Lecture 13: Methanogensis, Chemolithotrophy

Question 1

What is methanogenesis?

Answer 1

Methanogenesis is the production of methane in strict anaerobic conditions

Question 2

Describe the common electron donors in chemolithotrophy

Answer 2

Early Chemolithotrophs used:
-H2, and reduced nitrogen, sulfur, and iron compounds as electron donors (Not great donors, and do not generate as much energy as chemoorganotrophs)

Question 3

Explain reverse electron flow and how/why it is used in chemolithotrophy

Answer 3

• NADPH has high reducing power
• Donation of e- to NADP is unfavorable from their inorganic e- sources (ammonia, nitrite, H2S) as they have a more positive reduction potential
• Generation of NADPH relies on reverse electron flow

Energy in the form of the proton motive force must be diverted from performing other cellular work (e.g., ATP synthesis, transport, motility) to push the electrons from molecules of relatively positive reduction potentials to those that are more negative.

-Chemolithotrophs can afford this inefficiency, as they have no serious competitors for their unique energy sources.

Question 4

Defend why chemolithotrophy is a useful metabolic strategy for some organisms

Answer 4

Depending on the environment being a chemolithotroph could be a selective advantage to the conditions that are in the current environment.

Question 5

Piece together reactions in the nitrogen cycle, know important enzymes

Answer 5

1. Nitrogen Fixation: N2 (g) to NH3
   - This requires the nitrogenase enzyme
   - Only performed by bacteria, needs to be in anaerobic environment
   - NH3 -> Organic N (nucleic and amino acids)
   - reductive (released into environment)
2. Dissimulation and Mineralization: NH3 to NH4
3. Step 1: Nitrification:NH4+ to NO2-
   - Nitrosomonas
4. Step 2: Nitrification: NO2 to NO3
   - Nitrobacter

5.Fate of NO3- : Assimilation or denitrification

Assimilation: NO3 to Organic N

Denitrification: NO3 to N2
-When gases N2 and N2O are produced and released into the atmosphere.

Anammox:
NH4 to N2

Only performed by chemolithotrophs

Question 6

Explain how nitrogen metabolism is connected to chemolithotrophy

Answer 6

-All the little organisms that work together in the electron transfer chain make up nitrogen metabolism

Nitrogen cycling is an important function of chemolithotrophs and anaerobic respiration

Question 7

What are potential start products for methanogenesis?

Answer 7

Potential start products:

• CO2 + H2
• Methanol
• Acetate

Question 8

What organism performs methanogenesis

Answer 8

One of the earliest forms of metabolism

Performed by archaea

Question 9

What is the role of methanogenasis

Answer 9

Role of methanogenesis in Earth’s biogeochemistry

Important in carbon cycling (decay)
-Methane is a greenhouse gas

-Methane producers are coupled with organisms that can metabolize methane (methanotrophy)

Question 10

What do chemolithotrophs use as terminal electron acceptors?

Answer 10

-Sulfate (SO4-2) and nitrate (NO3-) as terminal electron acceptors

Question 11

What are the methanogenesis reactions?

Answer 11

ATP produced by sodium motive force

CO2 (electron acceptor) + 4H2(Electron donor) -> CH4(methane) + 2H2O

ATP produced at terminal step by PMF.

4CH3OH(electron acceptors) -> 3CH4 (product: methane) + CO2 + 2H2O

CH3COO- (electron acceptors) + H+(electron donor) -> CO2 + CH4(methane)