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Week 4 Lectures November 2001 Microbial Ecology and Geochemical Cycles This Week’s Lecture Microbial Ecology Importance of Oxic/Anoxic Environment Geochemical Cycles Applications Microbial Ecology Understand the biodiversity of microorganisms and how different metabolically diverse organisms interact Monitor the activities of microorganisms and their impact on ecosystems Important Terms Environment: everything surrounding microorganism including the physical, biological, and chemical factors that act on the organism Populations of individual microbial species Guilds are metabolically related populations The microbial community is made up of guilds Guilds and Communities Community 1 Photic zone algae cyanobacter Community 2: Oxic Zone Oxic Anoxic Sediments Chemoorganoheterotrophs Chemolithoautotrophs Guild 1: nitrifiers Guild 2: sulfur oxidizing bacteria Community 3: Anoxic Zone Chemoorganoheterotrophs Guild 3: denitrifiers Guild 4: sulfate reducers Guild 5: fermenters Chemolithoautotrophs Guild 6: methanogens Guild 7: sulfate reducers Microbial Habitats and the Oxic/Anoxic Interface Oxygen clearly plays an important role in determining the range of microbial mediated reactions that occur in any environment It is important to understand the relationships between these two environments and the factors that lead to the formation of both Oxygen Relationships in Lake Ecosystems Oxic Anoxic Sediments { Epilimnion: oxygen concentration relatively uniform and may be as higher as near saturation { { Thermocline: zone of sharp temperature gradient that separates the epilimnion and hypolimnion Hypolimnion: zone of unmixed water having low oxygen content Dissolved Oxygen mg/L Oxygen Relationships in Surface Waters (Streams and Rivers) Wastewater Discharge Low Dissolved Oxygen Distance Downstream Oxygen Relationships in Groundwater Groundwater constituents that consume oxygen include: dissolved organic carbon (plant exudates, etc. methane inorganics reduced nitrogen reduced iron Typically oxygen concentrations decrease with travel distance Terrestrial Ecosystems O horizon: layer of undecomposed plant material A horizon: surface soil high in organic matter and high microbial activity B horizon: subsoil; minerals and humus leached from A horizon accumulate, little organic matter C horizon: soil base with low microbial activity Interrelationship Between Moisture Content and Oxygenation in Soils Soils that retain water tend to be more susceptible to anaerobic conditions Clays and silts Distance, mm Microenvironments Using Soil as an Example Distance, mm Geochemical Cycles oxidation/reduction reactions that describe the changes in an element as it passes through an ecosystem geochemical cycles then are of interest for elements that undergo oxidation/reduction reactions (C, S, N, Fe, and others) as shown before, oxygen plays a key role in metabolic reactions and is a major consideration in the description of geochemical cycles Carbon Geochemical Cycle Nitrogen Geochemical Cycle Sulfur Geochemical Cycle Coupling of Sulfur and Carbon Cycles: Concrete Corrosion S= SO4 + H+ acid Aerobic sulfur oxidation in crown where condensation occurs Aerobic atmosphere sewage with organics H 2S H 2S anaerobic sulfate reduction SO4 S= Summary by Example: Pipe Corrosion organics in sewage are used as energy source to convert SO4 to S= by sulfate reducers (chemoorganoheterotrophs) S= in equilibrium with dissolved H2S Dissolved H2S in equilibrium with gaseous H2S Example Continued Gaseous H2S dissolves into condensate at crown of sewer pipe and is used as energy source by sulfide oxidizers (chemolithoautotrophs) As H2S metabolized, acid is produced which dissolves concrete crown causing pipe to collapse Coupling of Carbon and Mercury Cycles: Mercury Cycling oxidized in atmosphere Coupling of Sulfur and Iron Cycles: Acid Mine Drainage Significant problem in areas where coal has been mined What happens? Why does it only happen after mining is started? Where does the yellow and reddish stain in contaminated streams come from? Acid Mine Drainage: Sulfur Oxidation and the Propogation Cycle for Iron Most coal contains some pyrite Pyrite has the formula of FeS2 When exposed to oxygen, pyrite undergoes the following slow spontaneous reaction which may be also biologically catalyzed FeS2 + 3 O2 + H2O Fe3+ + 2 S042- + 2H+ Acid Mine Drainage: Now what happens? FeS2 + 3 O2 + H2O Fe2+ + 2 S042- + 2H+ Under acidic conditions, ferrous iron (Fe2+) is then oxidized biologically to ferric iron (Fe3+) . This reaction does not proceed without the presence of iron oxidizing bacteria because ferrous iron is stable under acidic pH Fe2+ + O2 + H+ Fe3+ + H2O Generation of more acid and reduced iron Ferric iron produced by iron oxidizing bacteria reacts with more pyrite to form more reduced iron to accelerate the cycle and increase acidity FeS2 + 14 Fe3+ + H2O 15 Fe2+ + 2 S042- + 16H+ Putting it All Together Slow spontaneous reaction that initiates process FeS2 + 3 O2 + H2O Fe2+ + 2 S042- + 2H+ Fe2+ + H2O O2 + H+ Fe3+ + Fe2+ Not much O2 energy so iron oxidizing organisms Fe3+ oxidize large amounts of iron FeS2 FeS2 + 14 Fe3+ + H2O 15 Fe2+ + 2 S042- + 16H+ note production of large amounts of reduced iron and acid Answers to Questions When does it start? Stains? Jarosite [HFe3(SO4)2(OH)6]