Life Cycles Moro Nitrogen Fixation

Question 1

What is nitrogen fixation?

Answer 1

Nitrogen fixation is the process of converting atmospheric nitrogen (N2) into a form usable by living organisms, typically into ammonia (NH3).

Question 2

Why is nitrogen essential for life?

Answer 2

Nitrogen is a constituent of amino acids and nucleotides, which are essential for the formation of proteins and nucleic acids, respectively.

Question 3

What organisms can fix atmospheric nitrogen?

Answer 3

Many cyanobacteria, known as diazotrophic cyanobacteria, can fix atmospheric nitrogen under aerobic or anaerobic conditions.

Question 4

What is the enzyme complex involved in nitrogen fixation?

Answer 4

The enzyme complex is called nitrogenase, which consists of two metalloprotein units: dinitrogen reductase (Fe protein) and dinitrogenase (MoFe protein).

Question 5

In which types of organisms is nitrogenase enzyme found?

Answer 5

Nitrogenase is found in many groups of prokaryotes, including Bacteria and Archaea, but not in any eukaryotic genomes.

Question 6

What conditions activate nitrogenase enzyme?

Answer 6

The enzyme is activated when the organism does not have easy access to other inorganic nitrogen sources due to the high energy requirement of the process.

in other words: because the costs of the nitrogenase are very high (16 ATP per 1mol NH3), it is only activated when ther is no other source of Nitrogen.

Question 7

How is nitrogenase enzyme classified?

Answer 7

Nitrogenase enzymes are classified based on the metals present in the active site, including iron, molybdenum, vanadium, or additional iron ions

Question 8

Why is nitrogenase highly sensitive to oxygen?

Answer 8

• Oxygen affects genes coding for nitrogenase components
• causing repression
• causes rapid and irreversible oxidative damage to the enzyme’s proteins

Question 9

How do cyanobacteria separate nitrogenase activity from oxygen?

Answer 9

Cyanobacteria use two strategies:

1. spatial separation of nitrogen fixation cells from oxygen-producing cells
2. temporal separation of nitrogen fixation from photosynthesis.

Question 10

What are heterocysts and their function in nitrogen fixation?

Answer 10

Heterocysts are specialized, non-photosynthetic cells in filamentous cyanobacteria that create a micro-anaerobic environment for nitrogenase to function.

Question 11

How do heterocysts differ from vegetative cells?

Answer 11

Heterocysts are

• larger
• have additional wall layers
• a less dense cytoplasm
• lack oxygen-evolving photosystem II

Question 12

What structures help heterocysts protect nitrogenase from oxygen?

Answer 12

• glycolipid laminated layer (gas barrier)
• thick homogeneous polysaccharide layer (damage protection)
• micro-plasmodesmata channels (transfer)

Question 13

How is nitrogen transferred between heterocysts and vegetative cells?

Answer 13

Through micro-plasmodesmata channels, allowing transfer of nitrogen and ammonia from heterocysts and sucrose and maltose from vegetative cells.

Question 14

What are cyanobacterial mats and their significance?

Answer 14

Cyanobacterial mats are dense communities formed by nitrogen-fixing cyanobacteria, which develop in low-nitrogen environments, significantly contributing to nitrogen fixation in marine sediments

Question 15

Describe the temporal separation mechanism in cyanobacteria.

Answer 15

Temporal separation involves photosynthesis during the day and nitrogen fixation at night, common in unicellular species like Gloeothece and non-heterocystous filamentous species like Oscillatoria.

Question 16

What are diazocytes and their role in Trichodesmium?

Answer 16

Diazocytes are nitrogen-fixing cells in Trichodesmium that handle nitrogen fixation during the day by reducing oxygenic photosynthesis during peak nitrogenase activity.

Question 17

What environmental conditions favor Trichodesmium blooms?

Answer 17

High water temperatures (>21°C), low nutrient concentration, high water transparency, and weak or absent winds.

Question 18

How long does it take for nitrogenase complex to form in Trichodesmium?

Answer 18

The formation of the nitrogenase complex takes around 27 hours, with stages of pro-diazocytes appearing between 8-27 hours and mature diazocytes forming after 27 hours.

Question 19

What are diazotrophic cyanobacteria and how do they fix nitrogen?

