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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