Ecology and evolution

Amanita muscaria, c. Alison Pouliot

We now know that fungi evolved about 1.5 billion years ago from a single common ancestor with animals and as a separate group of living organisms. They are neither plant nor animal!

The first fungi were likely to be aquatic and to have first colonised the land 460 million years ago; about the same time as plants. 250 million years ago they were abundant in many places on earth and likely to have been the dominant life form. Now they live in ecosystems worldwide; from rainforests to deserts, streams and oceans and at the poles and the equator.

Fungi were originally thought to be plants and not very well developed ones at that.  The evolutionary ancestry of fungi was not well understood because the relatively soft tissues of fungi are generally not preserved as fossils. Fungi also have relatively simple morphologies and lack embryos which follow a definite sequence of development stages – features biologists often use to determine evolutionary relationships.

New molecular techniques comparing the DNA sequences of different species have changed our understanding of the evolution of fungi and suggest fungi are more closely related to animals than plants and that there are far more species than first thought.

Scientists estimate that 25% of the world’s biomass is fungal. A recent estimate of global fungal diversity is 2.2 to 3.8 million species, of which a mere 80,000 have been studied and described.

The long evolutionary history of fungi explains their great diversity, their adaptation to every ecosystem on Earth and their important interrelationships with other life forms.

Fungi perform vital ecological roles in the carbon cycle as the primary decomposers of organic matter. They cycle nutrients, provide shelter and sustenance to animals, invertebrates and microbes, promote disease resilience, conserve soil and have highly symbiotic relationships with many plants and algae.

However, one thing is certain – the world would be a very different place without fungi!

Fungi are neither plants nor animals. Here are some of the similarities and differences:

  • Plants are autotrophic, having chlorophyll that facilitates photosynthesis – the process of converting sunlight and carbon dioxide into oxygen and sugar. Fungi are heterotrophic which means they do not have chlorophyll and must absorb nutrients, including sugars, from organic matter.
  • Fungi can easily absorb elements essential for life such as phosphorus and nitrogen. Plants often have difficulty doing this, especially when conditions are less than ideal.
  • Both plants and fungi are made up of cells that contain nuclei and membrane-bound organelles. Plant cells, however, have hard protective walls of cellulose and fungal cell walls are made of chitin – a tough polysaccharide found in insect exoskeletons and similar to animal hair and fingernails.
  • Plants, fungi and animals are eukaryotic meaning, unlike bacteria, there is a membrane around the nucleus.

Their parts:

Hyphae and mycelia

The feeding body of a fungus (the mycelium) is made up of a mass of tiny microscopic filaments (hyphae) that grow and spread throughout dead organic material or soil, or wrap around plant roots. Hyphae also produce the sporing stage (fruit-bodies).

The size of individual mycelia can vary considerably, from microscopic to those that extend over several hectares. The mycelium of some Armillaria species may be the largest, oldest and heaviest single organisms on the planet. An Armillaria solidifies in the Malheur National Forest in Oregon, USA is the current record-holder at 2,400 years old and 5.6 kilometres across!

Hyphae grow outwards, branching and anastomosing (linking by cross connecting), and produce enzymes that soften and break apart the chemicals in organic material.  Hyphae absorb these simpler chemicals to fuel further growth.

While mycelia are usually present in the substrate where a fungus exists, they are fragile and microscopic and almost impossible to study and identify to species level without specialised equipment.

Spores, through which the fungus reproduces, are either produced directly by the mycelium, or by specialised structures (the fruit-bodies) that are periodically produced by the mycelium. Spores are always microscopic but can be seen en masse in the interior of puffballs, or by taking the spore print of an agaric.

The sporing-body is a dense aggregation of hyphae that appears on the surface when conditions are right, and can grow quickly and last only as long as it takes to disperse spores to produce the next generation. Others, like bracket fungi, have tough, perennial sporing-bodies. Sporing-bodies come in many forms, such as mushrooms, puffballs, brackets, corals, jellies and cups, and are often brightly coloured. Sporing-bodies are usually 90% water.