This is the second installment of this year's Fungus Week, our more-or-less annual celebration of this exciting realm of life.
Maybe you have to be a mushroom enthusiast or a fungal ecologist to give this a thought, but counting the number of mushrooms in a tract of forest will not tell you the size of the fungal biomass therein. The mushrooms you see are only the fruit bodies. The whole fungal organism consists of an extensive growth and accumulation of invisible hyphae, the mycelium. Measuring fungi by counting mushrooms is like weighing an orchard by counting the apples on apple trees, only here not all “trees” produce fruit. To the consternation of wild mushroom collectors, the copious amounts of mycelial filaments existing in the soil and decaying wood may or may not produce mushrooms. What determines which mycelia will fruit, and how prolifically? In earlier times, this conundrum seemed difficult to unravel, but now, with high throughput sequencing available, this has become amenable to investigation.
A group of Norwegian and Finnish researchers carried out an intensive study to correlate the number of fruit bodies emerging from decaying tree logs with the abundance of the mycelia in the wood. The general conclusion was that for most fungal species, the more mycelial mass at a site, the greater the number of visible fruit bodies. This may not seem surprising, but the details, based on careful measurements, matter. For example, fewer fruit bodies were produced by those species whose fruiting is more energetically costly, such as the ones that display a cap sticking out from the surface of a tree (called pileated in the trade) as compared to those whose fruit bodies lie flat along the surface (known as resupinate). The quantities of both mycelial DNA and visible fruit bodies increased linearly with the increasing decay of the wood until the decay became quite advanced. From that point on, the amount of mycelial DNA continued to increase, whereas the fruit body count decreased. In other words, the mycelium goes on developing as the tree decays but this does not result in the concomitant formation of fruit bodies. These fungi find growing easier than differentiating under these conditions.
These measurements involved a great deal of work. The investigators used 100 large (20-42 cm in diameter and some 20 meters in length) Norway spruce logs whose stage of decomposition was duly evaluated. The work involved qPCR, high throughput sequencing, and tons of statistics. They found 198 species from the DNA data and 137 from the fruit body count. The mean number of species per log was ca. 16 and 9, respectively. Almost all the species encountered were bracket fungi. (Calling them mushrooms is a matter of usage. Some people don’t. I do.) All of the more abundant ones were woody or leathery in consistency, thus inedible. I presume that was a source of disappointment to the investigators, being that edible mushrooms such as shiitake and maitake do grow on trees. The pie chart shows that a few mushroom species dominated while most species were represented by relatively few finds. In their words: “…. species that are able to obtain a dominating position in the mycelial community possess a high fruiting rate, produce abundant fruit bodies, and have a high prevalence both as fruit bodies and as DNA, suggesting a positive feedback-loop.”
Until the advent of readily available DNA techniques, the study of fungal communities depended largely on identifying and enumerating fruit bodies. Now that the quantity of subterranean or tree-dwelling mycelia can be readily determined, a truer picture of fungal abundance emerges, thus revealing actual ecological relationships. For example, based on what the eye tells you, inclusion of some fungal species in red lists of threatened organisms may turn out to have been pessimistic. Some species are just stingy in fruiting. Their mycelia may be doing quite well and not be endangered.
Being able to measure the actual mass of the organisms can expand our knowledge of the presence and activities of fungi in the environment. As the authors say: “An interesting avenue for future research would be to examine what makes some species wait even decades until they form fruit bodies, and what triggers fruit body production.” Even those mushroom hunters who return from the woods with a nearly empty basket can take heart; the mycelium is lying awaiting.
Ovaskainen O, Schigel D, Ali-Kovero H, Auvinen P, Paulin L, Nordén B, Nordén J (2013). Combining high-throughput sequencing with fruit body surveys reveals contrasting life-history strategies in fungi. The ISME journal, 7 (9), 1696-1709. PMID 23575372