Lecture 3C: Anaerobic Respiration and Fermentation: Nitrate Reduction and Denitrification, Sulfate and Sulfur Reduction

Question 1

Nitrate Reduction and Denitrification

What is the common electron acceptor in nitrate reduction?

Answer 1

Nitrate (NO₃⁻)

Question 2

Nitrate Reduction and Denitrification

What is the first product in nitrate reduction?

Answer 2

Nitrite (NO₂⁻)

Question 3

Nitrate Reduction and Denitrification

What gaseous products are formed in denitrification?

Give three (3)

Answer 3

• nitric oxide (NO)
• nitrous oxide (N₂O)
• dinitrogen (N₂)

Question 4

Nitrate Reduction and Denitrification

What microorganism reduces nitrate (NO₃⁻) to nitrtite (NO₂⁻) but does not denitrify fully?

Answer 4

Escherichia coli

Explanation:
*Escherichia coli *reduces NO₃⁻ (nitrate) to NO₂⁻ (nitrite) through nitrate reduction, but it does not perform complete denitrification (which would convert nitrite to N₂ gas). This is because *E. coli *lacks the necessary enzymes (e.g., nitrite reductase, nitric oxide reductase, and nitrous oxide reductase) required for the full denitrification pathway. Instead, it primarily uses nitrate reduction as an anaerobic respiration process.

Question 5

Nitrate Reduction and Denitrification

What enzyme catalyzes nitrate (NO₃⁻) to nitrtite (NO₂⁻)?

Answer 5

Nitrate reductase

Explanation:
- It catalyzes the reduction of nitrate (NO₃⁻) to nitrite (NO₂⁻) by facilitating the gain of electrons (reduction) in the chemical reaction.
- nitrate reductase enables the conversion of nitrate to nitrite by adding electrons, often using electron donors like NADH or NADPH in the process.

Question 6

Nitrate Reduction and Denitrification

What enzyme catalyzes nitrite (NO₂⁻) to nitric oxide (NO)?

Answer 6

Nitrite reductase

Question 7

Nitrate Reduction and Denitrification

What enzyme catalyzes nitric oxide (NO) to nitrous oxide (N₂O)?

Answer 7

Nitric oxide reductase

Question 8

Nitrate Reduction and Denitrification

What enzyme catalyzes nitrous oxide (N₂O) to dinitrogen (N₂)?

Answer 8

Nitrous oxide reductase

Question 9

Nitrate Reduction and Denitrification

What type of bacteria are strong denitrifiers?

Give one (1); what species?

Answer 9

Pseudomonas species

Question 10

Nitrate Reduction and Denitrification

What phylum do many denitrifying bacteria belong to?

Answer 10

Proteobacteria

Question 11

Nitrate Reduction and Denitrification

What ecological impact does denitrification have on agriculture?

Answer 11

Removes nitrate fertilizers

Explanation: While denitrification helps prevent nitrate accumulation in water bodies (thus reducing water pollution and eutrophication), it can also lead to decreased soil fertility and reduced crop yields due to the loss of available nitrogen, which is essential for plant growth.

Question 12

Nitrate Reduction and Denitrification

What greenhouse gas is released during denitrification?

Additional question: How does it contribute to climate change?

Answer 12

Nitrous oxide (N₂O)

• N₂O is a potent greenhouse gas, with a global warming potential approximately 298 times greater than CO₂ over a 100-year period. This means it traps significantly more heat in the atmosphere than carbon dioxide.
• Nitrous oxide has a long atmospheric lifetime, lasting over 100 years. This allows it to persist in the atmosphere and contribute to warming over an extended period.
• N₂O also contributes to the depletion of the ozone layer when it breaks down in the stratosphere, releasing nitrogen oxides that can destroy ozone molecules. A healthy ozone layer is crucial for protecting the Earth from harmful ultraviolet radiation.

Question 13

Nitrate Reduction and Denitrification

What environmental issue is caused by NO reacting with ozone?

Answer 13

Acid rain

Question 14

Nitrate Reduction and Denitrification

How is denitrification beneficial in sewage treatment?

