Fungus

• 1. Introduction

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  A fungus carries chitin in its cell walls, the same tough material found in the shells of crabs and beetles. That single trait helped pull fungi out of the plant kingdom and into a category of their own, the biological kingdom Fungi. For centuries people assumed fungi were plants. They grow in soil, they stay rooted in place, and a mushroom can resemble a moss. But the truth runs the other way. Fungi are genetically closer to animals than to plants, and the two groups appear to have diverged around one billion years ago. This is the story of an enormous and largely hidden kingdom. Somewhere between 2.2 and 3.8 million species are thought to exist, yet only about 148,000 have been described. So how does an organism without leaves, roots, or a mouth feed itself? Why does over 90% of plant life depend on these creatures to survive? And what makes a single fungus capable of poisoning a person, healing a person, or quietly spreading across nearly nine thousand years and several square miles of forest floor?

• 2. Neither Plant Nor Animal

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  Heterotrophs is the word biologists use for organisms that cannot make their own food, and fungi belong firmly in that group. They absorb dissolved organic molecules, usually by secreting digestive enzymes into their surroundings and drinking back the broken-down result. A fungus performs no photosynthesis. It has no chloroplasts at all. Growth itself is how most fungi move, since only a few spores can swim or drift through air and water. The fungal cell wall combines two structural molecules, chitin and glucan, and fungi are the only organisms that pair these two together. Glucans also turn up in plants, and chitin forms the exoskeletons of arthropods, but the specific combination is theirs alone. Their walls contain no cellulose, which separates them from plants and from the water molds they superficially resemble. Many features tie fungi to the rest of life. With other eukaryotes they share membrane-bound nuclei, mitochondria, and storage compounds like the sugar alcohol mannitol and the disaccharide trehalose. With animals they share a heterotrophic appetite for preformed organic compounds. With plants they share cell walls, vacuoles, and the ability to make spores. More than one hundred fungal species even glow with bioluminescence. The opisthokont lineage, which groups fungi with animals, is broadly marked by a single posterior flagellum, yet every fungal phylum except the chytrids and blastocladiomycetes has lost that tail entirely.

• 3. The Threads That Build A Body

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  Hyphae are the cylindrical, thread-like filaments that make up most of a fungal body, typically 2 to 10 micrometers wide and sometimes several centimeters long. Each hypha grows from its tip, where a cluster of vesicles called the Spitzenkorper drives the advance. New tips branch off existing threads, and when two hyphae touch they can fuse together in a process called anastomosis. Together these growing, branching, fusing threads weave a mycelium, an interconnected network that is the true working form of a fungus. Septate hyphae are partitioned into compartments by cross walls called septa, each compartment holding one or more nuclei. Those septa carry pores that let cytoplasm, organelles, and even nuclei slip through, such as the dolipore septum found in the phylum Basidiomycota. Coenocytic hyphae skip the partitions entirely and act as multinucleate supercells. Some fungi never bother with threads. They live as single-celled yeasts that reproduce by budding or fission, and dimorphic fungi can flip between a yeast phase and a hyphal phase depending on their environment. Mechanical force is one of the mycelium's quiet talents. A plant pathogen like Magnaporthe grisea builds a structure called an appressorium that punctures plant tissue, and the pressure it generates against the plant surface can exceed 8 megapascals. The fungus Purpureocillium lilacinum uses a similar tool to drill into the eggs of nematodes.

• 4. Sex, Spores, And Survival

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  A third of all fungi are estimated to reproduce using more than one method of propagation. A single species may pass through two distinct stages: the teleomorph, its sexual phase, and the anamorph, its asexual phase. Asexual reproduction runs on vegetative spores called conidia, or on plain mycelial fragmentation, where a broken piece of mycelium simply grows into a whole new one. The Fungi imperfecti, also called Deuteromycota, gathers all the species that lack any observable sexual cycle, though it is no longer treated as a real taxonomic group. Sexual reproduction has been directly observed in every fungal phylum except Glomeromycota. Compatible hyphae fuse, and many ascomycetes and basidiomycetes then enter a dikaryotic stage, where two parental nuclei share a cell without merging. In ascomycetes this produces an ascus, a sac in which nuclear fusion is followed at once by meiosis and the release of ascospores. Basidiomycetes form a clamp connection at each septum to keep the two nuclei properly sorted, and their club-shaped basidia generate basidiospores. Getting a spore airborne can be violent. The forcible discharge of a single ballistospore relies on a tiny bead of liquid called Buller's drop, and the launch can reach an initial acceleration of more than 10,000 g. The spore travels only 0.01 to 0.02 centimeters, just enough to clear the gills or pores and fall into the air below. Other fungi take stranger routes. Stinkhorns wear lively colors and a putrid odor to lure insects, while truffles produce strong-smelling fruitbodies that tempt voles and other small mammals to eat them and scatter the spores in their droppings.

