Lichens which are a composite organisms arose due to a mutual symbiotic relationship between the green algae /cynobacteria and fungi . While the algal or cyanobacterial cells helps in the photosynthesis by reducing the atmospheric carbon dioxide into organic carbon sugars , the fungi also helps in the absorption of this sugars . More than 98% of this highly diverse group of lichen forming fungi is concentrated within the Ascomycota , the largest fungal phylum .

Majority of lichens are made of:
- Green algae and ascomycetes
Explanation:
Lichens are unique organisms that result from a mutualistic symbiotic relationship between a fungus and a photosynthetic partner, which is usually a green algae or a cyanobacterium. In most lichens, the photosynthetic partner is a green alga. The fungal partner belongs to the group of ascomycetes, which are fungi that produce spores in specialized structures called asci.
Here's a detailed explanation of why the majority of lichens are made of green algae and ascomycetes:
1. Mutualistic relationship: Lichens are formed when a fungus and a photosynthetic partner come together and form a mutually beneficial association. The fungus provides a protective environment and absorbs water and nutrients, while the photosynthetic partner provides food through photosynthesis.
2. Photosynthetic partner: The photosynthetic partner in most lichens is a green alga. Green algae are capable of photosynthesis, producing food (carbohydrates) using sunlight, water, and carbon dioxide. They provide the fungus with a source of energy and nutrients.
3. Fungal partner: The fungal partner in lichens belongs to the group of ascomycetes. Ascomycetes are a diverse group of fungi that produce spores in specialized sac-like structures called asci. These spores are responsible for the dispersal and reproduction of the fungus.
4. Structure of lichens: Lichens have a unique structure that reflects their dual nature. They consist of a thallus, which is the body of the lichen, and can have different forms such as crustose, foliose, or fruticose. The thallus is composed of fungal hyphae intertwined with the cells of the photosynthetic partner.
5. Other lichen associations: While the majority of lichens are made of green algae and ascomycetes, there are other combinations as well. Some lichens may have a cyanobacterium as the photosynthetic partner instead of a green alga. There are also lichens that have a fungal partner belonging to the group of basidiomycetes, which are fungi that produce spores on club-shaped structures called basidia. However, these combinations are less common compared to green algae and ascomycetes.
In conclusion, the majority of lichens are made of green algae and ascomycetes. This mutualistic partnership allows lichens to survive in diverse habitats and play important roles in ecological processes such as soil formation and nitrogen fixation.