Answer 14

Removes excess nitrogen

Explanation:
Denitrification in sewage treatment is beneficial because it removes excess nitrogen by converting nitrate and nitrite into nitrogen gas (N₂). This process helps prevent eutrophication, improves water quality, and supports sustainable treatment practices by reducing nitrogen levels in wastewater before it is released into the environment.

Question 15

Nitrate Reduction and Denitrification

What element is present in nitrate reductase?

Answer 15

Molybdenum

Explanation:
Molybdenum acts as a cofactor in the enzyme’s active site, facilitating the reduction of nitrate (NO₃⁻) to nitrite (NO₂⁻) by participating in the electron transfer process during the reaction. This element is crucial for the enzymatic activity of nitrate reductase and plays a key role in nitrogen metabolism in various organisms.

Question 16

Nitrate Reduction and Denitrification

What represses the synthesis of nitrate reductase?

Answer 16

Oxygen (O₂)

• In the presence of oxygen, bacteria prefer to use aerobic respiration, which is more efficient for energy production.
• Nitrate reduction is often used under anaerobic or low-oxygen conditions.

Question 17

Nitrate Reduction and Denitrification

What bacterium performs full denitrification?

Give two (2)

Answer 17

• Paracoccus denitrificans
• Pseudomonas stutzeri

Question 18

Nitrate Reduction and Denitrification

1. What nitrogen compound has an oxidation state of +5?
2. What nitrogen compound has an oxidation state of -3?
3. What nitrogen compound has an oxidation state of 0?

Answer 18

1. Nitrate (NO₃⁻)
2. Ammonia (NH₃)
3. Nitrogen gas (N₂)

Explanation:
1. Nitrate (NO₃⁻, +5): Its high oxidation state means it can act as an electron acceptor in anaerobic respiration (denitrification).
2. Ammonia (NH₃, -3):The -3 oxidation state represents the most reduced form of nitrogen, making it readily usable by organisms.
3. Nitrogen Gas (N₂, 0): The oxidation state of 0 signifies that nitrogen atoms in N₂ are neither electron-rich nor electron-deficient. This, combined with the strong triple bond between the nitrogen atoms, makes N₂ relatively unreactive (inert). This inertness is why N₂ makes up about 78% of the Earth’s atmosphere.

Question 19

Nitrate Reduction and Denitrification

What is the significance of gaseous products in denitrification?

Answer 19

They escape into the environment

Question 20

Sulfur and Sulfate Reduction

What is the primary electron acceptor in sulfate reduction?

Answer 20

Sulfate (SO₄²⁻)

Question 21

Sulfur and Sulfate Reduction

What is the main product of sulfate reduction?

Answer 21

Hydrogen sulfide (H₂S)

Question 22

Sulfur and Sulfate Reduction

Which group of bacteria is known for sulfate reduction?

Answer 22

Sulfate-reducing bacteria (SRB)

Question 23

Sulfur and Sulfate Reduction

1. What sulfur compound has an oxidation state of +6?
2. What sulfur compound has an oxidation state of -2?

Answer 23

1. Sulfate (SO₄²⁻)
2. Hydrogen sulfide (H₂S)

Explanation:
Sulfate (SO₄²⁻, +6):
* The +6 oxidation state represents the most oxidized form of sulfur.
* Sulfur in sulfate has a +6 oxidation state, meaning it has lost electrons compared to its neutral state.
* This makes sulfate an oxidizing agent, meaning it tends to accept electrons from other substances.

Hydrogen Sulfide (H₂S, -2):
* The -2 oxidation state represents the most reduced form of sulfur.
* Sulfur in hydrogen sulfide has a -2 oxidation state, meaning it has gained electrons compared to its neutral state.
* This makes hydrogen sulfide a reducing agent, meaning it tends to donate electrons to other substances.

Question 24

Sulfur and Sulfate Reduction

Which electron donor is used by all sulfate-reducing bacteria?

Answer 24

Hydrogen (H₂)

Question 25

# Sulfur and Sulfate Reduction What are common organic electron donors for freshwater sulfate-reducing bacteria? (2)

Answer 25

- Lactate - Pyruvate ## Footnote Lactate and pyruvate are common organic electron donors used by freshwater sulfate-reducing bacteria to drive the reduction of sulfate into hydrogen sulfide in oxygen-free conditions.

