Metabolism:
Dissimilatory (energy, catabolic) metabolism
Assimilatory (anabolic) metabolism
Energy budget of a bacterium grown on glucose:
(% Energy (ATP) expended
____________________________________________
Synthesis:
Polysaccharide
6.5
Protein
61.1
Lipid
0.1
Nucleic acids
13.5
Transport into cells
18.3
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Most energy is spent on growth:
growth rate is almost proportional to energy generation.
Additional energy requirement for assimilatory reduction.
1
Autotrophy: solely dependent on inorganic compounds
Heterotrophy: dependent on organic substrates
Distinction rarely absolute: ”autrotrophs” may require organic growth
factors (vitamins). Usually refers to carbon source for assimilatory
metabolism (autotrophs depend on C-1 compounds, that is CO2 or CH4).
Photoautotrophs use light as energy source
Chemoautotrophs depend on the oxidation of inorganic compounds as
energy source.
Metabolism depends on redox processes of the type
AH2 + B → BH2 + A
Coupled to ATP synthesis:
ADP + Pi + energy → ATP + H2O
Substrate level phosphorylation: 1 mol ATP generated per mole
substrate.
Electron transport phosphorylation
2
Fermentation:
Redox balance
Substrate phosphorylation
Dismutation of organic substrate (e.g. C6H12O6 → 2CH3·CHOH·COOH)
Anaerobic processes
Low energy yield
Respiration:
External electron acceptor
Electron transport system
Organic or inorganic susbstrates
Electron transport system
Methanogenesis:
CH3COOH → CH4 + CO2
CO2 + 4H2
→
(formally a fermentation process)
CH4 + 2H2O (formally a respiration process)
Phototrophy:
Cyclic phosphorylation (but dependent on org. C for assimilatory
metabolism).
Photosynthesis: uses energy + external electron donor to reduce
CO2 to organic matter.
3
Fermentation
Fermentation to lactate or ethanol:
Mixed acid fermentation:
Fermentation to butyrate and to acetate
4
Other types of fermentation:
Propionic acid fermentation
Stickland reaction (fermentation of amino acids)
Thiosulphate disproportionation:
S2O32- + H2O → SO42- + HS- + H+
Homoacetogens:
CO2 + 4H2 → CH3COOH + 2H2O
Respiration:
Aerobic respiration: O2 is the terminal electron acceptor.
Substrates: all kinds of organic matter, depending on presence
of hydrolytic enzymes, reduced inorganic molecules (e.g., H2,
HS-, NH4+, Fe2+, etc)
Anaerobic respiration: other electron acceptors (e.g., NO3-,
Fe3+, SO42-, etc).
Substrates: usually only low molecular weight compounds
(volatile fatty acids), H2)
5
Aerobic respiration – organic substrate:
(Hydrolysis) → Glycolysis → Citric acid cycle →Electron transport system
Chemoautotrophs:
Hydrogen oxidisers (knallgas bacteria): 2H2 + O2
→
H2O
Methane oxidisers: CH4 + 2O2→ CO2 + 2H2O (Methylobacter, etc)
Nitrifiers: 2NH4+ + 3O2 → 2NO2- + 4H+ + 2H2O (Nitrosomonas)
2NO2- + O2 → NO3(Nitrobacter, etc.)
Sulphide oxidisers: HS- + 2O2 → SO42- + H+ (Thiobacillus, Beggiatoa,
etc.)
(So and S2O32- occur as intermediate metabolites)
Fe and Mn oxidisers: 4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H2O
6
Anaerobic respiration
Possible electron acceptors:
NO3-, NO2-, (denitrification, principal end product: N2)
Fe3+, Mn4+ (end product: Fe2+, Mn2+)
SO42- , So, (sulphate reduction, endproduct: HS-1)
Amanox-reaction:
NO2- + NH4+ → N2 + 2H2O
Some organic electron acceptors (e.g., trimethyl amine oxide
that is reduced to trimethylamine)
7
Denitrification:
Sulphate reduction in Desulfovibrio
Substrates: e.g., H2,lactate, but not acetate
Acetate utilisation
requires a citric acid
cycle, e.g. in Desulfobacter
8
Methanogenesis
Acetoclastic-
CO2 + H2- methanogenesis
Phototrophy:
Oxygenic photosynthesis:
CO2 + 2H2O + light → (CH2O) + O2 + H2O (cyanobacteria,
chloroplasts)
Two photosystems, chlorophylls + accessory pigments
Anoxygenic photosynthesis:
CO2 + 2H2S + light → (CH2O) + 2S + H2O
One photosystem, bacteriochlorophylls, various electron
acceptors (HS-1, H2, Fe2+) Several unrelated groups of
bacteria.
Purple membrane (halophilic archaebacteria, perhaps
wider occurrence).
9
10
Diversity among phototrophs:
Oxygenic phototrophs:
Cyanobacteria: Chorophyll a (b), Phycobillins (phycocyanin, phycoerythrin, photosystem I+II
Anoxygenic phototrophs:
Single photosystem, various electrondonors.
Purple sulphur bacteria (HS-, So, bacteriochorophyll a or b)
Purple non-sulphur bacteria (H2, Fe2+, HS-, bacteriochlorophyll a or b)
Green sulphur bacteria (HS-, bacteriochlorophyll c or d, strict anaerobes)
Green non-sulphur bacterua (H2, HS-, multicellular filamentous forms,
bacteriochlorophyll c or d)
Heliobacterium (H2, bacteriochlorophyll g)
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Assimilative metabolism
A bacterial cell contains about 70% H2O
Dry weight composition (%):
C: 55
O: 20
N: 10
H: 8
P: 3
S: 1
Remaining each <1%
Protein: 55%, Nucleic acids 23%,
lipids, polysaccharides, monomers 22%
12
Principal mineral forms:
In oxic habitats In anoxic habitats
_________________________________
C
CO2
CO2, CH4
N
NO3 , NO2 ,N2
NH4+, N2
2S
SO4
So, HSP
PO43PO43__________________________________________________
Inorganic carbon assimilation:
Calvin cycle: most chemoautotrophs, purple bacteria, cyanobacteria
Inverse citric acid cycle: green sulphur bacteria, some Archaebacteria
Synthesis of acetyl-CoA: sulphate reducers, H2/CO2 acetogens,
methanogens
Via HCHO: methanotrophs
N2-fixation:
Depends on dinitrogenase and denitrogenase reductase. The latter
(Fe and Mo containing) enzyme is O2-sensitive. N2-fixation is
energetically costly (18-24 ATP per N2). Widespread among anaerobic
bacteria and aerobes (cyanobacteria, Azotobacter) with special
adaptations for creating anaerobic conditions.
13