Exsudoporus frostii is another eye-catching species growing from the forest floor throughout North and Central America. Formally known as Boletus frostii, the species has been placed in the newly circumscribed genus in 2014-a process aspiring mycologists are all too familiar with. I feel like as soon as I learn a new mushrooms scientific name, taxonomists move the species in a new genus, or synthesize a new genus all together. The fungal kingdom is the most dynamic when it comes to shifting nomenclature. Although I’m biased, this kingdom is most dynamic in other ways, once you realize all of the different ecological interactions fungal species fill.
This particular species fills a mycorrhizal niche, supplying suitable trees with rare soil nutrients. In return for its resource gathering services, it receives sugars from its plant host. This mutualism represents ancient line of species interactions stemming all the way back to when the first species started inhabiting land around 470 million years ago. The most primitive land plants carry genes that allow lower fungi of the glomeromycota to colonize their roots. These genes have been conserved through millennia and even reside in the genomes of higher plants. Just like the plants conservation of genes associated with symbiosis, mushroom forming higher fungi too have a subset of these ‘sym genes’ from their glomeromycotan cousins. Though, just because one has these sym genes doesn’t make it a mycorrhizal species. For example, humans have millions of genes we don’t use. Much of our genome isn’t functional, is silenced and is retained simply because not enough time has passed for those genes to be lost.
So, you might be wondering how we designate a fungal species as mycorrhizal in the first place. Looking at these sym genes is a good place to start, but in higher fungi, this mycorrhizal ecology has evolved independently around 80 times so these genes can show up in a variety of positions on the genome. One can gently remove the soil and other forest debris from the base of the fruiting body to see where the mycelium is growing from. Even if you do find hyphal threads surrounding plant roots, that fungus could be parasitizing that plant and not engaging in a mutualism. Probably the coolest, most state of the art way to see the specific ecology of a fungus is to use isotope-ratio mass spectrometry.
An isotope is an atom with a differing number of neutrons. Isotopes of all elements occur throughout the universe and we have realized the ‘normal’ ratio of isotopes that occur here on Earth. Luckily for us, biological mechanisms discriminate against certain isotopes. For example, when a mycorrhizal fungus is packaging nitrogen to send to its plant host, the isotope Nitrogen 15 is discriminated against. Very little Nitrogen 15 makes its way into the plant, and there is an elevated amount of Nitrogen 15 within the fruiting body. On the other side, fungi that break down dead plant material accumulate the isotopes that those plants had access to. So a saprotrophic fungus that breaks down a mycorrhizal tree will have fewer Nitrogen 15 molecules than a saprotroph breaking down a non-mycorrhizal plant. For this reason, using Carbon isotopic ratios can elucidate these interactions.
Plants also discriminate against isotopes too. The heavier Carbon 13 atoms do not make its way into the sugar solution the plant sends to its fungal symbiote. Analogous to the Nitrogen 15 interaction, the plant accumulates the heaver Carbon 13 molecules in its own tissue, while its fungal counterpart becomes enriched with lighter Carbon molecules with just 12 protons and neutrons.
If we use isotope-ratio mass spectrometry on Exsudoporus frostii, we would see that the fruiting body would have elevated levels of Nitrogen 15 and lower amounts of Carbon 13 compared to its specific environment. Throughout North and Central America, the oaks Exsudoporus frostiiengages with will have elevated levels of Carbon 13. The use of stable isotope as an ecological tool has increased our ecological understanding significantly. Isotopic technologies have aided the challenging categorization of fungal functioning profusely. Atomic biases within forest floor inhabitants makes the saying “you are what you eat”, actually have ecological merit.