Fungi
Fungi are eukaryotic organisms such as mushrooms, molds, and yeasts that absorb nutrients from their surroundings and act as major decomposers.
Definition
Fungi are living things like mushrooms, mold, and yeast that, unlike plants, cannot make their own food from sunlight, so they get it by breaking down materials around them, and the mushroom you see aboveground is only a small part of the organism, most of which is a hidden web of threads spreading through soil or wood. Formally, fungi are a kingdom of eukaryotic heterotrophs that secrete digestive enzymes onto their food and absorb the dissolved products, mostly multicellular and built from thread-like hyphae forming a network called a mycelium, with unicellular yeasts as the exception, chitin cell walls, reproduction by spores, and roles as the main decomposers of wood and leaf litter and as partners with plant roots in mycorrhizae. At the phylogenetic level, fungi form a monophyletic kingdom within Opisthokonta, sister to animals, spanning phyla such as Chytridiomycota, Zygomycota, Glomeromycota, Ascomycota (including yeasts and Penicillium), and Basidiomycota (mushrooms and rusts), with growth by apical hyphal extension and complex life cycles that include a prolonged dikaryotic stage where two genetically distinct nuclei share one cell before fusing, and they produce antibiotics, organic acids, and industrially important enzymes.
Example
Around $90$ percent of plant species form mycorrhizae, trading soil phosphorus and water from fungal threads for sugar from the plant, and a honey fungus in Oregon spreads over more than $9\ \text{km}^2$, one of the largest organisms known. Penicillin, discovered from Penicillium mold, blocks the enzymes that cross-link bacterial peptidoglycan, harming bacteria but not fungi or humans, and baker's yeast was the first eukaryote to have its genome sequenced, in 1996, with about $6{,}000$ genes on $16$ chromosomes.
Key Insight
Without fungi, dead trees would pile up forever, since fungi are among the very few organisms able to break down lignin; because fungi and animals are close relatives sharing much of their cell biology, fungal infections are hard to treat, and the few safe antifungals work only by targeting chitin and ergosterol, molecules animal cells do not make.