• 5. A Billion Years In Stone

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  Ourasphaira giraldae is the name scientists gave to a fossilized fungus reported from the Canadian Arctic in May 2019, an organism that may have grown on land a billion years ago, long before plants did. The fungal fossil record is otherwise meager. Fruiting bodies are soft and easily degraded, and most fungal structures are microscopic, so their remains are hard to tell apart from those of other microbes. The earliest fossils with clearly fungal features date to the Paleoproterozoic era, multicellular organisms with filaments capable of anastomosis. The oldest fossilized mycelium identified by its molecular makeup is between 715 and 810 million years old. Fungi do not become common and uncontroversial in the record until the early Devonian, between 416 and 359.2 million years ago, when they appear in abundance in the Rhynie chert. Around that same moment, roughly 400 million years ago, the Ascomycota and Basidiomycota split apart. Mushroom-forming fungi arrive much later. Two amber-preserved specimens of the extinct Archaeomarasmius leggetti show the earliest known mushroom-makers from the late Cretaceous, 90 million years ago. Catastrophe favored them. After the Cretaceous-Paleogene extinction that killed most dinosaurs, sixty-five million years ago, evidence of fungi surges, as the death of so many plants and animals left behind what one description likens to a massive compost heap.

• 6. The Great Alliances

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  Over 90% of all plant species engage in mycorrhizal relationships with fungi, and depend on them to survive. In this partnership fungal hyphae lace through or around plant roots and trade for sugars, often boosting the plant's uptake of nitrate and phosphate from poor soils. The arrangement is ancient, dating back at least 400 million years. Fungal threads can even shuttle carbohydrates from one plant to another through what are called common mycorrhizal networks. Lichens are a second great alliance, binding a fungus to a photosynthetic algae or cyanobacteria known as a photobiont. The photobiont hands over sugars, and the fungus supplies minerals and water, until the two function almost as a single organism. Around 27% of known fungi, more than 19,400 species, live this lichenized life, and lichens colonize bare soil, rock, tree bark, shells, and barnacles across every continent. Insects strike their own bargains. Ambrosia beetles farm fungi in the bark of the trees they infest, and females of the wood wasp genus Sirex inject their eggs alongside spores of the wood-rotting fungus Amylostereum areolatum into pine sapwood, so the growing fungus feeds their larvae. Termites on the African savannah cultivate fungi too. Not every relationship is friendly. Ophiocordyceps unilateralis is a fungus that alters the behavior of its animal host to spread its spores more effectively, a strategy known as active host transmission.

• 7. Poison And Cure

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  Amatoxins are among the deadliest substances a mushroom can carry, and the death cap, Amanita phalloides, is the most common cause of fatal mushroom poisoning. The section Phalloideae of Amanita holds dozens of amatoxin-containing species, including the destroying angels. Ergot alkaloids have an even longer record of harm. Produced by the ergot fungus Claviceps purpurea, they caused serious epidemics of ergotism, known as St Anthony's Fire, in people who ate rye contaminated with the fungus. Aflatoxins, made by certain Aspergillus species on grains and nuts, are insidious liver toxins and highly carcinogenic. Yet the same kingdom is a pharmacy. Since the 1940s fungi have produced antibiotics, beginning with the penicillins, a group of beta-lactam antibiotics built from small peptides. Penicillin G comes from Penicillium chrysogenum, and modern penicillins are semisynthetic versions reshaped for specific traits. Fungi also yield ciclosporin, used as an immunosuppressant during transplant surgery, and the statins that inhibit cholesterol synthesis, such as lovastatin from Aspergillus terreus. The shiitake mushroom supplies lentinan, a drug approved for cancer treatment in several countries including Japan. Some fungi feed us instead. Saccharomyces cerevisiae, baker's yeast, raises bread and ferments alcoholic beverages, the mold Aspergillus oryzae brews soy sauce and sake, and the meat alternative Quorn is made from Fusarium venenatum.

• 8. Decomposers And Workhorses

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  Decomposition is the role fungi were built for, and alongside bacteria they are the major decomposers in most terrestrial and aquatic ecosystems. By breaking organic matter down into inorganic molecules, they keep biogeochemical cycles turning and feed countless food webs. A single fungus can also grow to astonishing size. A clonal colony of Armillaria solidipes spreads over more than 900 hectares, about 3.5 square miles, with an estimated age of nearly 9,000 years. Fungi thrive where little else will. They live in deserts, in highly salty water, under ionizing radiation, and in deep sea sediments, and some can survive the intense ultraviolet and cosmic radiation of space travel. In a handful of species the pigment melanin may even help extract energy from gamma radiation, a poorly understood ability called radiotrophic growth. Humans have put this resilience to work. White-rot fungi degrade insecticides, herbicides, creosote, coal tars, and heavy fuels, turning them into carbon dioxide, water, and basic elements, and fungi can biomineralize uranium oxides for cleaning up radioactive sites. In the laboratory the bread mold Neurospora crassa helped scientists formulate the one gene-one enzyme hypothesis. The reach extends to climate. According to a study in Current Biology, fungi can soak up around 36% of global fossil fuel greenhouse gas emissions, the very mycorrhizal networks now threatened by herbicides, fungicides, fertilizers, deforestation, and soil paved over with concrete.