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Lecture #18:
Something from Almost Nothing:
Chemoautotrophy and
Chemolithotrophy
Background Reading for Lecture #18
Physiology & Biochemistry of Prokaryotes 3rd Ed.:
Ch. 12, pp. 345-356; Ch. 13, pp. 367-374
Brock Biology of Microorganisms 11th Ed:
Ch.17, pp. 548-555, 583-585, 560-562, 564-568, 575-577
Brock Biology of Microorganisms 12th ed.:
Ch. 20, pp. 591-602, 622-623, 627-635
Article(s) to be discussed  Berg review (Autotrophic
carbon fixation)
Topics:
 Chemoautotrophy
 Hydrogen Bacteria
 Sulfur-oxidizing Bacteria
 Sulfur-reducing Bacteria
 Iron-oxidizing Bacteria
 Methanogenesis
 Methanogenesis and Syntrophy
Chemolithotrophy Supports Chemoautotrophy
& NAD(P)H
Chemolithotrophy  ability of microbes to obtain energy
from the oxidation of inorganic compounds
Chemoautotrophy  ability of microbes to use CO2 as a
sole source of carbon
Major Pathways of CO2 Fixation: CalvinBenson-Bassham (CBB) Pathway
 CBB is most common method
of CO2 fixation on Earth
 CBB is only used by Bacteria
and Eukaryota
 The CBB pathway requires 3
ATP for the fixation of 1 CO2 (net
result of CBB is formation of 1
triose-P from 3 CO2 at the
expense of 9 ATP & 6 NAD(P)H)
 RuBisCO (ribulose-1,5bisphosphate
carboxylase/oxygenase is the
key enzyme of the pathway and
catalyzes the carboxylation or
oxygenation of RuBP
Major Pathways of CO2 Fixation:
Reductive Tricarboxylic Acid (rTCA) Cycle
Chlorobium
chlorochromatii
Frd
Light driven
rxns provides
reduced Fd
Acl
Pyruvate
synthase
Oor
 rTCA cycle was originally discovered in green sulfur phototrophs and has
since been identified in a variety of chemoautotrophs
 rTCA cycle specific enzymes are 2-oxoglutarate:ferredoxin oxidoreductase
(Oor), fumarate reductase (Frd), and ATP citrate lyase (Acl)
 rTCA cycle pathway tends to be in organisms that live in low O2
Major Pathways of CO2 Fixation: 3hydroxypropionate (3-HP) Cycle
 Originally identified from the
anaerobic phototroph Chloroflexus
Chloroflexus
 Key pathway enzymes are
acetyl-coA/propionyl-coA
carboxylase, malonyl-coA
reductase, propionyl-coA synthase
 An additional enzyme 4hydroxybutyryl-coA dehydratase
was identified in some Archaea
 Requires 7 ATP and 6 NAD(P)H
Most microbes that use the 3-HP
cycle can also grow
heterotrophically
Major Pathways of CO2 Fixation: Reductive
Acetyl-coA (Wood-Ljungdahl) Pathway
 The only non-cyclic CO2 fixation
pathway
Clostridium
ljungdahlii
 Considered the first evolved CO2
fixation pathway
 This pathway is only found in
anaerobic chemoautotrophs
 In this pathway one CO2 is
captured on a dedicated cofactor
and is reduced to a methyl group
pyruvate
CO2, Fdred
 The 2nd CO2 is reduced to a
carbonyl group by the carbon
monoxide dehydrogenase/acetylcoA synthase
 Requires 1 ATP and 3 Fd to
make 1 pyruvate
*
*
*
*
Chemolithotrophic Options to Support
Chemoautotrophy
 Hydrogen oxidation
 Sulfur oxidation
 Metal oxidation (Fe, Mn, As, Cu)
 Ammonia oxidation
Hydrogen Bacteria: Oxidation of H2 to
Generate Energy
 Capable of growing with H2 as the
only e- donor and O2 as the eacceptor
 H2 + ½ O2  H2O G0 = -237 kJ
 Many hydrogen bacteria can grow
autotrophically
Ralstonia eutropha, Gram
negative bacteria
 All hydrogen bacteria contain one
or more hydrogenases to generate
ATP or produce reducing power
 Most hydrogen bacteria are
considered to be facultative
chemolithotrophs
Aquifex Sp. marine
hyperthermophile
Bioenergetics of Hydrogenases in HydrogenOxidizing Bacteria
H+
NADH
+ 2H+
Sulfur-Oxidizing Bacteria
H2S + 2 O2  SO42- + 2 H+
G0 = -798.2 kJ
HS- + ½ O2 + H+  S0 + H2O
-209.4 kJ
S0 + H2O 1 ½ O2  SO42- + 2 H+
-587.1 kJ
S2O32- + H2O + 2 O2  2 SO42- + 2 H+ -818.3 kJ
Deposition of internal sulfur granules by
Beggiatoa
Sulfur spring colonized with Sulfolobus
Sp.
Attachment of the sulfur-oxidizing
archaeon Sulfolobus acidocaldarius to a
crystal of elemental sulfur
Oxidation of Reduced Sulfur Species by
Sulfur Chemolithotrophs
Generation of PMF in Sulfur-Oxidizing Bacteria
Iron-Oxidizing Bacteria
+ 0.77 V
 Iron oxidation (Fe2+  Fe3+)
releases only a small amount of
energy
½ O2  H2O (+ 0.82)
 At neutral pH under oxic
conditions, Fe will be
abiologically oxidized Fe(OH)3
 Most Fe-oxidizing bacteria live
in acidic environments
Empty iron-encrusted sheaths of
Sphaerotilus collected from seepage at
the edge of a small swamp
 Some Fe-oxidizing bacteria
such as Thiobacillus ferrooxidans
are autotrophic
Thiobacillus ferrooxidans
Electron Flow in the Fe-Oxidizing
Bacterium Thiobacillus ferrooxidans
Methanogens
 All methanogens belong to the Domain Archaea
 Methanogens are obligate anaerobes
 Most methanogens are mesophilic (there are some
thermophilic/hyperthermophilic & psychrophilic methanogens)
 Methanogens can convert a variety of substrates to methane grouped as (1)
CO2 type, (2) methyl substrates, (3) acetate in order to produce ATP
 Methanogens contain a variety of unusual coenzymes that are used for
methane production
Methanococcus
janaschii
Methanosarcina
barkeri
Methanobacterium Methanothermus
thermoautotrophicum fervidus
Coenzymes of Methanogenic Archaea
Coenzymes of Methanogenic Archaea Continued
Pathway of Methanogenesis from CO2
Energy Conservation in Methanogenesis
Methanophenazine (MPH)
Methanogenesis and Syntrophy
Ethanol
Acetate
Methane
+19 kJ/rxn
-137 kJ/rxn
-111.3 kJ/rxn
Syntrophy is also referred to as interspecies H2 transfer
Syntrophy Between Hydrogen-generating
Bacteria and Methanogenic Archaea
+
Hyperthermophilic bacterium
Thermotoga maritima
Hyperthermophilic
archaeon Methanococcus
jannaschii
Ferments sugars producing
CO2 and H2, (growth becomes
inhibited by high [H2])
Produces methane from
CO2 and H2