Ocean VL 5

Question 1

What are the main processes in the nitrogen cycle?

Answer 1

1. Nitrogen Fixation: Conversion of atmospheric N₂ to NH₃ by nitrogen-fixing bacteria.
2. Nitrification: Conversion of NH₄⁺ to NO₂⁻ and then NO₃⁻ by bacteria.
3. Denitrification: Conversion of NO₃⁻ to N₂ gas, returning nitrogen to the atmosphere.
4. Ammonification: Decomposition of organic nitrogen compounds into NH₄⁺.

Question 2

What is nitrogen fixation, and why is it important?

Answer 2

• Nitrogen fixation is the process where N₂ (Dinitrogen) is converted into ammonia (NH₃), which can then be used by living organisms.
• It is essential because atmospheric nitrogen is unavailable to most organisms, and fixation provides biologically available nitrogen.
• Cyanobacteria Archaea and other bacteria perform this process using the enzyme nitrogenase, which is sensitive to oxygen.
• Some microorganisms have special cells for nitrogen fixation (heterocystes)
• N2-fixation process is energy demanding
• Cyanobacteria Mats show high fixation rates, but a restricted areal distribution
• unclonised marine sediments also fix nitrogen, but to a much lesser extant

Question 3

NItrogen fixation in marine symbioses

Answer 3

• N2- fixing cyanobacteria form symbioses with diverse protist
• 2 groups:
  1) DDA (Diatom. diaotroph associations)
  2) UCYN-A (unicellar cyanobacterium A.)
• both systems co-occur
• Planktonic N2- fixin symbioses are significant contributors to both the N and the C cycles

Question 4

What happens during ammonification?

Answer 4

• Ammonification is the decomposition of organic matter
• Intermediate products (DON): amino acids, short polypeptides, amines, nucleic acids, urea
• Final product: ammonia, can be toxic for fish!

Protein →(proteinases)→ peptides →(peptidases)→ amino acids

Amino acids →(deamination)→ organic acid + NH4+ (Ammonium)

Question 5

What is the process of nitrification, and what organisms are involved?

Answer 5

Nitrification is the conversion of ammonium (NH₄⁺)/ or ammonia (NH₃) to nitrite (NO₂⁻) and then to nitrate (NO₃⁻).

It involves two types of bacteria:
1. Ammonium-oxidizing bacteria (e.g., Nitrosomonas), which convert NH₄⁺ to NO₂⁻.
2. Nitrite-oxidizing bacteria (e.g., Nitrobacter), which convert NO₂⁻ to NO₃⁻.
3. nitrifying archaea show an even higher abundance sometimes

First step:
NH3 + O2 + 2e- + 2 H+ -> NH2OH + H2O Ammonium monooxygenase (AMO)

Second step:
NH2OH+H2O -> NO2- + 5H+ +4e- Hydroxylamine oxidoreductase (HAO)

Question 6

What is denitrification, and where does it occur?

Answer 6

• Denitrification is the reduction of nitrate (NO₃⁻) to nitrogen gas (N₂), which is released back into the atmosphere.
• It occurs in anoxic environments, such as marine sediments, and is performed by bacteria like Pseudomonas and Paracoccus.
• This process closes the nitrogen cycle by removing bioavailable nitrogen from ecosystems.

Question 7

What are some new discoveries in the nitrogen cycle?

Answer 7

1. ANAMMOX: Anaerobic ammonia oxidation where NH₄⁺ is converted to N₂ without oxygen, performed by specialized bacteria.
2. COMMAMOX: Complete nitrification by a single organism, where Nitrospira inopinata carries out both ammonium and nitrite oxidation.
3. Ammonia-oxidizing archaea: Archaea that oxidize ammonia in marine sediments, contributing to the nitrogen cycle.

Question 9

How does ocean deoxygenation affect biogeochemical cycles?

Answer 9

Reduced oxygen levels in oceans (ocean deoxygenation) limit processes like nitrification and denitrification, especially in oxygen minimum zones (OMZs).

It also shifts microbial processes, increasing the role of anaerobic processes like sulfate reduction and methanogenesis, which further impact nutrient cycling and greenhouse gas emissions.

Question 10

(autotrophic aerobic) nitrification-
1) Name four groups of organsims.
2) In what do organisms differ

Answer 10

1) ammonia-oxidizing bacteria,
2) nitrite-oxidizing bacteria,
3) ammonia-oxidizing archaea,
4) commamox bacteria

Organisms differ in their affinity for ammonia and their growth yield per mole of ammonia –> different ecological niches.

Question 11

Why is the Nitrogen cycle so important?

Answer 11

• N is an essential nutrient for all organisms
• Probably the best bio-signature for life on other planets (Capone et al. 2006)
• 4 x 1011 kg of N cycles through the ocean each year
• Human activities influence the N cycle even in the oceans
• Some products of microbial N metabolism are greenhouse gases