They range in size from the single-celled organism we know as yeast to the largest known living organism on Earth — a 3.5-mile-wide mushroom.
Dubbed “the humongous fungus,” this honey mushroom (Armillaria ostoyae) covers some 2,200 acres in Oregon’s Malheur National Forest.
The only above-ground signs of the humongous fungus are patches of dead trees and the mushrooms that form at the base of infected trees. (See image on left.)
It started out 2,400 years ago as a single spore invisible to the naked eye, then grew to gargantuan proportions by intertwining threads of cells called hyphae.
Under a microscope, hyphae look like a tangled mass of threads or tiny plant roots. This tangled mass is called the fungal mycelium, and is the part of the famous honey mushroom that spreads for miles underground.
If mushrooms and other fungi can get so huge, why mention them on a site about microorganisms?
Visible fungi such as mushrooms are multicellular entities, but their cells are closely connected in a way unlike that of other multicellular organisms.
Plant and animal cells are entirely separated from one another by cell walls (in plants) and cell membranes (in
animals). The dividers between fungal cells, however, often have openings that allow proteins, fluids and even nuclei to flow from one cell to another. A few fungal
species have no cell dividers: just a long, continuous cell dotted by multiple nuclei spread throughout.
The zoospores have no cell wall, are uniflagellated, and may swim for 24 hours on endogenous energy reserves. On contact with a suitable surface (e.g., a nematode cuticle), the zoospore encysts by withdrawing its flagellum and surrounding itself with a thick cell wall and then adhering to the surface. The fungi Arthrobotrys oligospora can capture a nematode when it merely touches the outside of its trap.
(Click on image above to view animation.)
Fungi are eukaryotic organisms. This means that their DNA-containing chromosomes are enclosed within a nucleus inside their cells. (The chromosomes of bacteria and archaea are not walled off inside nuclei, making them prokaryotic organisms.)
Many decades ago, scientists thought that fungi were primitive kinds of plants. New studies looking at the DNA of fungi have confirmed that these organisms are not plants.
Fungi are basically static. But they can spread either by forming reproductive spores that are carried on wind and rain (see the page Microbial Reproduction for more details) or by growing and extending their hyphae. Remember that hyphae are chains of fungal cells.
Fungi absorb nutrients from living or dead organic matter (plant or animal stuff) that they grow on. They absorb simple, easily dissolved nutrients, such as sugars, through their cell walls. They give off special digestive enzymes to break down complex nutrients into simpler forms that they can absorb.
Fungi can be found in rising bread, moldy bread, and old food in the refrigerator, and on forest floors. Most decompose non-living things, but some damage crops and plants. A few cause problems in people, such as Candida, which causes yeast infections.
If you’ve ever baked bread, you’re probably familiar with the only fungi that’s truly a single-celled organism: yeast.
Hyphae grow by adding cells at the tip. Hyphae are very tiny, measuring only a few microns in diameter in some cases. But they can also be incredibly strong, punching through not only the soft membranes of animal cells, but also the tough, woody walls of plant cells and the hard chitin that makes up insect bodies.