Question 26

# Sulfur and Sulfate Reduction What are marine sulfate-reducing bacteria's common electron donors? (2)

Answer 26

- Acetate - Long-Chain Fatty Acids ## Footnote In marine environments, sulfate-reducing bacteria (SRB) are abundant in anoxic sediments where they play a key role in breaking down organic matter. * Acetate * Long-chain fatty acids (e.g., palmitate, oleate) These are: * More prevalent in marine sediments due to the degradation of complex organic material. * Stable and energy-rich substrates suitable for long-term microbial respiration. * Used by SRB to transfer electrons to sulfate, which is then reduced to hydrogen sulfide (H₂S).

Question 27

# Sulfur and Sulfate Reduction What enzyme catalyzes the reaction of sulfate with ATP?

Answer 27

ATP sulfurylase

Question 28

# Sulfur and Sulfate Reduction What is the product of sulfate activation?

Answer 28

Adenosine phosphosulfate (APS)

Question 29

# Sulfur and Sulfate Reduction What is the process called where one substance is oxidized while another is reduced?

Answer 29

Disproportionation

Question 30

# Sulfur and Sulfate Reduction What bacterium disproportionates thiosulfate to produce H₂S and sulfate?

Answer 30

*Desulfovibrio sulfodismutans*

Question 31

# Sulfur and Sulfate Reduction What is the process where phosphite oxidation is coupled with sulfate reduction?

Answer 31

Phosphite oxidation

Question 32

# Sulfur and Sulfate Reduction Which bacterium can oxidize acetate while reducing sulfur to H₂S?

Answer 32

*Desulfuromonas acetoxidans*

Question 33

# Sulfur and Sulfate Reduction What is the role of cytochrome c₃ in sulfate reduction?

Answer 33

Electron transfer

Question 34

# Sulfur and Sulfate Reduction What does APS reductase reduce APS to?

Answer 34

Sulfite (SO₃²⁻)

Question 35

# Sulfur and Sulfate Reduction What is formed when sulfite is reduced?

Answer 35

Hydrogen sulfide (H₂S)

Question 36

# Sulfur and Sulfate Reduction 1. What process has the Gibbs free energy change (∆G0′) of -152 kJ? 2. What process has the Gibbs free energy change (∆G0′) of -21.9 kJ 3. What process has the Gibbs free energy change (∆G0′) of -364 kJ

Answer 36

1. sulfate reduction 2. thiosulfate disproportionation 3. phosphite oxidation coupled with sulfate reduction ## Footnote The different ∆G0′ values imply that phosphite oxidation with sulfate reduction releases the most energy, making it highly favorable. Sulfate reduction alone is moderately favorable, while thiosulfate disproportionation yields much less energy, indicating it's less efficient for energy conservation.

Question 37

# Sulfur and Sulfate Reduction What is the ecological significance of sulfate-reducing bacteria?

Answer 37

Nutrient cycling

Question 38

# Sulfur and Sulfate Reduction What type of environments do sulfate-reducing bacteria thrive in?

Answer 38

Anoxic environments

Question 39

# Sulfur and Sulfate Reduction What happens to the energy generated during dissimilative sulfate reduction?

Answer 39

Drives ATP synthesis ## Footnote The energy generated during dissimilative sulfate reduction is used to drive ATP synthesis through chemiosmotic mechanisms, supporting the cell’s metabolic activities and growth under anaerobic conditions.

Question 40

# Sulfur and Sulfate Reduction What is the oxidation state of elemental sulfur (S⁰)?

Answer 40

0 | because it is in its pure, uncombined form

Question 41

# Sulfur and Sulfate Reduction What is thiosulfate's average oxidation state?

Answer 41

-2/+6 ## Footnote Thiosulfate (S2O32−) contains two sulfur atoms with different oxidation states: * One sulfur is in the -2 oxidation state (like in sulfide), * The other is in the +6 oxidation state (like in sulfate).