Chapter 21 - Nutrient cycles

What determines the purpose for which microbes use certain elements

• The chemical state

• Environmental conditions

Major carbon reservoirs on earth

• Rocks and sediments

• Oceans

• methane hydrates

• Fossil fuels

Atmosphere composition

• N₂ = 78%

• O₂ = 21%

• CO₂ = 0.039%

• Other gasses

How much ppm of CO_2, is in the air.

400 ppm.

Carbon cycle

• CO₂ goes in the air from fossil fuels, animals and microbes.

• Is taken up by plants

• Is taken up by the water for aquatic plants.

Relation between organic matter and CO2 in carbon cycle

KNOW THIS

Methanogens vs. Methanotrophs

• Methanogens produce CH₄ by reducing CO₂

• Methanotrophs use CH₄ as an electron donor and oxidize it to form CO₂

Methanogenesis

• All are archaea

• Strictly anaerobic

• Are diverse in morphology and circumstances under which they occur.

• Can use H₂ (donor) as an energy source and then breathe with CO₂ (acceptor) creating CH₄

The three methanogenic pathways and their substrates

1. CO₂ reduction pathway, electrons are typically derived from H₂

2. Acetolactic pathway: Acetate or pyruvate is used

3. Very few: electron donor is methylated single carbon molecules.

Methane vs. CO₂

Methane contributes twice as much to pollution compared to CO₂

Largest methane sources

• Ruminants

• Termites

• Paddy fields (freshwater sediments)

• Natural wetlands (freshwater sediments)

• Landfills

• Tundra (freshwater sediments)

Anaerobic breakdown of organic matter.

1. Start with complex polymers

2. These are broken down into monomers by exo-enzymes

   • The exoenzymes are produced by fungi and bacteria

3. Monomers are fermented (fermentation because no oxygen and no other electron acceptors) into:

   • H₂, CO₂

   • Acetate

   • Propionate, butyrate, succinate, alcohols

4. The alcohols are broken down further by secondary fermentation. Which is syntropy. Producing H₂, CO₂ and Acetate

5. Acetogens turn the H₂ and CO₂, into acetate.

6. The last step is that methanogens convert everything into CH₄ and CO₂ by methanogenesis.

Syntrophy

Process in which at least two different microorganisms must work together to break down/use a compound: they cannot do it on their own.

• For example one reaction has a positive delta G, and won’t happen by itself

• Therefore it is combined with a reaction that has a negative delta G.

• So that when they are combined the overall change in Gibbs energy is still negative.

Where can you find methanogens?

• Termites

• Ruminants

• Rice fields

• Puddles and marshes

Therefore these are large contributers to all the methane in the air.

What happens to methanogenesis in salt-water sediments/marine water?

Contains sulfate too that can accept electrons.

• Therefore there is less methane produced because sulfate reduction is much more energy efficient.

• There is a competition between methanogens and sulfate reducers.

• The sulfite that is produced can also keep reacting with iron to make insoluble iron for example.

Methane hydrates

Methane ice

Methanotrophs

• organisms which are able to obtain energy by oxidizing methane

• Are dependent on methanogens

• Require different environmental conditions compared to methanogens

Ammonification

Organic N → NH₄⁺

Nitrogen cycle

The sulfur cycle