New Zealand's array of lichen species represents about 10% of the world’s known lichens.
A lichen is an amalgamation of a fungus and one or more photosynthetic organisms (those that make food from sunlight, water and carbon dioxide). No fungi can make their own food, but they have evolved ways to barter or steal it from other living things. A fifth of the world’s fungi do that by partnering with photosynthetic algae and/or cyanobacteria – their partnership is called a lichen.
Lichens function similarly to green plants. Historically they have been treated as plants and studied by botanists.
Lichens are made up of at least two organisms, a fungus and either a green alga or a cyanobacterium (sometimes both). The lichen’s body (thallus) is built by the two organisms working together. How they do that is still poorly understood. Most of the thallus is made of the tangled, thread-like strands of the fungus. The algal partner is usually restricted to a thin layer of single cells below the upper surface of the thallus.
Lichenised fungi receive sugar from their photosynthetic partner. The alga or cyanobacterium within the thallus is protected from the outside environment, and receives water and minerals from the fungus. This mutual aid improves the survival of both partners in habitats where neither could easily survive alone. Most lichens can be found growing on soil, rock, tree bark, and leaves, but some have managed to colonise man-made surfaces such as asphalt, glass, plastic and metals.
Extreme hot and cold
Lichens can survive in temperatures that would kill most other creatures. Desert-dwelling lichens can endure heat up to 70°C. In laboratories, they have survived being dry-frozen at close to absolute zero (-273°C), and in Antarctica they can photosynthesise at -23°C and overwinter in total darkness at -70°C.
Lichens thrive in places that unpredictably alternate between being wet and dry. They have a competitive advantage over most land plants, as they shut down their metabolism during dry spells. When the rain comes, they absorb water readily and restart their metabolism.
Many lichens produce unique chemical compounds called lichen substances. Some are brightly coloured sunscreens that protect the lichen from ultraviolet light, while others are poisons that fend off grazers such as slugs and insects, or invaders like bacteria and rot fungi. Some protect against sulfur and other air pollutants.
Lichen bodies have three main forms:
- Crustose: half the world’s lichens are described as crustose, and form a powdery, scaly, or tough coating on soil, rock or bark.
- Foliose: a foliose lichen looks like a leaf or flat sheet loosely attached to a surface. Often the leaf-like thallus is divided into lobes. Large foliose lichens are a feature of New Zealand’s forests, and are commonly found clothing tree trunks.
- Fruticose: erect or hanging, fruticose lichens look like miniature shrubs, strands of hair, or snarled balls of wool.
Most lichenised fungi belong to the class of cup fungi (Ascomycetes), and reproduce by shedding fungal spores from splash cups. Splash cups are often brightly coloured and range from thumbnail-size to so small you need a hand lens to see them. When a raindrop hits the cup, the spores bounce out, are lofted into the air, and land faraway. The germinated fungal spore must find an algal partner quickly to form a new lichen.
Most lichens also reproduce without spores. Many shed tiny bits of thallus containing both fungus and alga, so a new lichen can grow without the need for the fungus and alga to find each other first.