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Topic 7: BioGeoChemistry – Anaerobic Respirations Overview Anaerobic respirations of inorganic electron acceptors Aerobic oxidation of the endproducts of anaerobic respirations Cycles (C, N, S, Fe) Industrially and environmentally relevant reactions 1 Topic 7: BioGeoChemistry – Anaerobic Respirations Examples of Examinable Material (will need own complementation of knowledge gaps from internet) The processes: •Sulfate reduction, oxidation of sulfur/sulfides •Nitrate reduction (denitrification), nitrification •CO2 reduction (methanogenesis), methane oxidation •Iron reduction (Geobacter), Iron oxidation •Any others? •Ecological role of the processes •Economic, commercial, applied role of the processes •Reaction, Organism, 2 Simple carbon cycle R1: Photosynthesis CO2 + H2O CH2O + O2 Electron donor: H2O (the oxygen atom) Oxidation state from -II 0 Electron acceptor: CO2 (the C atom) Oxidation state from +4 0 Reaction is endergonic. How does it work? Light energy to drive reaction “uphill” Terrestrial plants and marine microalgae CO2 + H2O <--> H2CO3<--> HCO3- <--> CO32H+ H+ 3 Simple Carbon cycle R2: Respiration CH2O + O2 CO2 + H2O + new biomass Electron donor: organic carbon Electron acceptor: O2 Exergonic, releasing energy (ATP) for growth Reactions stoichiometrically reverts photosynthesis. For mature ecosystems (e.g. rain forest) respiration balances exactly photosynthetic activity Sustained Net O2 production (or CO2 consumption) needs deposition of organics 4 Role of Bacteria in Nature CO2 CH2O O2 Electron Donor H 2O Electron Acceptor Energy Oxygen cycle and simplified carbon cycle More complex carbon cycle also involves: • Methane cycle • Anaerobic respirations How Can Life without Oxygen Work? O2 = principal electron acceptor of aerobic life. Without O2 a different e- acceptor needs to be found. Fermentations (e.g. lactic, ethanolic) have used internally created e- acceptors for no gain in ATP (no respiration). Now we will deal with e-acceptors that allow ATP generation via respiration (ETC, proton gradient, ATPsynthase) Bacteria are the only life forms capable of using electron acceptors other than O2 (anaerobic respiration). The use of alternative electron acceptors dramatically changes the chemistry of the environment 7 What are the Typical Electron Accepting Reactions? O2 H2O (aerobic respiration) + 4e- H2S (sulfate reducing bacteria) SO42- + 8e- Fe3+ Fe2+ (iron reducing bacteria) + 1e8 What are the Typical Electron Accepting Reactions? S H2S (sulfur reducing bacteria) + 2e- NO3- N2 (denitrifying bacteria) + 5e- NO3- NH3 (nitrate ammonification) + 8e- CO2 + 8e- CH4 (methane producing bacteria) 9 What are the Typical Electron Accepting Reactions? O2 + 4e- H2O (aerobic respiration) SO42- + 8e- H2S (sulfate reducing bacteria) Fe3+ + 1e- Fe2+ (iron reducing bacteria) S + 2e- H2S (sulfur reducing bacteria) NO3- + 5e- N2 (denitrifying bacteria) NO3- + 8e- NH3 (nitrate ammonification) CO2 + 8e- CH4 (methane producing bacteria) 10 What Happens with the Electron Acceptors after Accepting Electrons? By accepting electrons, the acceptors they turn into reduced species. Reduced species are reducing agents that dramatically change the chemistry of the environment If in contact with O2 , reduced species can become electron donors for specialised lithotrophic bacteria The continued cycle of electron acceptors to reduced species and back to electron acceptors is a typical part of biogeochemical cycling. The S, N, Fe cycle are typical examples. 