Lichens and Mycorrhiza: Two Masterpieces of Symbiosis
Imagine two very different organisms deciding to live together — not as enemies, not as indifferent neighbours, but as partners who each bring something the other lacks. That is symbiosis, and lichens and mycorrhiza are two of the most elegant examples you will meet in biology.
Lichens: The Pioneer Painters
Walk past an old stone wall, a tree trunk, or a bare rock in a clean mountain region. That crusty, flaky, often colourful patch you see — grey, orange, greenish — is a lichen. It looks like a single plant, but it is not. A lichen is a partnership between a fungus (the mycobiont) and an alga (the phycobiont). Sometimes the alga is replaced by a cyanobacterium.
The fungus builds the structure — a protective, spongy body that holds water and minerals. The alga, tucked safely inside, does what algae do best: photosynthesis. It makes food (sugars) using sunlight, carbon dioxide, and water. The fungus cannot photosynthesise; it has no chlorophyll. So the alga feeds the fungus. In return, the fungus gives the alga a home, shelter from drying out, and a steady supply of mineral nutrients it absorbs from the rock or bark.
The lichen is not a single species. It is a dual organism — a stable, self-contained ecosystem in miniature.
Because lichens absorb everything from the air and rain — including pollutants — they are extremely sensitive to air quality. Sulphur dioxide, heavy metals, and acid rain kill them quickly. That is why they are used as bioindicators: a place rich in lichens has clean air. A place with no lichens likely has serious pollution.
In exams, remember: lichens = fungus + alga. The fungus provides structure and minerals; the alga provides food. Pollution kills the alga first, so the whole lichen dies.
Mycorrhiza: The Underground Network
Now go underground. Most plants — over 80% of all land plants — do not rely only on their own roots to absorb water and minerals. They team up with fungi in a partnership called mycorrhiza (literally "fungus-root").
The fungus grows into or around the plant's root cells, forming a dense network of threads (hyphae) that extend far beyond the root into the soil. These hyphae are incredibly thin — much thinner than root hairs — so they can reach into tiny soil pores where water and phosphate ions are trapped. The fungus absorbs these nutrients and passes them to the plant.
What does the fungus get in return? The plant is a photosynthetic machine. It sends sugars and organic compounds down to the roots, and the fungus takes its share. The plant feeds the fungus; the fungus feeds the plant.
| Feature | Lichen | Mycorrhiza |
|---------|--------|------------|
| Partners | Fungus + alga (or cyanobacterium) | Fungus + plant root |
| Who photosynthesises? | The alga | The plant |
| Who provides minerals? | The fungus (from rock/air) | The fungus (from soil) |
| Who provides food? | The alga | The plant |
| Where found? | On surfaces (rocks, bark, soil) | Underground, on/in roots |
| Key role | Pollution indicator, pioneer on bare rock | Nutrient absorption (especially phosphorus) |
A common mistake: thinking mycorrhiza is a disease or a parasite. It is not. Both partners benefit. The plant grows better with mycorrhiza than without it, especially in poor soil.
The Big Picture
Both lichens and mycorrhiza are mutualistic symbioses — both partners gain. But they solve different problems. …