Fungi are found easily if you know where to look – from mushrooms in forests to mould on bread – but many are inconspicuous or invisible to the naked eye.
This might explain why they’re so often overlooked and were once grouped together with plants, despite being a kingdom with their own branch on the tree of life.
How do fungi differ from animals and plants?
Like plants, many fungi grow in soil, can’t move from place to place and their cells have walls (reinforced by cellulose in plants and by chitin in fungi). But these are only superficial similarities. The most fundamental difference is nutrition: while most land plants are ‘autotrophs’ that use light to produce carbohydrates via photosynthesis, fungi are ‘heterotrophs’ that eat molecules made by others.
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So, fungi aren’t producers, they’re consumers – like animals. But whereas animals ingest food before breaking it down inside their bodies, fungi release enzymes to digest food externally then absorb the resulting nutrients into their cells.
Why are fungi important?
The majority of fungi are terrestrial and many are ‘saprotrophs’ that cause decay while feeding on organic matter. And because they decompose dead organisms to recycle carbon and other molecules, they’re essential to ecosystems.
Fungi are also food to humans – edible mushrooms, moulds in blue cheese and yeasts used in baking and brewing. They’re important to our health, too: fungi can cause infections such as thrush (yeast) and ringworm (mould) but they’ve also given us medicines such as statins and penicillin.
What do fungi look like?
A mature individual might be a single-celled yeast or (like all animals and plants) have a body made of multiple cells. The bulk of a multicellular fungus comes from branching, thread-like hyphae – microscopic filaments that resemble plant roots (but hundreds of times thinner) and create a mass called mycelium. Mycelium can feel wispy, as in moulds, or form more solid structures such as the stalk and cap/cup of mushrooms. The word ‘fungus’ stems from Greek for sponge.
How do fungi reproduce?
Many species produce spores that, like pollen or seeds, are dispersed – usually by wind – before they settle and grow hyphae. Around 98 per cent of known fungi come from two groups, sac fungi (ascomycetes) and club fungi (basidiomycetes).
A sac fungus’ spores are enclosed in a bag-shaped structure called an ascus, while a club fungus’ spores are released from club-like stalks – both can be within a ‘fruiting body’ below the surface (as in truffles) or above ground (mushrooms).
Some fungi reproduce asexually as clones: a yeast copies itself by fission (the cell splits in two) or by budding, while a mycelium can fragment to establish new colonies.
Other fungi may meet and exchange genetic material through sexual reproduction. Note that while fungi have sex, they don’t have male and female sexes but ‘mating types’ (the split-gill mushroom has more than 23,000!)
What about symbiotic relationships?
Lichen are fungi living in close association with algae or bacteria. Another well-known symbiosis occurs when hyphae penetrate a plant root system to form ‘mycorrhizae’ (fungus-roots). This is often a mutually beneficial partnership: the plant gets help absorbing water and minerals and the fungus gets carbohydrates.
Mycorrhizae have been found in more than 90 per cent of plant families, and they can connect neighbouring trees to create a common mycorrhizal network or ‘wood-wide web’ that some claim enables different plants to communicate.
But symbioses can be parasitic, too. Armillaria ostoyae uses root-like hyphae twisted into cords (rhizomorphs) to steal nutrients from host plants, and one specimen in Oregon is arguably the world’s largest living organism: nicknamed the ‘humongous fungus’, it covers an area of almost 10km2.
How many species of fungi are there?
About 155,000 fungi have been described but the true figure has been estimated at two to three million species. To recognise the kingdom’s ecological importance and the fact that biodiversity goes beyond animals and plants, the IUCN recently adopted a new phrase: fauna, flora and funga.
