Cyanobacteria are a group of photosynthetic bacteria that can perform nitrogen fixation, which is the conversion of atmospheric nitrogen (N2) into a form that can be used by other organisms. Remembering examples of cyanobacteria capable of nitrogen fixation can be made easier by following a trick:

Trick: Remember the acronym ANABAENA

ANABAENA is an acronym that stands for different cyanobacterial genera capable of nitrogen fixation. Each letter represents a specific genera. Let's go through each of them:

1. Anabaena:
- Anabaena is a filamentous cyanobacterium that forms long chains of cells.
- It is commonly found in freshwater environments such as lakes, ponds, and rivers.
- Anabaena has specialized cells called heterocysts that are responsible for nitrogen fixation.
- These heterocysts have thick cell walls and lack photosynthetic pigments, allowing them to create an anaerobic environment for nitrogenase, the enzyme responsible for nitrogen fixation.

2. Nostoc:
- Nostoc is another filamentous cyanobacterium that forms colonies or gelatinous masses.
- It is commonly found in soil, moist rocks, and freshwater habitats.
- Similar to Anabaena, Nostoc also possesses heterocysts for nitrogen fixation.
- The colonies of Nostoc can vary in color, ranging from green to brownish-black.

3. Aulosira:
- Aulosira is a filamentous cyanobacterium that forms unbranched chains of cells.
- It is commonly found in freshwater habitats such as rivers, ponds, and ditches.
- Aulosira also possesses heterocysts for nitrogen fixation.
- The cells of Aulosira are often surrounded by a sheath, which helps protect them from environmental stresses.

4. Calothrix:
- Calothrix is a filamentous cyanobacterium that forms either unbranched or branched chains of cells.
- It is commonly found in soil, both terrestrial and aquatic environments.
- Calothrix possesses heterocysts for nitrogen fixation.
- The sheath surrounding the cells of Calothrix is often thick and mucilaginous.

5. Anabaenopsis:
- Anabaenopsis is a filamentous cyanobacterium that forms unbranched chains of cells.
- It is commonly found in freshwater habitats such as rivers, lakes, and ponds.
- Anabaenopsis also possesses heterocysts for nitrogen fixation.
- The cells of Anabaenopsis can be either solitary or in clusters.

Remembering the acronym ANABAENA can help you recall the different genera of cyanobacteria capable of nitrogen fixation. By associating each letter with its corresponding cyanobacterial genus and their key characteristics, you can easily remember examples of cyanobacteria capable of nitrogen fixation.