11 Classification of Microbial Metabolic Types Energy type Photo Chemo Electron donor Organo Litho Carbon source Hetero Auto Electron acceptor Aerobic Anaerobic Examples: Algae: Photo-Litho-Autotroph, Bacteria, Fish: Chemo-organo-heterotroph Thiobacillus: Chemo-litho-autotroph Photosynthetic bacteria: Photo-Organo-heterotroph Our focus : Anaerobic Heterotophs and Aerobic lithotrophs 12 Life without O2: Alternative Electron Acceptors 1 Electron Acceptors O2 NO3- NO3- Fe3+ SO42- So CO2 CO2 HumOx H 2O N2 NH4Fe2+ H 2S H 2S CH4 Acetate HumRed Measure for Energy Released with H2 as Electron Donor (log K) 21 21 15 8 5 3 2 ? ? adapted from Stumm & Morgan Observation: There is a sequence of use of electron acceptors which is according to the energetic usefulness (redox potential) O2 , nitrate, humic acids, ferric iron, sulfate, CO2 13 14 Electron Acceptors are Reduced and can become e- Donors Electron Donors Electron Acceptors organic O2 H2S So NO3- So SO42- NO3- NH4+ Fe2+ Fe3+ Fe3+ Fe2+ NO2- NO3- SO42- H2S H2 H+ So H2S CO CO2 CO2 Acetate NH4 NO2- CO2 CH4 HumOx HumOx HumR H2O N2 HumR 15 Interconnection Between Different Electron Donors and Acceptors Electron Donors H2 H+ Electron Acceptors CH4 So CO2 H2S CH4 CO2 SO42- H2S H2S So Fe3+ Fe2+ SO42- NO3- NH4 Fe2+ Fe3+ NO3- N2 NO2- NO3- O2 H2O So 16 Simple Sulfur cycle Competition for Electron Donor by Different Acceptor Systems General observations with anaerobic respirations: Threshold level for minimum degradable substrate concentrations decreases with the redox potential of the electron acceptor Using H2 as a model substrate (accounting for about 30 % of energy flow in anoxic environments): Organisms with more “powerful” electron acceptor out-compete others by keeping the H2 concentration below “detection limit” of competitors. This explains the apparent preference for using best electron acceptors (most positive redox potential) first. CO2 CH4 H2 SO4 2- H2S NO3- NH4+ Time 18 How does the threshold for electron donor (e.g. H2) affect the kinetics of uptake rate (not growth rate)? What is the relationship between substrate concentration (S) and its uptake rate (v) ? vmax (h-1) v (h-1) v = vmax substrate limitation kM S (g/L) S ------S + kM Effect of threshold (e.g. H2) because of backreaction) And 1919 Growth- Michaelis Menten model Effect of Maintenance Coefficient (mS) on growth Rate The negative specific growth rate (µ) observed in the absence of substrate (when S = 0) (cells are starving, causing loss of biomass over time) µ (h-1) 0 S(g/L) is the decay rate mS*Ymax - mS*Ymax And 2020 21 Sulfate Reduction (SRB) Sulfate is a suitable and abundant alternative electron acceptor Typical reactions: 4 H2 + SO42- + H+ CH3-COO- + SO42- HS- + 4 H2O HS- + 2 HCO3- Organisms: Sulfate Reducing Bacteria (SRB), strictly anaerobes, Desulfovibrio, Desulfobacter, etc. Electron donors: small molecules (breakdown products from fermentations, or geologically formed, e.g. H2, acetate, organic acids, alcohols) Reduce also elemental sulfur, sulfite and thiosulfate to H2S Ubiquitous 22 Bacterial Sulfate Reduction When Does it Occur ? In the presence of organic substances , after depletion of oxygen nitrate and ferric iron sulfate reduction is next Initially in sediments, Rates from 0.01 to 10 mM/day Typically in sediments but also on surfaces (ships) underneath biofilms Within flocs or intestines of marine animals Sulfide reacts chemically as a reducing agent (e.g. with O2 or Fe3+) elemental sulfur formation Formation of FeS and FeS2 black color of sediment In typical reduced sediments (e.g. mangroves, estuaries SR may be higher than O2 use) Thiosulfate disproportionation (SO32-) into sulfate and sulfide 23 Dissimilatory Sulfate Reduction by SRB Organisms: Sulfate Reducing Bacteria (SRB), strictly anaerobe Desulfovibrio, Desulfobacter, etc. Use of small compounds (H2, acetate, other organic acids alcohols but not polymers, proteins, carbohydrates, fats) Cooperation with fermentative bacteria needed to degrade detritus End product sulfide (H2S HS- + H+) is toxic, reactive, explosive Typical reactions: 4 H2 + SO42- + H+ HS- + 4 H2O CH3-COO- + SO42- HS- + 2 HCO3- Reduce also elemental sulfur, sulfite and thiosulfate to H2S 24 SRB Significance in Marine Environments Ecologically: playing a major role in sulfur cycle and sediment activities sulfide = O2 scavenger “negative oxygen concentration” responsible for sulfur deposits (H2S + O2 S + H2O) P-release from sediments Economically: End product H2S: poisonous, explosive, corrosive, malodorous Corrosion of submersed steel structures (e.g. platforms, bridges) Corrosion of oil pipelines (inside and outside) Lethal gas emissions on offshore platforms Petroleum degradation (burning sour gas: H2S + O2 SO2) 25 S-ox: Volcanic Sulfur Springs E.g. New Zealand Yelllowstone National Park 26 SRB Morphology Typical shape of sulfate reducing bacteria (SRB) of the type Desulfovibrio. 