This is the second in a two-part series about the history of mycology and the basics of fungal reproduction. If you’re interested in learning more about how sexy mushrooms can be, take a look at the last post. Although this post sort of rambles through mushroom biology, it has enough foundational material to give you a sense of the ins and outs of mushroom hunting. Hope you enjoy it!
Yours In Fungal Fancy,
Mushrooms are Sexy, Part Deux
Before the advent of microscopy, our understanding of fungal reproduction was limited at best. In his Natural Philosophy, Aristotle uneasily concluded that mushrooms were plants, despite the fact that they did not have noticeable seed and erupted rapidly in the wake of storms rather than honoring a predetermined and observable flowering season. Most Greeks agreed that Aristotle was onto something, but they also revered the more enchanting and magical aspects of mushrooms, and some ethnobotanists believe that the Greeks used magic mushrooms or the hallucinogenic fungus ergot in the Eleusinian Mysteries, to power visions at the Oracle of Delphi, and to fuel the ecstatic Dionysian sex parties that were all the rage among individuals that occupied the upper crust of classical Greek society. For more on this, see this blog post from several weeks ago.
Aristotle and his contemporaries called mushrooms “sons of the gods” and “seed of the gods” because of their mysterious nature, and because of their affinity for mighty Zeus’s thunderstorms. In addition, the Greeks dubbed mushrooms cryptogams because fungal reproduction was so puzzling. In Greek, the word cryptogam breaks into two root words: crypto, which means “hidden,” and gammein, which means “to marry.”
For many hundreds of years, the idea that mushrooms were basically weird plants persisted, and many people still think of them as more closely akin to plants than other life forms, even though our understanding of fungal biology has grown enough that most mushroom lovers are aware that fungi are more like animals than plants. Our basic needs are the same (food, water, oxygen), and there is even a superkingdom that unites Animalia with Fungi as a way of demonstrating our common evolutionary roots.
In 1969, microbiologist Robert Whittaker proposed that fungi be given their own kingdom within the Linnean classification system. Naturalist Carl Linneaus created the field of taxonomy in 1735, and it established the rank-based system that scientists use to classify organisms down to genus and species.
Prior to Whittaker’s realignment, fungi had been bounced around botanical taxonomy like unwanted stepchildren; first they were assigned to the plant kingdom, then they were lumped in with the protists, a grab-bag of organisms that do not produce specialized tissues. Whittaker clarified the distinction between fungi and other organisms based on fungal feeding habits.
Like animals, fungi cannot produce their own food, and must obtain nutrition from the surrounding habitat. Some species decompose dead plant material using powerful digestive enzymes that are secreted into the mycelium’s habitat. Other fungi team up with plants, sharing water and nutrition with a symbiotic partner. Still other species are parasitic, attacking and stealing resources from other organisms. To complicate matters, many fungi are not strictly decomposers, mutualists, or parasites. Fungi have a tricky way of changing behavior and lifestyle to suit the environmental conditions.
The oyster mushroom, for instance, is normally described as a wood-decomposing fungus. However, the oyster has a sinister side that manifests when the fungus has the opportunity to commit murder. Oyster mushroom mycelium secretes a specialized enzyme that paralyzes nematodes, microscopic bugs that live in soil and wood. Once it immobilizes its prey, the fungus devours the stunned (and, as it happens, protein-rich) nematodes. Thus, the oyster mushroom transforms itself from an innocuous decomposer momentarily into a hungry carnivore.
To Mushroom or Not to Mushroom: Which Fungi Make Mushrooms?
Most fungi do not reproduce using mushrooms. In fact, only about 10% of the fungal kingdom does so. Molds, yeasts and other species that reproduce asexually are called the fungi imperfecti (also known as the Deuteromycota). Compared to mushroom-making species, these fungi imperfecti are dominant in almost every habitat on earth. Furthermore, many of these fungi are potent decomposers of complex hydrocarbons and other large, durable molecules. This is why some of the most exciting applications of fungi as bioremediation agents focus on fungi imperfecti, including several yeasts and white rotting molds that, when left unchecked, can cause all kinds of problems for human and environmental health.
Fungi imperfecti have a variety of reproductive strategies, from budding, wherein a new yeast cell spontaneously springs from an identical parent, to asexual sporulation, in which germinated spores do not have to recombine with a genetically distinct partner in order to grow into a new fungus. Mycelium fruits when certain conditions in the environment are just right. Usually, elevated moisture levels, a bit of sunlight, and cool air do the trick.
Although mycelium and mushrooms do not need sunlight to grow, many species of fungi are photosensitive and only fruit when exposed to light. This is because the presence of sunlight indicates to the mycelium that it’s reached the surface of its growing medium. Mushroom spores are spread via air currents, and so fungi have evolved to detect outdoor environments. The shiitake, for instance, will not fruit unless the mycelium senses certain natural light spectra. When these conditions are not present, the mycelium focuses its energies on expanding its network into new sources of food and moisture.
This is why mushrooms are not always present when the actual fungus is living right underfoot. The fruiting cycle is very taxing on the mycelium, and so it only fruits when the timing is just perfect. For many species, fall is the best time to make mushrooms, but this varies, and mushroom season typically is the “rainy season,” no matter when on the calendar that time occurs. In North Carolina, our hot and wet summers are perfect for mushrooms of all kinds, and the mushroom season extends into the fall and tapers off by the time the relatively cold and dry winter commences. Strands of mycelium coalesce into dense knots of tissue called primordia. Primordia are baby mushrooms, and cultivators call them “pinheads” or “buttons.” Many mushrooms are pre-determinate—the primordia have all the features of the fully developed mushroom, just in miniature.
