Fungi

Kingdom

Food, Animal source foods

Consumption area(s): Earth

Introduction

Fungi constitute a distinct kingdom of eukaryotic organisms, encompassing both unicellular and multicellular forms. Scientists have formally identified over 700,000 species, yet estimates suggest that the true diversity may exceed three million species. Only a small fraction of these are considered edible.

Description of Fungi

Organisms belonging to the Fungal Kingdom share several defining features. They exhibit heterotrophic nutrition, meaning they obtain nutrients from organic matter produced by other organisms. They lack differentiated tissues and do not possess vascular systems for fluid transport. Their reproduction occurs via spores, bypassing an embryonic stage typical of plants and animals.

Fungi range from extremely simple unicellular forms to highly complex multicellular organisms. Their vegetative body can develop into filamentous networks called hyphae, without forming true tissues. Unlike plant cells, whose walls are mainly cellulose, fungal cell walls are composed of β-glucans and chitin, a substance also found in the exoskeletons of arthropods.

In terms of energy storage, fungi accumulate glycogen similarly to animals, in contrast to plants which store starch. The hyphal cells can be uninucleate or multinucleate and are sometimes separated by septa. The presence or absence of septa helps distinguish different fungal groups. For instance, in Zygomycetes, hyphae lack septa, whereas in Ascomycetes, Basidiomycetes, and Deuteromycetes, septa are present.

Septa can take different forms: they may be complete, without a pore; perforated, with a central pore allowing cytoplasmic flow and organelle movement; or dolipore, with a chitinous cylindrical border surrounding the pore, as observed in some Basidiomycetes. Hyphae grow within their substrate, eventually forming an interwoven mycelial network.

Reproduction

Multicellular edible fungi reproduce through spores, which can be generated either asexually or sexually. Asexual reproduction allows fungi to multiply efficiently without the need for gamete fusion. In edible species, this commonly occurs via fragmentation or sporogenesis, where portions of the mycelium or specialized cells detach and develop into new individuals.

Sexual reproduction in fungi relies on the formation of spores, which are produced in vast numbers by each organism and dispersed primarily through wind, water, or insects. In many species as Ascomycota male and female spores merge to create a structure with multiple nuclei. The nuclei within this structure do not fuse immediately; only after karyogamy occurs does meiosis take place, producing haploid spores. When environmental conditions like humidity and temperature are favorable, these spores germinate, giving rise to new mycelia.

In Basidiomycota species, a spore of a specific sexual polarity lands on a suitable substrate and, under optimal conditions, germinates into a hypha (primary mycelium) . To complete the life cycle and develop reproductive structures, the hypha must transition into a secondary mycelium, the true vegetative body of the fungus. Here, a hypha from a spore of one sexual type fuses with a hypha of the opposite sexual type—a process called somatogamy—forming the secondary mycelium that will eventually produce the fruiting body (carpophore) containing new spores.

Mushrooms

Mushrooms are the fleshy, spore-producing fruiting bodies of fungi, typically emerging above the ground on soil or other substrates. They originate from tiny primordia, less than two millimeters across, which form within the mycelium, the network of threadlike hyphae making up the fungus. The primordium grows into a round button surrounded by the universal veil, which may rupture as the mushroom expands, leaving remnants at the base of the stalk called the volva, or on the cap and. Some mushrooms lack this veil. A second layer, the partial veil, often covers the gills; when it breaks, remnants can form a ring on the stalk or fringes on the cap margin, varying in shape and prominence across species.

The stalk or stipe may be central, lateral, or absent, depending on the type of mushroom. Some mushrooms lack a stalk entirely. The gills—the blade-like structures where spores develop—attach to the stalk in diverse way, with attachment sometimes changing during growth.

At the microscopic level, the hymenium is the layer of spore-bearing cells covering gills, tubes, or spines. In Ascomycota, spores form inside sac-like asci, usually containing eight spores. In Basidiomycota, spores—typically four—develop on projections called sterigmata extending from basidia. Spore characteristics such as shape, color, size, ornamentation, and chemical reactions are crucial for identifying mushrooms. Each spore has an apiculus, the point of attachment to the basidium, through which hyphae emerge during germination.

Mushroom growth can be rapid, giving rise to the expressions “to mushroom” or “to pop up like a mushroom,” though fruiting bodies take days to develop from primordia before swelling quickly by water absorption. Certain species, such as Parasola plicatilis, may grow and release spores in just a few hours, then collapse, while others, like Pleurotus nebrodensis, grow slowly and are vulnerable to overharvesting.

Although fruiting bodies are short-lived, the underlying mycelium can be ancient and vast. For instance, colonies of Armillaria solidipes can persist for thousands of years, spanning huge areas and forming massive underground networks in decaying roots. These long-lived mycelial networks support repeated fruiting over centuries, sustaining fungal populations far beyond the visible lifespan of individual mushrooms.

Nutrition

All fungi are heterotrophic, meaning they obtain their nutrients from the environment by absorbing them through their cell walls. They play a crucial ecological role by decomposing organic matter, recycling it into available forms, and thus maintaining the nutrient cycles in ecosystems.

Fungi display different lifestyles depending on their relationship with the substrate. These include saprotrophy, parasitism, and mutualistic symbiosis, and fungi are classified accordingly as saprotrophs, parasites, or mutualists.

Saprotrophic fungi break down non-living organic material, such as plant debris or animal remains, into simpler compounds. They rely on enzymes to decompose complex polymers like cellulose, gradually producing residues that can be reabsorbed. Each species occupies a specific position in this decomposer chain, and if one link is lost, dependent organisms may fail to survive.

Parasitic fungi feed on living organisms, often causing gradual death of their hosts. Certain species, may initially act as mutualists before becoming parasites, eventually continuing as saprotrophs after the host dies. Obligatory parasites, in contrast, cannot survive without their host. Parasitic fungi can affect plants, insects, vertebrates, and even other fungi. Some are highly specialized, targeting only specific tissues or species, while predatory fungi may use traps to capture prey.

Mutualistic fungi engage in symbiotic relationships where both partners benefit. A classic example is the mycorrhizal association with plant roots, in which the fungus provides water, minerals, nitrogen, phosphorus, and potassium, while receiving sugars from the plant. This exchange enhances plant growth and survival, especially during drought.

Classification of Fungi

Fungi are primarily classified into phyla, including:

• Ascomycetes (Ascomycota) (truffles)
• Basidiomycetes (Basidiomycota) (all other edible Fungi)