Metabolism V (Anaerobic respiration) Flashcards
Name organisms that respire using terminal electron acceptors other than molecular oxygen.
Examples include Hyphomicrobium sulfonivorans, which is found in garden soil and is significant in nitrate respiration for wastewater treatment. Other examples of organisms include Acidithiobacillus ferrooxidans, involved in sulfur oxidation and iron reduction, and Geobacter spp., capable of reducing metals like uranium and participating in anaerobic respiration. These organisms play important roles in industrial processes such as landfill management, sewage treatment, and biogas production, as well as in biogeochemical cycles such as denitrification.
List some examples of terminal electron acceptors
there are a lot! MANY more than this!
* molecular oxygen (O2) reduction to water (H2O)
* O(O) to O(-II) needs 2 ε-, but 4 ε- per O2 molecule.
* nitrate (NO3-) reduction to molecular nitrogen (N2)
* N(V) to N(O) needs 5 ε-.
* sulfate (SO42-) reduction to hydrogen sulfide (H2S)
* S(VI) to (S(-II) needs 8 ε-.
* cyclo-octasulfur (S8) reduction to hydrogen sulfide (8H2S)
* S(O) to S(-II) needs 2 ε-, but 16 ε- per S8 molecule.
* ferric iron (Fe3+) reduction to ferrous iron (Fe2+)
* Fe(III) to Fe(II) needs 1 ε-.
* uranyl uranium (UO22+) reduction to uranous uranium (U4+)
* U(VI) to U(IV) needs 2 ε-.
* perchlorate (ClO4-) reduction to chloride (Cl-)
* Cl(VII) to Cl(-I) needs 8 ε-.
* chlorate (ClO3-) reduction to chloride (Cl-)
* Cl(V) to Cl(-I) needs 6 ε-.
* manganic manganese (Mn4+) reduction to manganous manganese (Mn2+)
* Mn(IV) to Mn(II) needs 2 ε-.
* carbon dioxide (CO2) reduction to methane (CH4)
* C(IV) to C(-IV) needs 8 ε-.
* pertechnetate (TcO4-) to technetium (IV) (Tc4+)
* Tc(VII) to Tc(IV) needs 3 ε-.
* selenate (SeO42-) reduction to selenite (SeO32-)
* Se(VI) to Se(IV) needs 2 ε-.
* arsenate (AsO43-) reduction to o-arsenite (AsO33-)
* As(V) to As(III) needs 2 ε-
metals aren’t as good terminal electron acceptors
native sulfur and all commercial sulfur is >90 % cyclo-octasulfur.
native uranyl sulfate (adolfpateraite)
Rio Tinto, Spain –waters are high in ferric iron.
polymetallic nodules from the Abyssal Plain are mostly
manganic oxide
Explain the characteristics of metal cation reduction in anaerobic respiration
Metal cations involved in anaerobic respiration typically accept only a few electrons per mole. For example:
Ferric iron (Fe3+) reduction to ferrous iron (Fe2+) requires 1 electron.
Uranyl uranium (UO2+) reduction to uranous uranium (U4+) requires 2 electrons.
Manganic manganese (Mn4+) reduction to manganous manganese (Mn2+) also requires 2 electrons.
These reductions demonstrate that metal cations are weak electron acceptors. Additionally, other metal cations such as Co(III) and various actinides have been observed as accept
ors in anaerobic respiration. Compared to oxygen, which requires 16 electrons for reduction to water, the reductions of metal cations involve significantly fewer electrons. Consequently, the Gibbs free energy change (ΔG) for anaerobic respiration reactions involving metal cations is generally either positive or weakly negative, indicating limited energy availability for microbial growth.
Describe examples of reductions for comparison in anaerobic respiration.
Acetate at the expense of molecular oxygen:
CH3COO- + H+ + 2O2 → 2CO2 + 2H2O
ΔG° = -893.7 kJ/mol acetate oxidized
ΔG° = -446.9 kJ/mol molecular oxygen reduced (requiring -111.8 kJ/mol per electron)
Acetate at the expense of nitrate:
CH3COO- + 5H+ + 2NO3- → 2CO2 + N2 + 4H2O
ΔG° = -1,145.5 kJ/mol acetate oxidized
ΔG° = -572.7 kJ/mol nitrate reduced (requiring -114.5 kJ/mol per electron)
Acetate at the expense of selenate:
CH3COO- + H+ + 4SeO42- → 2CO2 + 4SeO32- + 2H2O
ΔG° = -607.7 kJ/mol acetate oxidized
ΔG° = -151.9 kJ/mol selenate reduced (requiring -76.0 kJ/mol per electron)
Acetate at the expense of uranyl uranium:
CH3COO- + H+ + 2UO22+ → 2CO2 + 2U4+ + 2H2O
ΔG° = -50.1 kJ/mol acetate oxidized
ΔG° = -25.1 kJ/mol uranyl ions reduced (requiring -12.6 kJ/mol per electron)
Front side: Describe examples of metal reduction by specific organisms.
