The answer to the question "A lichen is best characterized as a symbiosis between" is option 'C', which is "Fungus and an algae".



Lichens are composite organisms that result from a mutualistic relationship between a fungus and an algae or a cyanobacterium. This symbiotic relationship is known as mutualism, where both organisms benefit from each other's presence.


- Lichens are found in diverse habitats, including deserts, forests, tundras, and rocky surfaces.
- They have a unique structure consisting of a fungal partner (mycobiont) and a photosynthetic partner (photobiont).
- The fungal partner provides a protective environment and absorbs water and nutrients from the surroundings.
- The photosynthetic partner, which can be an algae or a cyanobacterium, produces organic compounds through photosynthesis, providing nutrients for both partners.
- Lichens have a slow growth rate and can survive in extreme conditions, including high temperatures, low nutrient availability, and high levels of pollutants.
- They play a crucial role in ecosystem functioning, such as soil formation, nitrogen fixation, and providing habitat for other organisms.


- The fungal partner in a lichen is usually an ascomycete or a basidiomycete fungus.
- The fungal hyphae provide a protective structure called the thallus, which surrounds and holds the algae or cyanobacteria.
- The algae or cyanobacteria, also known as the photobiont, reside within the thallus and perform photosynthesis to produce organic compounds.
- These organic compounds are then shared with the fungus, which cannot produce its own food through photosynthesis.
- In return, the fungus provides a suitable environment for the algae or cyanobacteria to grow and protects them from desiccation and excessive light.


In conclusion, lichens are best characterized as a symbiosis between a fungus and an algae. The fungus provides structure and protection, while the algae or cyanobacteria provide organic compounds through photosynthesis. This mutualistic relationship allows lichens to survive in diverse environments and perform important ecological functions.