases when reproductive growth commences

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Labile C compounds released early due to mobilization of rhizome carbohydrates, and later due to translocation of fresh photosynthate Aboveground biomass completely removed by ice

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Plant Growth Affects Sulfide Production and Consumption A M J J A S 3000 2000 1000 0 Sulfate Reduction (nmol ml-1 day-1)

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A M J J A S 3000 2000 1000 0 Sulfate Reduction (nmol ml-1 day-1) Iron Behaves Similarly to Bioturbated Sediments

Culturable SO4 Reducers are Much More Abundant in Marsh: 

Culturable SO4 Reducers are Much More Abundant in Marsh Viable SRB (x 104) capable of growing on various substrates 12 New Species and 3 New Genera Isolated in Pure Culture

Vegetated Sediments: 

Vegetated Sediments Unvegetated Sediments Vegetation Affects Bacterial Composition Desulfovibrios are replaced by Desulfobacteriaceae *compared to total Bacteria

SO4 Reducers use Fermentation Products of Other Bacteria: 

SO4 Reducers use Fermentation Products of Other Bacteria

Roots Produce Fermentation Products: 

Roots Produce Fermentation Products Hydroponic, anaerobic, with antibiotics Roots replace fermenting bacteria

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Iron Behaves Similarly to Bioturbated Sediments Bacteria on Roots Respond Rapidly to Change 30 20 10 0 % Desulfobacteriaceae Not noted in bulk sediment

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Sulfate reduction (nmol ml -1 d -1 ) Fe 2+ (µM) Residence time Sulfide (µM) Residence time 500 200 0.4 days <2 <3 min 1000 3 3 min 2000 2 days Fe S Fe S particle transport surface oxidation Fe(ox) POC DOC solute transport subsurface oxidation O 2 Comparison of Animal and Plant Effects FeS FeOOH Fe 2+ SO 4 2- ｷH 2 S

Hydrologic Effects: 

Hydrologic Effects

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Hg2+ (Hg(HS)2) CH4 + Hg0 CH3Hg+ SRB Reductive Process merA merB Hg0 Anoxic Sediments Oxic Water Hg2+ CO2 + Hg2+ Oxidative Process

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CH4 CO2 %day-1 Wet Dry 0 0.1 0.2 0.3 0.4 0.5 0 1 2 3 Methylation Demethylation Changing Environmental Conditions Affect Both Methylation and Demethylation in situ 2 m Wet Dry

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0 4 8 12 16 20 0 1 2 3 4 5 6 7 8 9 Days Methylation (% /day) Dry (Anoxic) Wet (Anoxic) Wet (Oxic) Oxygen exposure rapidly inhibits methylation. However, even deposits that have been dry and oxic for months retain the ability to methylate when anoxia is restored. Riverbanks Sediments 203Hg2+  CH3203Hg+ Methylation is an Anaerobic Process

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Changes in Environmental Conditions Affect Both Methylation and Demethylation Methylation (% per day) Demethylation (% per day) Days Anoxic 0 1 2 3 4 5 6 7 0 5 10 Oxic CH4 CO2 Hg Cycling No Hg Cycling Reductive demethylation (mer) is activated soon after O2 introduction. Riverbank Soils

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1 0 2 4 6 8 10 12 2 4 20 56 85 100 Demethylation (% per day) Reductive process seems to “out-compete” the oxidative process However, the reverse can take several weeks Anoxic soil was dried overnight, rehydrated, and then incubated anaerobically for over three months

Acknowledgements: 

Acknowledgements Marine W.B. Lyons J. Faganeli H. Gaudette W. Orem G. Jones D. Bazylinski J. Tugel Vegetation R. Devereux J. Rooney-Varga W. Dennison D. Capone A. Giblin J. Hobbie F. Short R. Kiene Mercury T. Barkay J. Weber J. Faganeli M. Horvat J. Gray

Spartina Seasonal Summary(deduced from 4 years of close time interval sampling): 

Spartina Seasonal Summary (deduced from 4 years of close time interval sampling) Immediately after spring growth begins, SO42- reduction increases up to 100x from DOC release, and Fe is removed During plant elongation, SO42- reduction remains high, but plants exude O2 that removes reduced S, but remoblizes Fe Onset of flowering leads to sharp decrease in SO42- reduction and O2 (and DOC) exudation Soon after flowering, sedimentary geochemistry returns to “unvegetated” state (i.e., porewater chemistry equals that predicted from microbial activity)