Fungal questions

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I got some impromptu questions asked of me during a recent classroom visit (ages: 13-14). About fungi, of all things. Here are the four I remember, with my polished answers (with none of the doodles I made in situ).

Why should we bother learning about fungi?

Fungi are extremely important in terrestrial ecosystems, for plenty of reasons. They are prominent decomposers, thus playing a big role in nutrient cycles. Over 90% of plants are symbiotic with fungi, symbioses that are important for growth and fruiting of the plants (Wang & Qiu, 2006); in fact, some have even speculated that fungi managed to become terrestrial by being endophytes in plants (Lewis, 1987). Some of these symbioses turn into parasitism and pathogenicity; the same applies for animals, where fungal pathogens are not uncommon and sometimes of medical importance.

They also serve as prey for myriad animals, from earthworms, to mites and ticks, to vertebrates. Among the invertebrates, there are often specialisations for fungal feeding. For example, some mites have highly-derived mouthparts shaped like stylets, used to pierce through fungal hyphae and suck out the contents. Griffin (1996) shows why this is so: fungi are very nutritious, containing high levels of glycogen and other polysaccharides, glycoproteins and peptidoglycans (up to 44% protein in dry weight!), and low concentrations of cellular DNA an order of magnitude less than what is found in bacteria and other eukaryotes. This nutritiousness has led to many fungi producing potent toxins for defence, collectively termed mycotoxins (this is why you shouldn’t eat any mushroom you haven’t surely identified).

What is the biggest misconception about fungi that you see?

That people use “yeast” to mean a specific taxonomic grouping. Yeast is actually a term used to refer to a specific morph of fungi – it’s not a phylogenetic grouping. Some yeasts are famous for their biotechnological uses (making bread and alcohol), others are known as significant parts of human skin or other flora (Candida, Malassezia) and potential allergens. But if you look at a phylogenetic tree of the fungi, those aren’t all so closely-related.

Also, the continued teaching of fungal biology as botany. This system is extremely antiquated, stemming back from the days when mushrooms were thought to be plants, and is recognised as bad practice by all botanists and teachers. I have no idea why it’s still persisting.

Is it true that fungi are related to animals?

Most likely, yes. Under the Opisthokonta hypothesis, fungi and animals belong to the Opisthokonta, along with diverse protists (Steenkamp & Baldauf, 2004): the choanoflagellates, ichthyosporeans, corallochytreans, nucleariids, and ministeriids. There are few morphological traits that they share, the only prominent one being well-developed flattened or plate-like mitochondrial cristae (Cavalier-Smith & Chao, 2003), although not even that one is universal (Zettler et al., 2001) and is also found in some non-opisthokonts (Sleigh, 1989). However, molecular phylogenies of all kinds overwhlemingly support opisthokont monophyly, and the clade is generally accepted.

Where is the most suprising place to find fungi?

In the oceans – we’re used to mushrooms growing in moist forests, so the idea of fungi growing in the oceans was pretty surprising when I first found out about it. I wrote a blurb about marine fungi here, and I’ve done more reading since that post. It turns out that fungi are important members of coral reefs, playing important roles in the bioerosion critical for setting up the rubble on which corals grow. They’re particularly effective as chemical borers, using organic acids to dissolve the calcareous skeleton. One such microborer is Dodgella, a chytridialid that can dig 40µm deep pits along the circumference of a coral skeleton, greatly reducing its stability (Wisshak, 2006). Marine fungi overall dominate the boring community in cold-water corals (Beuck & Freiwald, 2005).

More impressive is their apparently being able to live in subseafloor basalts (Ivarsson, 2012), but that still technically falls under marine.

References:

Beuck L & Freiwald A. 2005. Bioerosion patterns in a deep-water Lophelia pertusa (Scleractinia) thicket (Propeller Mound, northern Porcupine Seabight). In: Freiwald A & Roberts JM (eds.). Cold-Water Corals and Ecosystems.

Cavalier-Smith T & Chao EE-Y. 2003. Phylogeny of Choanozoa, Apusozoa, and Other Protozoa and Early Eukaryote Megaevolution. Journal of Molecular Evolution 56, 540-563.

Griffin DH. 1996. Fungal Physiology.

Ivarsson M. 2012. Subseafloor basalts as fungal habitats. Biogeosciences 9, 3625-3635.

Sleigh M. 1989. Protozoa and Other Protists.

Steenkamp ET & Baldauf SL. 2004. Origin and evolution of animals, fungi and their unicellular allies (Opisthokonta). In: Hirt R & Horner D (eds.). Organelles, Genomes and Eukaryote Phylogeny: An Evolutionary Synthesis in the Age of Genomics.

Wang B & Qiu Y-L. 2006. Phylogenetic distribution and evolution of mycorrhizas in land plants. Mycorrhiza 16, 299-363.

Wisshak M. 2006. High-Latitude Bioerosion: The Kosterfjord Experiment.

Zettler LAA, Nerad TA, O’Kelly CJ & Sogin ML. 2001. The Nucleariid Amoebae: More Protists at the Animal-Fungal Boundary. The Journal of Eukaryotic Microbiology 48, 293-297.