27 SRB Role in Corrosion of Steel Electrons on the steel surface produce stabilising H2 layer bacteria use H2 as the electron donor for sulfate reduction this removes electrons and leaves the iron positively charged The positive charge favours the release of Fe2+ into solution Ongoing process causes corroding electron flow and weight loss 2SO 4 HS Bacteria feed on electricity Cathodic protection SRB Fe2+ H2 FeS Steel H+ eCorrosion current 28 SRB Damage to Pipeline Microbially influenced corrosion of marine oil pipeline showing typical pitting corrosion. 29 “SRB in Petroleum Industry” Research at Murdoch Q: Where do SRB in oil pipelines come from? A: Mostly as a biofilm attached inside the pipes. Method SRB monitoring during pig runs. Q: Are SRB supported by corrosion ? A: SRB can grow on corroding iron. Cathodic protection enhances their growth. Q: Are treatments effective against SRB? A: SRB can degrade organic biocides. 30 Dissimilatory Nitrate Reducing Bacteria Dentrification (nitrate to N2) typically involves the aerobic bacteria Organic electron donor + NO3- N2 Bacteria use complex substrates Further details in lecture on N-cycle In sediments nitrate ammonification can play important role Organic electron donor + NO3- NH3 Nitrate ammonification is due to anaerobic bacteria (e.g. SRB) 31 Dissimilatory Iron Reducing Bacteria Organisms: Various anaerobic bacteria, no specific group e.g. Geobacter e- donors: mainly small compounds Typical reaction: H2 + 2 Fe3+ 2 Fe2+ + 2 H+ Reaction results in lowering of redox potential Reduce also Manganese, elemental sulfur and other metals (e.g. uranium) End product is magnetite (Fe3O4) and other compounds (black precipitates) Significance of iron reduction is still being underestimated Recent research: electricity production using ferric iron reducing bacteria 32 CO2 or HCO3- Reduction (Methane Producing Bacteria) CO2 is even more abundant than sulfate but difficult to use By Methane Producing Bacteria (Archeae) Strictly anaerobic requiring a redox potential of less than -350mV Highly oxygen sensitive: 4 H2 + HCO3- + H+ CH4 + 3 H2O Very limited substrate spectrum (H2, acetate, methanol) Syntrophic associations are formed with fermenting bacteria Because of poor solubility (bubble formation) some methane from sediments escapes into atmosphere (greenhouse gas) True removal of BOD from water. 33 Aerobic Re-oxidation Processes 1 Sulfide and Fe2+ Contact of reduced (black sediments) with O2 : bacterial oxidation of sulfide and Fe2+ occurs. Beggiatoa: 2 H2S + O2 2 S0 + H2O (white algae) Thiobacillus: H2S + 2 O2 H2SO4 (sulfuric acid) very low local pH values of <1. Further microbial pipeline corrosion. Also insoluble species are re-oxidized e.g. pyrite (FeS2) bio-leaching of minerals). Elemental sulfur is often produced as intermediate (white precipitate (“white smoker”, “white algae”) 34 Aerobic Re-oxidation Processes 2 - Ammonia NH4+ oxidation is energetically difficult and slow and requires oxygen as electron acceptor. Organisms: Nitrosomonas, Nitrobacter, two step process. NH4 uptake also possible by assimilation of phytoplankton. 35 Fate of Sulfide in the Presence of Oxygen Contact of reduced sulfide (H2S or FeS) with air spontaneous oxidation (H2S) to insoluble S Microbial Oxidation: (a) Beggiatoa: 2 H2S + O2 2 So + H2O (“white algae”) (b) Thiobacillus: H2S + 2 O2 H2SO4 (sulfuric acid) very low local pH values of <1. Further microbial pipeline corrosion. Also insoluble species are re-oxidized e.g. pyrite (FeS2) (bio-leaching of minerals). Elemental sulfur is often produced as intermediate (white precipitate (“white smoker”, “white algae”) Together, sulfate reduction and sulfide oxidation can close the sulfur cycle. 