Nitrogen fixation is a process by which atmospheric nitrogen (N2) is converted into ammonia (NH3) or other nitrogen compounds that can be utilized by plants. In rice fields, nitrogen fixation is important as it helps in maintaining soil fertility and improves crop productivity. Among the various nitrogen-fixing organisms, Aulosira fertilissima is the most active nitrogen fixer in rice fields.

Explanation:

Aulosira fertilissima is a filamentous cyanobacterium that can form symbiotic associations with rice plants. It forms heterocyst, a specialized cell that fixes atmospheric nitrogen. Following are the reasons why Aulosira fertilissima is the most active nitrogen fixer in rice fields:

1. Symbiotic association: Aulosira fertilissima forms a symbiotic association with rice plants, which means it can colonize the roots of rice plants and provide them with fixed nitrogen. This association is highly specific and efficient, and it helps in maintaining soil fertility.

2. Heterocyst formation: Aulosira fertilissima can form heterocysts, which are specialized cells that are responsible for nitrogen fixation. These cells have a thick cell wall that prevents oxygen from entering, as nitrogenase, the enzyme responsible for nitrogen fixation, is highly sensitive to oxygen.

3. Efficient nitrogen fixation: Aulosira fertilissima is known for its efficient nitrogen fixation ability. It can fix up to 70 kg of nitrogen per hectare per year, which is much higher than other nitrogen-fixing organisms.

4. Tolerance to adverse conditions: Aulosira fertilissima can tolerate adverse conditions such as low pH, high salinity, and waterlogging, which are common in rice fields. This makes it a suitable candidate for nitrogen fixation in rice fields.

In conclusion, Aulosira fertilissima is the most active nitrogen fixer in rice fields due to its symbiotic association with rice plants, ability to form heterocysts, efficient nitrogen fixation, and tolerance to adverse conditions.