Answer 19

Diazotrophic cyanobacteria are a type of cyanobacteria that can fix atmospheric nitrogen under aerobic or anaerobic conditions. They use an enzyme complex called nitrogenase, which consists of dinitrogen reductase (Fe protein) and dinitrogenase (MoFe protein), to convert N2 into NH3.

Question 20

Why is nitrogenase activity highly regulated and what triggers its activation?

Answer 20

Nitrogenase activity requires a large energy input and is regulated to ensure efficiency. The enzyme is activated only when the organism lacks accessible inorganic nitrogen sources.

Question 21

Explain the metal dependency of nitrogenase enzymes and which type is most common.

Answer 21

Nitrogenase enzymes are classified based on the metals in their active site: iron, molybdenum, vanadium, or an additional iron ion. The molybdenum nitrogenase is the most common in nature.

Question 22

How do oxygen levels affect nitrogenase activity?

Answer 22

Nitrogenase is highly sensitive to oxygen, which can cause oxidative damage and repress the expression of genes encoding the enzyme components. Oxygen inhibits nitrogenase activity and can irreversibly damage its proteins.

Question 23

What strategies do photosynthetic cyanobacteria use to protect nitrogenase from oxygen?

Answer 23

Cyanobacteria use two strategies to separate nitrogenase activity from oxygen:

1. Spatial separation: Heterocysts, specialized non-photosynthetic cells, provide an anaerobic environment for nitrogenase.
2. Temporal separation: Nitrogen fixation occurs at night, separated from daytime photosynthesis.

Question 24

What are heterocysts and their role in nitrogen fixation?

Answer 24

Heterocysts are specialized cells in filamentous cyanobacteria that fix nitrogen in an anaerobic environment. They lack photosystem II to prevent oxygen evolution and have thick cell walls to limit gas permeability.

Question 25

How do heterocysts differ structurally from vegetative cells?

Answer 25

Heterocysts are larger, have thicker walls, less dense cytoplasm, and lack phycobilisomes and ribulose. They contain stacked thylakoids and have specialized structures for nutrient exchange with adjacent vegetative cells.

Question 26

Describe the nutrient exchange between heterocysts and vegetative cells.

Answer 26

Heterocysts transfer fixed nitrogen (ammonia) to vegetative cells and receive reduced compounds (sucrose) from them. This exchange occurs via micro-plasmodesmata at the neck region where cyanophycin granules accumulate.

Question 27

How do non-heterocystous cyanobacteria fix nitrogen?

Answer 27

Non-heterocystous cyanobacteria fix nitrogen by creating microaerobic environments with low oxygen levels. They isolate nitrogenase in regions with reduced oxygen partial pressure, such as in dense cyanobacterial mats or specific trichomes

Question 28

Explain the role of diazocytes in Trichodesmium.

Answer 28

Diazocytes are specialized cells in Trichodesmium that fix nitrogen during the day. They reduce oxygenic photosynthesis during nitrogenase activity peaks to protect the enzyme from oxygen damage.

Question 29

What is Trichodesmium and where is it commonly found?

Answer 29

Trichodesmium is a marine cyanobacterial genus found in tropical and subtropical oceans, known for forming colonies that contribute significantly to the nitrogen cycle.

Question 30

What are the two types of Trichodesmium colonies?

Answer 30

Trichodesmium forms two types of colonies: single filaments that aggregate into spherical colonies called puffs (T. thiebautii) and filaments oriented in parallel to form spindle-shaped colonies called tufts (T. erythraeum).

Question 31

How does Trichodesmium fix nitrogen during the day?

Answer 31

Trichodesmium fixes nitrogen during the day by developing specialized cells called diazocytes, which reduce oxygenic photosynthesis during peak nitrogenase activity, allowing nitrogen fixation to occur despite the presence of light

Question 32

What are diazocytes and how do they function in Trichodesmium?

Answer 32

Diazocytes are specialized nitrogen-fixing cells within Trichodesmium trichomes. They are characterized by reduced gas vacuoles, more condensed thylakoids, and fewer storage compounds, allowing them to fix nitrogen during the day by reducing oxygen production.

Question 33

What environmental conditions favor the blooming of Trichodesmium?

Answer 33

Trichodesmium blooms are favored by high water temperatures (>21°C), low nutrient concentrations, high water transparency, and weak or absent winds.