36 Depth Profile of Aerobic Anaerobic Interface Brown high Eh NO3- O2 Fe3+ Microbial S conversion Sulfate NH4+ Fe 2+ HS- Sulfide Black Low Eh Highest chemical and biological activity at the interface (presence of electron donors and acceptors) Depth H2 , CH4 Concentration 37 Scheme of Ocean Hydrothermal Vent from Ocean Ridges Extreme Life Conditions: Anaerobic, hydrogen driven Strong temperature gradients High pressure Origin of life is thought to have been thermophilic, with H2 and So from volcanic sources as e-donor and acceptor. www.jamstec.go.jp/jamstec-e/ bio/subext/thergane.html 38 Sulfur Cycle at Hydrothermal Vents SO42- H2S + O2 S, SO42Biological oxidation H2S Geochemical Reduction Similar principle in sewer pipes 39 “Black Smokers” releasing reduced sufur and iron (e.g. FeS) as potential electron donors for bacteria. 40 •S oxidising Bacteria as primary producers White “snowblower” producing suspended sulfur bacteria in snow flake type aggregates during a volcanic eruption Woods Hole Oceanographic Institute East Pacific Rise. 41 Tubeworms (Riftia) living in association with sulfur oxidising bacteria 42 Tubeworms (Riftia) living in association with sulfur oxidising bacteria Micheal Degruy Dark food-chain Independent of Sunlight ? 43 Deep-sea mussel Bathymodiolus thermophilus using symbiotic sulfur bacteria. Photo by Richard A. Lutz 44 Galatheid crabs lining a fissure at a hot vent on the East Pacific Rise feeding on sulfur bacteria. Courtesy Woods Hole Oceanographic Institution 45 Tubeworms (Riftia) living in association with sulfur oxidising bacteria 46 Anaerobic Oxidation of Sulfide There are principally two conditions allowing sulfur cycle in the absence of oxygen: 1. Presence of light and phototrophic bacteria: Very colorful, play a role in microbial mats Can use light that is not suitable for algae Green sulfur bacteria (S outside, Chlorobium) Purple sulfur bacteria (S inside, Chromatium) 2. Presence of other “powerful e-acceptors (e.g. nitrate, Fe3+) are available Thiomargareta a recent discovery 47 Thiomargareta namibiensis 48 Nitrate storage 49 Thiomargareta namibiensis 50 Aerobic Re-oxidation Processes 2 - Methane CH4 is a highly energetic e-donor (fuel) in aerobic areas With electron acceptors O2, Nitrate, Fe3+ methane can be re-oxidized by methylotrophic bacteria Recently evidence has been found of CH4 oxidation linked to sulfate reduction. Those electron acceptors are usually made available by bioturbation, thus CH4 usually does not reach the water column Where benthic macrofauna has been killed methane production forms large CH4 bubbles that can escape the water column (does not occur in the open ocean) Under pressure, methane forms hydrates on the ocean floor around continental shelfes. These hydrates can be used as electron donors for aerobic bacteria food chain. 51 Gas Hydrate Molecules (dusk.geo.orst.edu/ oceans/lec14.html) 52 Methane hydrate mount under flashlight 53 Methane hydrate outcrop from continental shelf. approximately 250 miles east of Charleston, S.C courtesy Carolin Ruppel 54 “Chemosynthetic mussel from methane hydrate Tubeworms (Riftia) living in association with sulfur oxidising bacteria Tube worms Methane hydrate outcrop Micheal Degruy 55 Methane hydrate sample 56 “Chemosynthetic mussel from methane hydrate Tubeworms (Riftia) living in association with sulfur oxidising bacteria Tubeworm collected from gas hydrate seepage areas Micheal Degruy 57 Tubeworms (Riftia) living in Spider crab looking for food between the tubeworms growing on association with sulfur a methane hydrate outcrop oxidising bacteria Micheal Degruy 58 “Chemosynthetic mussel from methane hydrate Tubeworms (Riftia) living in association with sulfur oxidising bacteria Mussels Micheal Degruy with bacterial slime living on methane hydrate 59 “Chemosynthetic mussel from methane hydrate” Tubeworms (Riftia) living in association with sulfur oxidising bacteria Micheal Degruy 60 “Chemosynthetic mussel from methane hydrate “Iceworm” living on Tubeworms (Riftia) living in gas hydrate by association with sulfur ustilising oxidising bacteria methylotophic bacteria Micheal Degruy NOAA: The Deep East Expedition Blake Ridge Photos from NOAA Alvin div Sept. 23-28,2001National Oceanic and Atmospheri Administratio 61 “Chemosynthetic mussel from methane hydrate Tubeworms (Riftia) living in association with sulfur oxidising bacteria Methane hydrate with ice worms Micheal Degruy 62 “Chemosynthetic mussel from methane hydrate Tubeworms (Riftia) living in association with sulfur oxidising bacteria Micheal Degruy 63 Gas Hydrate at the Surface Under Atmospheric Conditions: gas hydrate separates into flammable CH4 and water (here cooling hands) 64 Example Locations of Gas Hydrate 65 66 Methane Hydrate (Summary) Methane hydrate, a curiosity or a significant global phenomenon? Needed for formation: low temperature and high pressure Why are hydrates mainly on the continental shelfes ? Deep oceans lack organic material High biologic productivity (CH4) Rapid sedimentation rates (bury the organic matter) 67 BioLeaching Example pyrite ore Fe2S or FeS (Fe2+ S2-) OS or Fe= +2, of S -2, both are reduced Bacteria oxidise both Example microbe: Thiobacillus thio-oxidans, Thiobacillus ferro-oxidans Initial steps: Oxidise sulfur to S0 and Fe2+ to Fe3+ Secondary steps: S0 to H2SO4 (pH to 1) Indirect leaching 2 Fe3+ + S2- 2 Fe2+ S0 68 Indirect Reactions As shown from the indirect effect of oxising ores via Fe3+ Also other metal ions can be oxidised 69 Heapleaching Tank-leaching Example ores: Chalcopyrite Arsenopyrite Example Metals: Gold, coppyer, Zinc, 70 What is Microbial Fuel Cell? Chemical Fuel Cell (e.g. H2 FC) Microbial Fuel Cell Electric current V External electric circuit e- e- e- External Resistor 2 Acetate 4 ATP 2 CoA PHB (18 e-) e2 Acetyl-CoA (16 e ) O H2O 2 CO2 + CH3 Bacteria 2 CoA Fe(CN)63n Fe(CN)64- 1 NADH + H+ e- 2H+ TCA Cycle O2 + 8 NADH + H Biomass Anode Cathode OR Medo Medred. Electrolyte ETC x 24 ATP Cation Exchange Membrane ETC H+ H2O Cathode + 1/2O2 + Anode H2 2e- 2e- O CH CH2 C - CH3 C S CoA 2H+ + eO - Applications for Microbial Fuel Cell • • • • • • • Powering monitoring devices in remote locations Powering electronic devices with renewable energy sources Self-feeding “Gastrobots”- (Air Force ‘SPIDERS’ Project) Converting astronaut waste to electricity (NASA) Decentralized domestic power source Novel sensing devices Conversion of waste organic matter to electricity instead of methane • Conversion of renewable biomass to electricity instead of ethanol • Bioremediation of contaminated environments (DOE-NABIR field trial) • Powering automobiles - collaboration with Toyota Microbial Fuel Cells for energy efficient Waste Water Treatment • Research interests are gaining momentums (Bullen et al., 2006; Davis and Higson, 2006; Logan and Regan, 2006; Lovley, 2006; Rabaey and Verstraete, 2005) • Energy and water supply are among the biggest challenges we will face in the future. • Recovery of valuable resources such as water, energy and nutrients. Adapted from Logan et al., 2006 ES&T Our MFC Research Focuses on: • Sophisticated computer process control of MFC • Highest power density in 2007 • Sustainable operation of MFC process To overcome the problems of non-sustainable operation of MFC (e.g. pH imbalance) • Practical application of MFC to solve problems of other bio-process (e.g. Anaerobic Digestion) Application of Computer for MFC Process Control Signal In Signal Out Laboratory Scale of the PC-Controlled MFC System Control and Monitoring: LabVIEW™ 7.1 / National Instrument Data Acquisition Card Plate 2. 