5. Biogeochemical cycles

Question 1

What is biogeochemical cycling?

Answer 1

It is when organic matter is decomposed into small inorganic molecules, which are then immobilized again by growing cells. i.e. cycled.

Question 2

What is an important characteristic in biogeochemical cycling

Answer 2

The exchange of compounds between aerobic and anaerobic environments in each of these cycles

Question 3

What are the three main biochemical cycles?

Answer 3

• carbon
• nitrogen
• sulfur

Question 4

Are biogeochemical cycles altered by human activity?

Answer 4

Yes

Question 5

Carbon cycle (CO2) (4)

Answer 5

• CO2 may be fixed into organic matter either under aerobic conditions by oxygenic photosynthetic organisms (i.e., cyanobacteria, algae, plants) and some chemolithoautotrophic bacteria (i.e., nitrifying, sulfur-oxidizing bacteria) or under anaerobic conditions by anoxygenic photosynthetic bacteria (i.e., Rhodospirillum, Chlorobium)
• Organic matter is oxidized back to CO2 via aerobic respiration, anaerobic respiration and fermentation
• In soil or sediment, carbon may be in the organic matter which may be active (living biomass) or inactive (dead biomass)
• The mineral reservoir of carbon is atmospheric or dissolved CO2 and calcareous rocks and corals (calcium carbonate)

Question 6

Carbon cycle (CH4) (2)

Answer 6

• Some of the organic matter as well as CO2 may be converted to CH4 by diverse groups of Archaea called methanogens
• CH4 is then oxidized to CO2 by a specialized group of bacteria, the methanotrophs
  
  • Examples: Methylosinus, Methylococcus

Question 7

What enzyme do methanotrophs use to oxidize CH4 to methanol?

Answer 7

Methane monooxygenase (MMO)

Question 8

Sulfur cycle (3)

Answer 8

• The many oxidation states of S in various compounds means that reduced S compounds are good energy sources while oxidized forms of S can be used as electron acceptors for anaerobic respiration
• Sulfur is also an important element in the cell i.e., S-containing amino acids such as methionine & cysteine
• Bacteria can degrade pollutants via anaerobic respiration if sulfate is abundant for use as terminal electron acceptor –> dissimilatory sulfate reduction

Question 9

What can do dissimilatory sulfate reduction?

Answer 9

Sulfate reducing bacteria (SRBs), e.g. Desulfovibrio

Question 10

In dissimilatory sulfate reduction, what is the energy source, and what is the TEA?

Answer 10

• H2 and/or organic carbon as energy source
• Sulfate as terminal electron acceptor

Question 11

Nitrogen cycle (5)

Answer 11

• N is an important element in cells (i.e., for proteins, nucleic acids)
• Growth of organisms (plants or pollutant-degrading bacteria) is often limited by nitrogen availability
• Bacteria can degrade pollutants via anaerobic respiration if nitrate is abundant for use as terminal electron acceptor
• dissimilatory nitrate reduction
• In bioremediation, addition of N-fertilizer (i.e., ammonium nitrate) often stimulates mineralization of organic matter by decreasing the C/N ratio
• Ammonia oxidation and denitrification in waste water treatment