by Skip Via
In casual conversation about the living inhabitants of an ecosystem, especially among those of us who took high school biology in the 60s, we generally rely on “flora” and “fauna” to categorize and conceptualize the organisms living there. If something’s not a mineral, then it must be a plant or an animal.
But it could be something else.
Current biology recognizes six kingdoms of living organisms. And there are other ways to conceptualize and categorize organisms. Check this Wikipedia article for a more in-depth discussion of Domains, Empires, Kingdoms, and other taxonomical specificities, including a very handy and easily understandable chart of all life. But for now, the six kingdoms:
- Eubacteria: prokaryotic (no nucleus) single cell organisms without organized nuclei, mitochondria, or other organelles protected by membranes; all bacteria
- Archaea: prokaryotic (no nucleus) single cell organisms without organized nuclei, but genetically distinct from bacteria (actually more closely related to humans than to bacteria) and an important part of our ecosystem. Their food sources are varied and extreme and include sugars, ammonia, metals, and hydrogen gas. They are important in carbon fixation and have been found in every habitat from the human gut to the volcanic hot springs of Yellowstone to ocean plankton.
- Protista: eukaryotic (having a nucleus) single celled organisms with nuclei and cell membranes, largely aquatic: amoebas, euglena, paramecium, and some colonial protists (multiple cells living together) such as certain species of kelp and green algae.
- Fungi: see below
- Plantae: multicellular organisms with specialized cells and no means of locomotion: trees, grasses, flowering plants, mosses, etc. Formerly Flora.
- Animalia: multicellular organisms with specialized cells and their own means of locomotion. Formerly Fauna.
Until the 1950s and 60s, the working assumption about living organisms was that if it wasn’t a plant, it must be an animal. Funguses were considered plants because they clearly weren’t animals. But by the mid-20th century, biologists recognized that some organisms–specifically funguses, protists, and bacteria–didn’t really fit into the kingdoms Animalia or Plantae.
This article is an introduction to one of those kingdoms—Fungi–and another group of organisms that are often associated with funguses–the lichens. I’ll try to clear up some misconceptions about distinctions between kingdoms, including my own.
Funguses, it turns out, are more closely related to animals than to plants. Like animals, funguses get the carbon and energy they need to live from outside sources (heterotrophism), being incapable of creating their own by processes such as photosynthesis. But they don’t exactly eat, either, nor can they move on their own. Rather, they absorb food directly into their cells by attaching themselves to a location that provides nutrition. How they do that occurs in three ways:
Biotrophic funguses get their nutrients from living plants or animals by establishing long-term relationships with their host. The host is impaired but usually not destroyed (at least immediately), and in some cases not harmed at all. Plant rusts, mildews, and molds are familiar examples of biotrophs.
Saprotrophic funguses obtain the nutrients they need from dead organic matter. These are the most common of the more than 3.5 million (!) species of funguses and the most familiar to us—mushrooms and shelf funguses being prominent examples. They can secrete substances that break down dead organic matter externally from a wide variety of sources (dead trees, animal feces, carcasses, etc.) that is then absorbed directly into the fungus’ cells.
Nectotrophic funguses invade their host and destroy it by killing the host’s cells and feeding off the dead matter. Think Dutch Elm disease, or the existentially terrifying cordyceps funguses that invade living insects and turn them into zombie slaves. Really.
Funguses are essential for life as we know it. They provide direct nutrition for animals (e.g., mushrooms), make other foods possible (bread, cheese, chocolate…) and accelerate the decay and repurposing of dead organic matter in our environment. They help animals digest their food in their gut and provide medications (e.g., penicillin, statins) that keep us healthy and cure diseases. They are essential for the growth and reproduction of many plant species, including local orchid species (see Calypso Orchids and Spotted Coralroot Orchids elsewhere on this site for more information). They occur in every ecosystem, from oceans to deserts to high mountains to rivers and streams.
Lichen are unique in that they are not a single organism at all but rather two or three different organisms—a fungus and a blue-green alga or a cyanobacteria, or sometimes both—existing in a stable symbiotic relationship that provides each organism with some advantage that it could not provide on its own. The alga/bacteria is the photobiont in this arrangement and the fungual host is the mycobiont.
Funguses cannot produce their own food, while algae and cyanobacteria can photosynthesize food internally. When the two associate in a lichen, the fungus (the more prominent of the two organisms) provides a sheltered place for the alga or bacteria to grow while the alga/bacteria provides food that the fungus can absorb. The association of the components, lichenization, does not happen by accident. The fungal element searches for and recognizes the proper alga/bacteria through chemical processes. If the alga/bacteria looks like a parasite or otherwise poor partner for growth, the fungus rejects it.
Each lichen component provides the other with the possibility of living in an environment that it could not survive in alone. For example, lichens are commonly found on rock surfaces where neither the fungus nor the alga/bacteria could establish themselves. These lichen are an important step in the “greening” of an environment, trapping dirt particles, organic matter, and moisture that can over time become a host for plants and a basis for soil creation.
Like funguses, lichens are common in every environment from sea level to high alpine ecosystems, sometimes in very extreme environments. They are often the first life form to grow back after a disaster. They take on many external forms that we commonly encounter. Fruticose lichens seem “plant-like” with small branches or shrub-like growths. Our locally common wolf lichen or goatsbeard lichen found hanging from tree branches are good examples. Foliose lichen have a leaf-like structure, and crustose lichens are the flat ones we commonly see growing on rocks or other surfaces. There are many other forms among lichens’ more than 20,000 species.
It’s important to remember that lichen are not parasitic–they do not draw nutrients from or harm their hosts. They don’t have roots as plants do—they just need a place to drop anchor–although the fungal element can actually penetrate between the crystals of rocks and break them down over time, again contributing to the process of creating new soil.
As is the case with funguses it might be easy to confuse lichens with plants, as many common lichen names include “moss.” The Spanish moss that adorns southern hardwoods is not a moss at all, but a form of fruticose lichen.
Lichen is an important food source for many animal species (reindeer “moss” provides the main diet for caribou, for example) including, in some cases, humans—although this is usually more common in times of hardship or emergency. Lichens are also the source of the antibiotic cephalexin (Suprax and Keflex), and are a potential source of other yet undiscovered pharmaceuticals.
As dual (or sometimes triple) organisms, lichens form a symbiotic relationship with members of other kingdoms and exhibit characteristics that are unique to that specific lichen species. But lichens are not recognized as a separate kingdom of organic life. Rather, they are considered “plantlike” by taxonomists, being essentially fungal based and therefore members of the Fungi kingdom.
So then next time you are playing 20 Questions and ask the inevitable question “Animal, vegetable, or mineral?” remember that that the correct answer could be “None of the above.”