Metabolic Diversity (III)
3-3-16
Overview
2. Anaerobic respiration
2.5 Methanogenesis
 CO2 is typically abundant in
anoxic habitats
- Major product of
chemoorganotrophs
 Two major groups of obligate
anaerobes use CO2 as e–
acceptor
- Acetogens
- Methanogens
Acetogens and Methanogens
 H2 is the major e– donor for both groups.
2.6 Methanogenesis
 Methanogens:
4H2 + CO2 → CH4 + 2H2O
ΔGº’= -136 kJ/rxn
- Obligate anaerobes
- All belong to Archaea
- Produce CH4 as the primary product of catabolism
- Obtain most or all of their energy from
methanogenesis (different from anaerobes that
produce small amount of CH4 as byproduct)
- Many are also autotrophs (fix CO2 by variations of the
acetyl-CoA pathway)
CH4 is produced biologically from many anaerobic environments
• CH4 is a greenhouse gas that may contribute ~ 20% of the total global warming
-
global warming potential of CH4 > 20 times of CO2
Current biological CH4 production: ~ 1 billion tons yr-1
Current CH4 emission:
~ 0.5-0.6 billion tons yr-1
Human
activities
~ 60%
Natural
sources
~ 40%
Natural gas
Fossil fuel
Wetlands
Landfills
Biomass
burning
Rice paddies
Termites
Ruminants
Oceans
Based on Bousquet et al (2006) Nature 443:439
Phylogenic Distribution of Methanogenic Archaea
 Methanogens are very diverse within the Archaea.

Major substrates for methanogenesis:
-
H2 + CO2 or (HCOOH): ~ 1/3 of the CH4 production
Acetate: ~ 2/3 of the CH4 production
Methyl-group C1 compounds: e.g. methanol, methylamine
Some neutral, some slightly higher pH. 3 types of substrates.
How to grow methanogenic archaea?
H2-consuming methanogens
require pressurized containers
and large headspaces for
optimal growth. Anaerobe
chamber. Only open tube in
chamber
Cofactors of Methanogenesis
DON’T
PANIC!!
THF
FAD or
NAD+
Methanogenesis
Pathway
4H2 + CO2 → CH4 + 2H2O
Keep in mind the methyl branch
of the acetyl-CoA pathway! CO2
reduced. And activates formyl
group. Doing 2 steps. Hydrogen
gas reduces f420. HS-CoB, free
thiol group functions as e- donor.
Reduce methyl group to methane
gas unlike acetogenesis.
Acetogenesis: CO2 reduced to
formate. Had formly group
activated 1 ATP linked.
Energy Conservation in
Methanogenesis
Electron bifurcation
ΔGº’= -30 kJ/mol
Methanogenesis Pathway
2H
2H
Acetogens in Wood-Feeding Termites
 Termite gut provides a
protective niche for
microbial symbionts
 Microbes near the
hindgut wall consume
O2 fast → anoxic
 Microbes degrade
sugars → acetate,
benefiting the host
Why acetogens outcompete
methanogens in wood-feeding
termite gut?
 Acetogens (mostly
spirochetes) are the
major H2-consuming
organisms → acetate
>> CH4
Anaerobic Food Chain
What would happen
if methanogens are
missing?
 The distribution of microbes in sediments of soils is
dictated by the free energy yield for each respiratory
process.
 Threshold: the lowest concentration of a substrate
which can be metabolized by an organism.
aA + bB ↔ cC + dD
𝐜
𝐂
𝐃
o
ΔG = ΔG + 2.303 RT log 𝐚
𝐀 𝐁
𝐝
𝐛

Log scale
When the [substrate] is very low, the
reaction no longer provides sufficient
energy to support growth.
How does the threshold concept explain
why certain e acceptors are used first in
anaerobic environment? Threshold for
methanogens is 104Use at lower level. Continue to use it until
105-.
 Threshold is set by the thermodynamics of the reaction.
Summary
 Acetogens: 4H2 + 2CO2 → CH3COOH + 2H2O
-
Obligate anaerobes
‘generalists’: use a wide variety of substrates
Reductive acetyl-CoA pathway
• Conserve energy
• Fix CO2
 Methanogens: 4H2 + CO2 → CH4 + 2H2O
Obligate anaerobes belonging to Archaea
‘specialists’: use three major types of substrates (CO2
+ H2, acetate, or methyl-group C1-compounds)
- Methanogenesis
• Conserve energy
- Important for the anerobic food chain: organic
matter → CH4
-
• Thursday (Oct 27)
‐ Example of paper presentation
‐ Each group
‐ Discussion leader (organize group discussion)
‐ Introduction/hypothesis (1 student)
‐ Method (1 student)
‐ Result/conclusion/future work (2 students)