Back in the 1940’s a new theory was synthesized called thermal melanism theory. This theory suggests that ectotherms (organisms that do not regulate their body temperature internally) like reptiles and insects, tend to be darker in cold climates, and lighter in warm climates. The darker pigments found in ectotherms in colder climates absorb solar radiation really well, while lighter colored ectotherms in hot environments reflect much of the same radiation. I really do love discovering ecogeophysical patterns like this in nature. So many times, these concepts can be utilized again, and bring more insight about our natural world. Just earlier this year, Franz-Sebastian Krah and his team revisited this theory from nearly 80 years ago, only this time tested it through a fungal perspective.
From the initial fruiting to proper spore development, the majority of a mushroom’s sexual development are temperature dependent. Not only did the authors of this 2019 study recognized that optimal temperatures are vital to fungal maturation, but that a mushroom that fruits above the forest is more exposed to the elements than its mycelia beneath the soil’s surface. They also took into consideration how energetically expensive pigments are for fungi to produce. Certain fungal pigments have been shown to provide the individual with fitness enhancing properties like disease resistance for example, but could these pigments play an even larger role? Krah and his team set off to study the possible analogous nature of thermal melanism theory, broadening this ecogeophysical pattern to the mycological realm.
Before addressing this fungal thermal hypothesis, these researchers gathered a mammoth data set of 3.2 million observations of 3,054 species across Europe. By measuring lightness values from digital images, mushrooms were scaled from 0 to 100. To test if mushroom pigmentation correlated with the thermal environment they fruited from, three measures of temperature were taken into account; mean temperature, variability of temperature and seasonality. This study revealed several patterns, but as a general rule of thumb, darker mushrooms were shown to dominate colder environments. Their original hypothesis was supported by these findings.
Taking this one step further, the ecology of each species was also investigated, and with this, another interesting pattern presented itself. Not only do mushrooms tend to have a darker coloration in colder places further from the equator, but mycorrhizal mushrooms too seem to have darker pigments worked into their tissue. Interestingly, lighter colored saprotrophic fungi actually carry out a seasonal shift, becoming darker throughout the colder months. Mycorrhizal mushrooms are significantly darker than their decomposer counterpart, which, the authors explain, could be a justified by the carbon availability delivered by their plant partners.
Since digestible plant sugars become abundant as a fungus enters a mycorrhizal relationship, more energy can be allocated to the production of ‘expensive’ pigments. Obtaining carbon as a saprotroph isn’t as lucrative with the production of digestive enzymes taking up much of the organism’s energy demand. Mycorrhizal mushrooms can simply afford being darker. Another possible explanation the authors bring up is that mycorrhizal species that pair up with tree roots are usually found in forests with a closed canopy. Mushrooms fruiting from the forest floor under dense vegetation receive less solar radiation, so dark pigments help the individuals get the most out of the fewer photons that don’t get intercepted by the plants above.
Seasonal shifts in fungal pigmentation and the darker shades mycorrhizal fungi are fitness enhancing adaptations that are analogous to the ecogoephysical patterns found in ectotherms. This new research reveals that thermal melanism theory is not only supported ‘cold blooded’ animals, but by fungi as well. Furthermore, this analysis revealed that fungal lightness is slightly increasing (though not statically significant) over the past 40 years. As global mean temperatures continue to rise, light colored mushrooms are likely to increase in abundance, as costly pigments aren’t required for fruiting bodies to absorb solar radiation. Dark colored mushrooms are at a clear advantage in cold habitats, and this study provides the first evidence of a morphological trait (fungal pigmentation) has an influence structuring fungal communities.