Biological nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into a usable form, such as ammonia (NH3), by certain microorganisms. This process is crucial for the cycling of nitrogen in ecosystems and plays a significant role in agriculture by providing a sustainable source of nitrogen for plants. Among the given options, blue-green algae, also known as cyanobacteria, are associated with biological nitrogen fixation.

Blue-green algae and nitrogen fixation:
Blue-green algae are a group of photosynthetic bacteria that are capable of carrying out nitrogen fixation. They possess specialized structures called heterocysts, which are responsible for nitrogen fixation. Heterocysts are anaerobic and provide an oxygen-free environment for the nitrogenase enzyme complex to function effectively.

Process of nitrogen fixation:
1. Nitrogen fixation occurs in the presence of specific enzymes called nitrogenases, which are found in the heterocysts of blue-green algae.
2. These enzymes catalyze the conversion of atmospheric nitrogen (N2) into ammonia (NH3).
3. The process requires a large amount of energy, which is obtained through photosynthesis.
4. Blue-green algae are photosynthetic organisms, capable of converting sunlight into chemical energy, which is utilized in the process of nitrogen fixation.
5. The ammonia produced by nitrogen fixation can be assimilated by plants and utilized for their growth and development.

Importance of biological nitrogen fixation:
Biological nitrogen fixation is essential for maintaining the nitrogen balance in ecosystems. It provides a sustainable source of nitrogen for plants, reducing the dependence on synthetic fertilizers. This process also contributes to the fertility of soil, as the fixed nitrogen becomes available to other organisms in the ecosystem through decomposition of plant and animal material.

Conclusion:
Among the given options, blue-green algae are associated with biological nitrogen fixation. These photosynthetic bacteria possess specialized structures called heterocysts, which house the nitrogenase enzymes responsible for converting atmospheric nitrogen into ammonia. This process plays a crucial role in the nitrogen cycle and is important for the growth and development of plants.