Over a few days or weeks, the mushroom develops and releases spores that are carried off by air currents, spreading the mycelium’s DNA far and wide. This is why picking mushrooms is not analogous to cutting a tree or even harvesting potatoes by digging up the entire potato plant. Mushrooms are like apples on a tree. They serve a reproductive function but are not the actual fungal organism, and just as picking an apple will not kill a healthy tree, so too gathering mushrooms will not harm the parent fungus.
There is significant debate about the impact of mushroom harvesting in America’s forests, and the wild mushroom trade is the biggest legal cash economy in the United States. However, several longitudinal studies are clear: picking mushrooms does not significantly damage the mycelium that produced them. However, human traffic can certainly damage fungal habitats and mycelium. Most mushroomers agree on a simple hunting ethic: walk softly, collect in moderation, and leave some mushrooms behind so that the mycelium thrives.
Fungi in Prehistory – The Giant Mushroom Prototaxites
Fungi were the first organisms to leave the primordial ocean and adapt to terrestrial living roughly a billion years ago, and they have led the evolutionary charge on this planet ever since. During the Paleozoic era (a time period that stretches from 541-252.2 million years in the past), giant mushrooms called Prototaxites festooned the landscape, and their successors survived the massive extinction event that brought the age of the dinosaurs to a close.
University of Chicago researcher Kevin Boyce verified that Prototaxites was a fungus in 2007 after nearly a century of academic controversy about the organism. The fossil of Prototaxites is approximately twenty feet tall and is shaped like a rounded, branchless tree trunk. Fossil remains of this fungus imply that it was a dominant and very successful life form; it has been located in places as widespread as Canada and Saudi Arabia. Fungi have survived two great waves of extinction, which is a testament to their resiliency and adaptability. First, they endured a cataclysmic moment 250 million years ago that denuded earth of 90% of its inhabitants, and 65 million years ago the fungi prevailed when a meteorite struck the earth and triggered another ecological catastrophe that heralded the end of the Mesozoic Era.
Animals and Fungi – We’re Frightfully Similar!
Human beings and mushrooms go way, way back. In terms of evolutionary history we share a common ancestor, making us more similar to one another than to plants. There are a few key biological characteristics that we share, indicating that we sprang from the same ancestor some 600 million years in the past. Specifically, neither of us can photosynthesize simple sugars, and both fungi and animals breathe oxygen and exhale carbon dioxide. The key difference between us is not, therefore, in what we eat and breathe, but how we eat and breathe.
Humans and other members of the animal kingdom developed stomach technology. We consume our food and envelope it inside a cellular sac (fancy biologist lingo for stomach), and then break it down with enzymes and acids. Fungi, on the other hand, secrete digestive juices into the environment around them, extract the nutrients they need, and draw them into the fungal body.
This critical difference gives us an idea of why our physical bodies are so different. Fungal mycelium evolved to be permeable, completely interconnected with its habitat and food source. This also explains why fungi are so responsive to environmental changes; because mycelium is completely exposed to its habitat, it must adapt constantly in order to survive. Throughout the ages, fungi shaped the ecosystems they inhabit profoundly. As decomposers of plant material, they build soil and provide sustenance for bacteria and other microbes that are critical to environmental health.
Furthermore, fungi forge partnerships with plants, growing around and through their roots with delicate filaments of mycelium. Rather than strangle the plant, the mycelium delivers essential nutrients to trees and grasses, effectively giving the plant a larger, more effective reach into the nourishing soil. In times past, the fungi that grew in association with plant root systems were called “root hairs” and botanists assumed that they were a mechanism to increase root system permeability. However, in time it became clear that these so-called root hairs were in fact a different organism living on the scaffolding provided by plants.
The absorptive capacity of mycelium is tremendous, and symbiotic fungi serve up a smorgasbord of helpful resources to plant partners. Two striking plants that are 100% reliant on fungal partners are Monotropa uniflora, commonly called Indian pipes because of their fragile white flowers and Sarcodes sanguinea or snow plant, a lurid red flower that grows in the western United States. These two beautiful plants are not able to perform photosynthesis, and draw their entire diet of simple sugars from root-dwelling fungi that support them.
Networks of mycelium also aerate soils and maintain moisture levels as a service to microscopic flora and fauna. The U.S. Department of Agriculture recommends no-till farming in order to preserve the vast landscape of fungi that support plants and soil alike. Recent studies of fungi in agricultural settings have shown time and time again that fungi are very important in growing plants of all sorts successfully, and it’s abundantly clear that even the most human-tamed habitats fundamentally need fungal organisms in order to flourish.
At the end of the day, I think it’s critically important for the scientific community to continue its work unraveling some of the mysteries of fungi. The tricky way in which these fascinating organisms pivot in the face of environmental change, and their uncanny ability to survive the most extreme circumstances, leads me to believe that there is tremendous opportunity in studying various applications of mycology. As our planet continues to undergo drastic environmental, social, and ecological changes, we are going to need all the help we can get, and I think that in fungi we may find the answers to some of our more pressing and present problems.