Metabolic Diversity and Fermentation

Metabolic Diversity

• Chemotrophs get energy from chemical compounds.
• Chemoorganotrophy involves using organic compounds as electron donors.
• Chemolithotrophy involves using inorganic compounds as electron donors.

Fermentation

• An energy source (like glucose) acts as the reductant, and a terminal electron acceptor (like pyruvate) acts as the oxidant.
• Electron transport chain (ETC) are not involved.
• NADH is oxidized to maintain redox balance.
• ATP is produced mostly through substrate-level phosphorylation.
• End products can include ethanol, lactic acid, acetic acid, and H2.
• Primary source of ATP for some prokaryotes.
• ATP source when respiration is not possible.

Homofermentative Fermentation

• Lactobacillus spp. carry it out.
• Glucose converts to 2 Lactate.

Heterofermentative Fermentation

• Lactobacillus spp. carry it out.
• Glucose converts to Lactate + ethanol + CO2 + H+

Mixed-Acid and Butanediol Fermentations

• Mixed-acid fermentation is done by E. coli.
• Butanediol fermentation is done by E. aerogenes.
• Pyruvate converts to 2,3-Butanediol.
• 2 Pyruvate + NADH converts to 2 CO2 + butanediol .

Secondary Fermentations

• Involves fermentation products from one organism being the substrate for another.
• Carried out by Propionibacterium.
• Gives Emmental cheeses their flavor.

Anaerobic Respiration

• Some prokaryotes utilize electron transport chains with inorganic final electron acceptors other than oxygen.
• Energy yields are lower compared to aerobic respiration.
• Major electron acceptors in order are Nitrate (NO3-), Ferric iron (Fe3-), Sulfate (SO42-), and CO2.

Nitrate Reduction in E. coli

• In the absence of O2, E. coli uses nitrate reductase instead of Cytochrome bo3 (Complex III/IV).
• Nitrate (NO3-) is reduced to nitrite (NO2-).
• Generates less PMF than using O2.

Denitrification in P. stutzeri

• In the absence of O2, Pseudomonads use nitrate reductase to reduce NO3- to NO2-.
• Then an extended ETC reduces NO2- to NO(g) to N2O(g) to N2(g).

Denitrification

• Acts as sewage treatment.
• Also leads to; loss of fertilizer to the atmosphere, N2O being a greenhouse gas and NO reacting with ozone and water to form nitric acid (acid rain).

Iron-Reducing Bacteria

• Geobacter uses acetate as an electron donor and ferric iron oxides as electron acceptors.
• Ferric iron (Fe3+) is reduced to ferrous iron (Fe2+).
• Several species can reduce other metals.
• Used for bioremediation of metal contamination and Uranium.

Sulfate Reduction in Desulfovibrio

• Desulfovibrio uses organic compounds as electron donors and sulfates as electron acceptors.
• Sulfate (SO42-) is reduced to sulfite (SO32-) and then to sulfide (H2S).
• If the reduced sulfur forms organic sulfur compounds, it is assimilative sulfate reduction.
• If the reduced sulfur is excreted, it is dissimilative sulfate reduction.

Methanogenesis and Acetogenesis

• Methanogens are anaerobic archaea.
• CO2 is reduced to CH4 (methane) or CH3CO2 (acetate).

Chemolithotrophic Respiration

• Lithotrophy oxidizes reduced inorganic compounds for energy.
• Prokaryotes utilize electron transport chains with inorganic electron donors to generate PMF.
• Major electron donors: ammonia (NH4+), ferrous iron (Fe2+), sulfide (H2S), and H2.
• O2 is the most used final electron acceptor in aerobic lithotrophic respiration.
• Most are autotrophs, generating reducing power to fix CO2.

Hydrogen Oxidation in Ralstonia

• Dihydrogen is oxidized with water formation.
• O2 is the preferred electron acceptor.
• Can also use CO2 (methanogenesis).
• H2 generates NADH for the Calvin cycle.

Iron Oxidation in Acidithiobacillus ferrooxidans

• Ferrous iron is oxidized, forming ferric iron.
• O2 is the preferred electron acceptor, but nitrate can be used.
• Uses reverse electron flow to generate NADH for the Calvin cycle.
• Acidic aqueous environments are where iron oxidizers are prevalent.

Nitrification in Nitrosomonas

• Ammonia is oxidized to nitrite.
• O2 is the final electron acceptor.
• Uses reverse electron flow to generate NADH for the Calvin cycle.
• Important in the global nitrogen cycle.
• Nitrifiers are present in NH3-rich soils, wastewater, and manure.

Nitrification in Nitrobacter

• Nitrite is oxidized to nitrate.
• O2 is the final electron acceptor.
• Utilizes reverse electron flow to generate NADH for the Calvin cycle.
• Important in the global nitrogen cycle.
• Common in NH3-rich soils, wastewater, and manure.

Sulfur Oxidizers

• Elemental sulfur (S0) and sulfide (H2S) are oxidized with subsequent formation of sulphite and then sulphate.
• O2 is the preferred electron acceptor and nitrate can be used.
• Reverse electron flow generates NADH for the Calvin cycle.
• Common name is "colourless sulfur bacteria".
• Commonly found in hydrothermal vents and hot springs.

Phototrophy

• Refers to using light energy to drive electron flow.
• Electron transport chains generate PMF for photophosphorylation.
• Can be oxygenic by splitting water to supply electrons, subsequently generating O2; performed by cyanobacteria, algae, protists, and plants.
• Anoxygenic forms don't split water or form O2, only carried out by prokaryotes.
• Most phototrophs are autotrophs, generating reducing power to fix CO2.

Anoxygenic Purple Sulfur Bacteria

• The common species is Chromatium spp.
• They use a Q-type photosystem.
• Cyclic electron flow generates PMF.
• They oxidize H2S and use reverse electron transport to generate reducing power.
• S0 accumulates in granules inside (periplasm) or outside of the cells, and used when [H2S] is low.
• This species fixes Carbon through the Calvin Cycle.

Anoxygenic Green Sulfur Bacteria

• The common species is _Chlorobium spp..
• Characterized by Chlorosomes to transfer light energy.
• They use a FeS-type photosystem.
• They have been speculated to use Cyclic electron flow.
• They oxidizes to Ferredoxin
• _H2S oxidises to either S0 or S042-
• They fixes Carbon by reverse of the citric acid cycle

Oxygenic Cyanobacteria

• (Prochlorococcus)
• It used both Q-type and FeS-type reaction centre in the photosynthesis
• It uses linear electron flow, thus making possible to generate PMF and reduce NAD(P)+
• Election, released from water are replaced after H2) oxidises to H2O
• It can undergo C1 fixation, also known as Calvin Cycle.