5 mm Plate 1. Plate 3. Acknowledgement: The Reactor was designed by Dr. Korneel Rabaey (Advanced Wastewater Management Centre, The University of Queensland, Australia) Conductive Granular Graphite Bio-Electrochemical device for the Potentiodynamic Study (refer to v3p127) i Potentiostat Counter Working Reference Outflo w V LabVIEW 7.1™ Inflow Graphite rod 5mm Ø (current collector) Ag/AgCl reference electrode (3M KCl) Biofilm coated granular graphite electrode Proton exchange membrane (Nafion 117) Platinum foil potassium chloride solution 1M Magnetic stirrer bar Compare Different Volatile Fatty Acids 100 90 Acetate Propionate Butyrate Current (mA) 80 70 60 50 40 30 20 10 0 0 1 2 3 4 Time (hour) 5 6 7 MFC for Microbial Biosensor Development - Potential Component for Anaerobic Digestion Process Control - Outcomes on MFC Research at Murdoch University in 2007-08 Journal Publications • Cheng, K. Y., Ho, G., Cord-Ruwisch, R. (2008). "Affinity of microbial fuel cell biofilm for the anodic potential." Environ. Sci. Technol., 10.1021/es8003969. • Cheng, K. Y., Cord-Ruwisch, R., Ho, G. (under review). A novel method for in situ cyclic voltammetric studies of a microbial fuel cell biofilm. J. Microbiol. Meth. • Cheng, K. Y., Cord-Ruwisch, R. Ho, G. (in preparation). Evidence for an optimum anodic potential for maximum current production in microbial fuel cell biofilms. Conference Presentations • Cheng, K. Y., Cord-Ruwisch, R., Ho, G. (2007). A mixed anodophilic biofilm exhibits saturation behavior with anodic potential in a microbial fuel cell. Microbial Fuel Cells: First International Symposium, Pennsylvania State University, Pennsylvania State, USA, May 27-29, 2008 • Cheng, K. Y., Cord-Ruwisch, R., Ho, G. (2007). Computer-controlled microbial fuel cell enables efficient electricity production from activated sludge. IWA Specialist Conference: 11th World Congress on Anaerobic Digestion: Bioenergy for Our Future – Renewable Energy from Waste. 23-27 Sep 2007 at Brisbane, Queensland, Australia Award • Cheng, K. Y. Winner of Huber Technology Prize 2008 (Munich, Germany): Enhanced Electricty Production from Wastewater in a Computer-Controlled Microbial Fuel Cell. (superivsors: Dr. Ralf Cord Ruwisch & Prof. Goen Ho) Summary • Anaerobic reduction processes and aerobic oxidation processes are increasingly found to go hand in hand. O2 H2S Sulfate E-donor 81 Summary • Anaerobic reduction processes and aerobic oxidation processes are increasingly found to go hand in hand. O2 Fe2+ Fe3+ E-donor 82 Summary • Anaerobic reduction processes and aerobic oxidation processes are increasingly found to go hand in hand. O2 CH4 CO2 H2 83 Summary • Anaerobic reduction processes and aerobic oxidation processes are increasingly found to go hand in hand. O2 NH3 NO3Organics 84 Iron reducing bacteria flowing electrons to ferric iron • Organisms: Geobacter and other anaerobic bacteria, no specific group •E- donors: mainly small organic compounds • Typical reaction: H2 + 2 Fe3+ --> 2 Fe2+ + 2 H+ • Reaction results in lowering of redox potential. • Reduce also Manganese, elemental sulfur and other metals (e.g... uranium). • Endproduct is magnetite (Fe3O4) and other compounds (black precipitates) • Significance of iron reduction is still being underestimated. •Recent research: electricity production using ferric iron reducing bacteria 85 Magnetite formation as the endproduct of iron reduction 86 How do IRB transfer electrons to insoluble iron ? ? Ideas: •solubilise iron (complexing, acid dissolving, etc.) •physically attach to ferric iron •excrete electron shuttling species (mediators) use e-carriers present in environment humic acids (quinone, analogue to NADH) 87 Humic acids as natural mediators ? Ideas: Geobacter transfers electrons to oxidised humic acid which diffuses to the iron to pass on the electrons (quinone, analogue to NADAH). Ox Humics Red Humics OH O OH Red. O Ox. 88 Other fancy tricks of Geobacter ? eCl Cl By transferring electrons to chlorinated hydrocarbons Reductive dechlorination Bioremediation potential “Chlorine respiration”? 