Lichen is a long term symbiotic association between which two types of organisms living together as a single visible unit?

Introduction / Context:
Lichens are fascinating composite organisms often seen on rocks, tree bark, and old walls. They can survive in harsh environments and are important ecological indicators. Lichens are not single organisms but rather stable symbiotic associations between two different partners. This question asks you to identify which two types of organisms together form a typical lichen.

Given Data / Assumptions:

• The organism discussed is lichen, seen as a crusty, leafy, or branching growth on surfaces.

• The options list possible pairings like algae and fungus, algae and tree, fungus and tree, bacteria and legume plant, and virus and bacterium.

• We assume basic familiarity with symbiotic relationships and examples such as lichens and root nodules.

• Only one pairing reflects the classical definition of a lichen.

Concept / Approach:
A lichen is formed by a symbiotic relationship between a fungal partner (mycobiont) and a photosynthetic partner (phycobiont), which is usually a green alga or sometimes a cyanobacterium. The fungus provides structure, protection, and helps retain water, while the alga or cyanobacterium performs photosynthesis, producing carbohydrates that feed both partners. The association is so close that they appear as a single organism. Lichens are therefore often described as fungi that have learned to farm algae. Hence, the correct pair is algae and fungus.

Step-by-Step Solution:
Step 1: Recall that the fungal component of a lichen forms most of the visible body or thallus. Step 2: Recognise that the algal or cyanobacterial component is embedded within this thallus and carries out photosynthesis. Step 3: Understand that the fungus gains food from the photosynthetic partner, while the alga gains protection and a moist environment from the fungus. Step 4: Consider the option algae and tree, which does not accurately describe the inner structure of a lichen, although lichens may grow on trees as a substrate. Step 5: Note that fungus and tree relationships exist, such as mycorrhizae, but they are not called lichens. Step 6: Bacteria and legume plant describe the nitrogen fixing root nodules, not lichens on rocks or bark. Step 7: Virus and bacterium describe bacteriophage interactions, unrelated to lichen formation. Step 8: Therefore, the correct symbiotic pair forming lichen is algae and fungus.

Verification / Alternative check:
Botany and ecology textbooks define lichens as associations between a fungus and an alga or cyanobacterium. Diagrams show algal cells or cyanobacteria embedded within fungal hyphae in cross sections of lichen thalli. These references use terms like mycobiont for the fungus and phycobiont for the algal partner, further confirming that a lichen consists of these two types of organisms living together as one unit.

Why Other Options Are Wrong:

Algae and tree: Algae may grow on trees but do not form the integrated internal structure of a lichen with the tree.

Fungus and tree: This more closely describes mycorrhizae, not lichens, and the tree is not part of the lichen body.

Bacteria and legume plant: These form root nodules for nitrogen fixation, a different kind of symbiosis.

Virus and bacterium: Describe infection of bacteria by viruses, not a constructive symbiotic lichen association.

Common Pitfalls:
Students may confuse lichens with mosses or simply think of any growth on tree bark as a plant tree partnership. Another confusion is between different kinds of symbiosis such as mycorrhizae and nitrogen fixing nodules. To remember lichens correctly, associate them with the phrase fungus plus alga living as one, and recognise their role as pioneer species on bare rocks.

Final Answer:
Lichen is a symbiotic association between Algae and fungus.