Lichens: Symbiosis Between Algae and Fungi Explained

Lichens are not single organisms at all but partnerships: a fungus and an alga living together so closely that they look and behave like one plant. You can see them as pale green, grey or orange patches on tree bark, walls and bare rocks. This symbiosis is one of nature's most successful collaborations, allowing lichens to survive in deserts, tundra and on mountaintops where neither partner could live alone.

Lichens matter for several reasons. They are excellent indicators of air pollution because they die out where the air is dirty, especially with sulphur dioxide. They are pioneer organisms that slowly break down bare rock and begin soil formation. Humans have also used them for centuries to make dyes, including litmus, and as traditional medicines.

Key Concepts

• Symbiosis (mutualism): a close association in which both partners benefit.
• Phycobiont: the algal partner, which is autotrophic and prepares food by photosynthesis.
• Mycobiont: the fungal partner, which is heterotrophic and provides shelter, anchorage and absorbed water and minerals.
• Crustose lichens: crust-like forms tightly attached to rock or bark.
• Foliose lichens: leaf-like, loosely attached forms.
• Fruticose lichens: shrubby, branched, upright or hanging forms such as reindeer moss.
• Reproduction: vegetative by fragmentation and special structures called soredia and isidia; the fungal partner can also reproduce sexually.

How It Works

Inside a lichen, fungal hyphae wrap around algal cells like a protective blanket. The alga photosynthesises and leaks sugars to the fungus, while the fungus shields the alga from drying winds and ultraviolet light, anchors the whole partnership to bark or rock, and absorbs mineral nutrients from rain and dust. Because each partner covers the other's weakness, lichens colonise bare rock surfaces where no plant can grow. Over decades their acids etch the rock and their dead parts build the first thin layer of organic matter, starting the long process of soil formation that later plants will use.

Worked Example

Question: Crustose lichens grow outward very slowly, roughly 1 millimetre per year in diameter. A circular lichen patch on a rock measures 24 millimetres across. Roughly how old is it?

Step 1: Growth rate = 1 mm of diameter per year (approximate, widely observed).

Step 2: Age = diameter / growth rate = 24 / 1.

Step 3: Age = 24 years.

Answer: Roughly 24 years old, which is why lichens are used to date rock surfaces.

Real-World Applications

• Pollution monitoring: lichens disappear from areas with heavy sulphur dioxide pollution, so their presence maps clean air.
• Ecological succession: as pioneers on bare rock, they start soil formation in primary succession.
• Dyes: lichens such as Roccella yield litmus and orchil dyes used in laboratories and textiles.
• Food and medicine: reindeer moss (Cladonia) feeds reindeer in the Arctic tundra, and several lichens are used in traditional remedies.

Lichens are symbiotic partnerships between an autotrophic algal phycobiont and a heterotrophic fungal mycobiont, growing as crustose, foliose or fruticose forms. They indicate clean air, pioneer bare rocks toward soil, and provide dyes, food and medicines, making them small but ecologically mighty organisms.