What are the conditions necessary for bacteria to fix nitrogen (10 points)? Describe the stages of infection and nodulation in rhizobium-legume symbiosis with respect to nitrogen fixation (15 points).

Answer 1

Conditions necessary for bacteria to fix nitrogen:

1. Adequate energy supply: Bacteria require a substantial amount of energy to fix nitrogen. This is typically obtained through photosynthesis in cyanobacteria or from organic compounds in non-photosynthetic bacteria.

2. Nitrogenase enzyme: Bacteria must possess the enzyme nitrogenase, which is responsible for the conversion of atmospheric nitrogen (N2) into ammonia (NH3). This enzyme complex is highly sensitive to oxygen, so bacteria must have mechanisms to protect it from exposure to oxygen.

3. Absence of oxygen: Nitrogenase is inactivated by oxygen, so bacteria that fix nitrogen must be able to avoid or reduce exposure to oxygen. This can be achieved through various adaptations such as anaerobic respiration or forming specialized structures called heterocysts (in cyanobacteria).

4. Availability of nitrogen source: Despite fixing nitrogen, bacteria require a source of combined nitrogen to meet their own nitrogen needs. This can be provided either by the host plant or by the surrounding environment.

5. Symbiotic association: Many bacteria that fix nitrogen form symbiotic associations with plants (most commonly legumes) or other organisms. This symbiotic relationship allows the bacteria to access the plant's carbon sources in exchange for fixed nitrogen.

6. Appropriate pH: Nitrogen-fixing bacteria generally prefer a pH range of 6.0-7.5 for optimal growth and nitrogen fixation.

7. Adequate temperature: Different nitrogen-fixing bacteria have different temperature preferences, but most perform optimally within a range of 25-35°C.

8. Appropriate moisture levels: Bacteria require suitable moisture levels not only for their growth and survival but also for maintaining proper oxygen levels within their cells.

9. Sufficient presence of essential nutrients: Nitrogen fixation requires the availability of certain essential nutrients, such as molybdenum, iron, and calcium, in sufficient amounts.

10. Physical proximity to the host: Bacteria need to come into contact with the host plant's root system in order to form the symbiotic association necessary for nitrogen fixation.

Stages of infection and nodulation in rhizobium-legume symbiosis with respect to nitrogen fixation:

1. Recognition and infection: Rhizobia (bacteria) colonize the soil surrounding the roots of legume plants. The plant releases flavonoid compounds that attract specific rhizobial strains. In response, rhizobia release signaling molecules called Nod factors, which are recognized by the plant's receptors. This recognition leads to the development of root hair curls or infection threads, which allow the rhizobia to enter the root cells.

2. Nodule formation: The infection threads guide the rhizobia deeper into the root tissue and eventually into the cortical cells. Within these cells, the rhizobia multiply and form structures called symbiosomes. The plant, in response to these symbiosomes, initiates cell division and differentiation, forming a nodule on the root's surface. The nodule provides an oxygen-limited environment that enables nitrogen fixation by protecting the nitrogenase enzyme from oxygen.

3. Nodulation regulation: Various plant hormones and genetic factors regulate the formation and development of nodules. These factors control the overall number, size, and distribution of nodules on the roots, ensuring an efficient allocation of resources and avoiding excessive nodulation.

4. Nitrogen fixation: Inside the nodules, rhizobia differentiate into bacteroids, which actively fix atmospheric nitrogen into ammonia using the nitrogenase enzyme. The ammonia is then converted into other nitrogen compounds such as amino acids and organic nitrogen, which the plant can utilize for its growth and development. In return, the plant provides the necessary carbon sources, such as carbohydrates, to the rhizobia.

5. Nodule senescence: Over time, some nodules can become less active or senesce due to various factors such as nutrient limitation, changes in plant hormonal balance, or environmental stresses. These senescent nodules cease nitrogen fixation, and the plant reabsorbs the stored nitrogen compounds before the nodule decomposes.

Overall, the rhizobium-legume symbiosis ensures the fixation of atmospheric nitrogen, providing both the plant and the bacteria with essential nutrients for their growth and survival.