89 Other fancy tricks of Geobacter ? Acetate Geobacter eWolinella e- Own work: Interspecies electron transfer from Geobacter to other strains How? Cytochromes? Wires? Nitrate 90 Geobacter Headlines (Geobacter.com) : Bioelectricity If Geobacter can transfer electrons to almost anything, why not to a carbon anode. Electricity generation by Geobacter Outback batteries Driving force: Organic wastes Key: Sugar degrading Geobacter type (Rhodoferax) 91 92 93 Simple approach to bio-electricity 94 Geobacter Headlines 2: Cleanup of Uranium Geobacters capability of reducing other metal species includes uranium. By reducing U(VI) to U(IV) which is less soluble limitation of contamination has been applied in situ (2003) Tod Anderson 95 Geobacter Headlines 3: Hottest Bug Strain 121 Geobacters presence of deep ocean vents (black smokers) has been shown. Temperature tolerance to 121 (autoclave) Interesting Genome. Suggestions of iron reducing Archeae to be one of the oldest lifeforms rather then sulfur reducers. Very old magnetite formations are seen to support this view. 96 Strain 121 97 Lecture Summary 1. Sulfate reduction P- release from sediments,Sulfur deposits, Corrosion of submerged steel, Lethal gas emissions 2. NO3- reduction to N2 black precipitate, magnetite (Fe3O4), is produced when Fe3+ is reduced to Fe2+ 3. CO2 reduction to CH4 Methane - highly energetic e-donor (fuel) in aerobic areas 4. Prolific life at the anaerobic/aerobic interface High activity at chemocline, Black smokers, Life on CH4 98 Lecture Summary 1. Sulfate reduction P- release from sediments,Sulfur deposits, Corrosion of submerged steel, Lethal gas emissions 2. Fe3+ reduction to Fe2+ black precipitate, magnetite (Fe3O4), is produced when Fe3+ is reduced to Fe2+ 3. CO2 reduction to CH4 Methane - highly energetic e-donor (fuel) in aerobic areas 4. Prolific life at the anaerobic/aerobic interface High activity at chemocline, Black smokers, Life on CH4 99 End of lecture, below only for personal interest 100 Methane hydrate • Methane hydrate, a curiosity or a significant global phenomenon? • Needed for formation: low temperature and high pressure • Why are hydrates mainly on the continental shelfes ? • Deep oceans lack • high biologic productivity (CH4) • rapid sedimentation rates (bury the organic matter) 101 Methane gas hydrate formation Gas hydrate stability zone on deep-water continental margins. A water depth of 1200 meters is assumed. 102 Sea floor slopes on continental margins are stable if the slope is less than 5°. However, many continental margins with shallow slopes have scars from underwater landslides. A potential trigger for shallow slope landslides is sudden gas release from the sediments. This can occur if the methane hydrate layer in the sediment becomes unstable. The hydrate layer can melt if the temperature rises or there is a drop in the confining pressure (below). Melting suddenly releases the methane trapped in the hydrate along with any natural gas trapped below the hydrate layer. Twenty thousand years ago an ice age resulted in the formation of large ice cap that covered much of northern Europe and Canada, and resulted in a 120m drop in sea level. The drop in sea-level reduced the pressure at the sea floor (due to the fact that there was less overlying water). Consequently the methane hydrate layer melted, causing many underwater landslides on the North American continental margin – the scares of which are still visible today and perhaps submarine slide scars recently mapped off Wollongong. 103 Gas hydrates and bubbles in the bermuda triangle ^ A drop in sea-level reduces the pressure at the sea floor and causes the melting of methane hydrate. The sudden release of gas results in landslides and slumps. It can also result in a plume of gas rapidly rising to the ocean surface. Gas in the water reduces the density of water leading to the loss of buoyancy of ocean going craft. Is this what causes the mysterious sinking of ships in the Bermuda Triangle? When sea level dropped during the last ice age, the destabilisation of hydrate and the release of methane may have been sufficient to heat the atmosphere via greenhouse effects and turn back the ice age. At atmospheric pressure the concentration of methane in hydrate is over 600 times greater than in the free gas form. Methane hydrate is also significantly denser than liquid natural gas. Methane hydrate may provide a cost effective way of transporting and storing methane. 104