Lecture 16

Question 1

Carbon reservoirs

Answer 1

• Includes atmosphere, land, oceans, sediments, rocks, and biomass
• Reservoir size and turnover time are important parameters in understanding the cycling of elements

Question 2

Carbon cycle

Answer 2

• CO2 in the atmosphere is the most rapidly transferred carbon reservoir
• CO2 is fixed by photosynthetic land plants and marine microbes
• CO2 is returned to the atmosphere by respiration as well as anthropogenic activities
• Microbial decomposition is the largest source of CO2 released to the atmosphere

Question 3

Methane hydrate

Answer 3

• Form when high levels of methane are under high pressure and low temperature
• Huge amounts of methane are trapped underground as methane hydrates
• Can absorb and release methane
• Fuel deep-sea vent ecosystems

Question 4

Light reaction

Answer 4

The electrons are used generate energy (ATP) and reducing power (NADPH), as well as a useful byproduct O2

Question 5

Dark reaction

Answer 5

ATP and NADPH are used to synthesize organic carbon from CO2

Question 6

CH2O

Answer 6

Refers generally to organic matter (OM) and not to a specific compound

Question 7

Aerobic respiration equation

Answer 7

CH2O + O2 –> CO2 + H2O

Question 8

Oxygenic photosynthesis

Answer 8

The dominant form of primary production, produces enough oxygen to oxidize all the organic matter produced by photosynthesis

Question 9

Anoxic habitats

Answer 9

Can vary in size and shape:

1) Some near the oxic world; intense aerobic heterotrophy in the top few mm of sediments stops oxygen from penetrating far, creating anoxic mud only mm away from oxic waters
2) Organic rich particles: anoxic microhabitats in otherwise oxic soils and waters
3) Bottom waters of lakes and oceanic oxygen minimum zones

Question 10

Sulfate reduction

Answer 10

• Crucial in marine environments, where there is lots of sulfate
• Microbial process important for organic matter mineralization in anoxic environment

Question 11

Carbon dioxide redution

Answer 11

• Crucial in freshwater environments, including wetlands and rice paddies, where sulfate is often in limited supply
• Microbial process important for organic matter mineralization in anoxic environment

Question 12

Anaerobic food chain

Answer 12

1) Hydrolysis of biopolymers
2) Primary fermentation of monomers
3) Secondary fermentation
4) Methanogenesis
- Acetate and H2 are central compounds in aerobic decomposition
- Methane and carbon dioxide are the ultimate end products

Question 13

Acetate and H2

Answer 13

• Central compounds in anaerobic food chain because:
  1) Both can be produced by fermentation pathways
  2) Also produced by another group of microbes in another step of the anaerobic food chain: the acetogenic bacteria (secondary fermenters)

Question 14

Interspecies hydrogen transfer

Answer 14

Connection between a hydrogen gas producer (acetogen) and a hydrogen gas consumer (sulfate or carbon dioxide reducer)

Question 15

Syntrophic

Answer 15

Mutually beneficial relationship where the organisms are physically very close together

Question 16

Nitrogen

Answer 16

• 4th most abundant element (after: C, O, and H) in organic matter
• Its supply often limits growth and biomass of terrestrial and aquatic organisms (strongly linked to C cycle)
• Redox from -3 to +5: Electron acceptor and electron donor for energy metabolism

Question 17

5 key microbial processes in the N cycle

Answer 17

• Nitrification
• Denitrification
• N2 Fixation
• Ammonification
• Anammox

Question 18

1) Nitrogen fixation (N2)

Answer 18

• N2 = most stable form of nitrogen and major reservoir for N on Earth
• Reduction of nitrogen gas to ammonia

Question 19

Diazotrophs

Answer 19

Nitrogen-fixing microbes:

• Phototrophs
• Chemotrophs
• Aerobic/anaerobic microbes
• Free-living/symbiotic microbes
• *No eukaryotic organism is known that can fix nitrogen

Question 20

2) Nitrification

Answer 20

An aerobic two step microbial process in which ammonia is oxidized to nitrate sequentially by two distinct groups of chemolithoautotrophic bacteria :

1) Ammonia oxidizing bacteria and archaea (AOB and AOA)
2) Nitrite oxidizing bacteria (NOB)

Question 21

Denitrification

Answer 21

• Oxidized N compounds are good electron acceptors
• NO3- reduction half reactions generate almost as much free energy as aerobic respiration
• Occurs where nitrate is present but O2 concentrations are reduced

Question 22

Annamox

Answer 22

• Anaerobic ammonia oxidation

- Ammonia can also be oxidized under anoxic conditions through the annamox process

Question 23

Human activity and imbalance

Answer 23

1) Development of industrial processses (Haber-Bosch) to reduce N2 to NH4+
2) Implementation of agriculture practices that boost crop yield
3) Burning fossil fuels

Question 24

Consequences of fertilizer use

Answer 24

• Around 90% of N fertilizer is NH4+ nitrifying bacteria convert it to highly mobile NO3- which leaches into rivers, lakes, and aquifers
• Results in N loss and eutrophication of coastal waters, creating huge hypoxic zones around the world