Acidithiobacillus ferrooxidans: These bacteria oxidize elemental sulfur (S8) while reducing ferric iron (Fe3+) at pH 2. The reaction is:
48Fe3+ + S8 + 32H2O → 48Fe2+ + 64H+ + 8SO4-
The Gibbs free energy change (ΔG) for Fe(III) reduction is -40.3 kJ/mol Fe(III) reduced.
Geobacter spp.: These bacteria reduce various metals, including U(VI) and Co(III). They are part of the Geobacteraceae family within the Desulfurimonadia class. Metal reduction by Geobacter spp. involves cell-surface cytochromes and extracellular reduction systems. However, their growth may be hindered by metal precipitates clogging their cell surfaces.
Describe nitrate reduction.
*Nitrate reduction is extremely widespread and is sometimes referred to as “denitrification,” though the latter term strictly denotes the industrial or biogeochemical process that nitrate respiration achieves.
*Workhorse example organisms include Paracoccus denitrificans and Hyphomicrobium spp.
*Nitrate reduction occurs stepwise, allowing intermediates to be used by some organisms:
1. Nitrate (NO3-) is reduced to nitrite (NO2-): N(V) → N(III)
2. Nitrite (NO2-) is further reduced to nitric oxide (NO): N(III) → N(II)
3. Nitric oxide (NO) is then reduced to nitrous oxide (N2O): N(II) → N(I)
4. Nitrous oxide (N2O) finally gets reduced to molecular nitrogen (N2): N(I) → N(O)
This process involves 4 enzymes and allied carriers, which replace part of the respiratory chain.
nitrate reduction
respiratory nitrate reductase, cytochrome c-linked (EC 1.9.6.1) (there are more than one enzyme)
NO3- + 2H+ + 2cyt c(red) → NO2- + H2O + 2cyt c(ox)
respiratory nitrite reductase, NO-forming (EC 1.7.2.1)
NO2- + 2H+ + cyt c(red) → NO + H2O + cyt c(ox)
respiratory nitric oxide reductase, cytochrome c-linked (EC 1.7.2.5)
2NO(gas) + 2H+ + 2cyt c(red) → N2O + H2O + 2cyt c(ox)
respiratory nitrous oxide reductase (EC 1.7.2.4)
N2O(gas) + 2H+ + 2cyt c(red) → N2 + H2O + 2cyt c(ox)
a lot just do partial denitrification (most common first and last steps)
Describe the importance of denitrification.
Denitrification is crucial in soil and aquatic biogeochemistry as it converts soluble nitrogen compounds into N2 gas.
It’s utilized in wastewater treatment plants (WWTP) to remove nitrogen from urine.
In sewage, organic-N (e.g., urea) is hydrolyzed to ammonia (NH3) by urease (EC 3.5.1.5).
Ammonia-oxidizing autotrophs oxidize NH3 to NO3- in two steps, converting it into hydroxylamine (NH2OH) and then to nitrate (NO3-).
Denitrifiers reduce nitrate (NO3-) to N2.
Historically, denitrification used CH3OH as a carbon source, favoring Hyphomicrobium spp.
Nowadays, glycerol is preferred as it’s non-toxic and non-flammable, leading to a mix of bacteria performing denitrification.
Define Anammox (membrane bound organelles)
Anammox is a specialized process limited to the Planctomycetota.
It involves the oxidation of NH3 (chemolithoautotrophic growth) using NO2- as the electron acceptor, termed “anammox” respiration.
Anammox bacteria are slow-growing but have been suggested for removing nitrogen from wastewater.
While unstable for conventional wastewater treatment plants (WWTP), they may work for tannery wastes.
Anammox consortia grow under anoxic conditions on NH3/NO2- in a sequencing batch reactor (SRB), mimicking WWTPs, but require a stable environment.
Anammox reduction of nitrite
nitrite reductase (NO-forming, EC 1.7.2.1)
NO2- + cyt c(red) + 2H+ → NO + H2O + cyt c(ox)
hydrazine synthase (EC 1.7.2.7)
NO + NH4+ + 3 cyt c(red) → N2H2 + H2O + 3 cyt c(ox)
hydrazine dehydrogenase (EC 1.7.2.8)
N2H4 (very explosive)+ 4 cyt c(ox) → N2 + 4 cyt c(red)
Cell colour is owing to the very high cyt c content.
Cells contain intracellular organelles inc.
anammoxosome with ladderane lipids that are highly
resistant to oxidation by hydrazine.
methanogenesis
- really just a very specialist example of anaerobic respiration that is only done by the Archaea.
- several types but H2 as electron donor and CO2
acting as both C-source and terminal electron acceptor would be chemolithoautotrophy.
CO2 + 4H2 → CH4 + 2H2O
ΔG = -130.8 kJ/mol C(IV) reduced [-16.4 kJ/mol electrons] - very strict anaerobes – just anoxia is not enough, but highly reducing conditions are also needed or they die.
- responsible for CH4 production from mouth and gut of the Animalia –cattle and termites are well-studied.
- can also be done using C1 compounds as the electron acceptor or acetate as the electron acceptor but more complex!
