Up until recently, basidiomycete fungal spore release was thought to be a passive process highly dependent on the microclimate surrounding the fruiting body. New research shows that these amazing organisms can actively change the temperature surrounding their gills, which creates air currents that ultimately lift spores several centimeters into the air, where they can hitch a ride on stronger winds. This new study exposes how fungi benefit from being tightly grouped together, and how important fungal-water relations truly are.
Spore release throughout the fungal kingdom is as diverse as it is bizarre. Many ascomycete fungi discharge all of their spores at once in a single puff! The spores passing through the air creates enough momentum to create its own air current. The bird's nest fungi (Nidulariaceae) depend on raindrops to rapidly compress pillows of spore filled tissue which launches spores past the boundary layer of still air surrounding the fungus. The stinkhorns (Phallaceae) attract carrion beetles and flies by the stench of its spore filled gleba. These insects land on the gleba and unknowingly carry spores to other suitable habitats.
Emilie Dressaire and her team figured that some gilled mushrooms must have a more complex mechanism than a simple, passive spore release. So, they set off to understand the physiology behind the spore dispersal of three separate species of basidiomycete (Lentinula edodes, Pleurotus ostreatus, and Agaricus californicus). First, they compared water loss in these living mushrooms to transpiring plants. They found that these mushrooms release much more water than the plants. Secondly, they compared water loss between cut mushrooms, and mushrooms still in contact with their substrate. Interestingly, mushrooms still connected to their substrate released double the amount of water, relative to their cut counterpart. Many plants and animals have evolved mechanisms to prevent water loss, so these scientists realized there must be an ecological function for this degree of evaporation in these gilled mushrooms.
It turns out that the rapid evaporation leads to air cooling around the fungus. They compared air temperatures around the different fungi and found that underneath the mushrooms cap, temperatures were 1-2°C cooler than the ambient air. The mushroom itself was around 4°C cooler than the surrounding air as well. This provides an improvement in spore dispersal, as the cooled air creates a zone of denser air which ultimately creates an air current. Now we’re not talking about a gust of wind, but this slight difference in air density is enough to provide enough air flow for these microscopic spores to escape the dead air covering the forest floor.
We must remember the dynamic nature of the biological world. You can search the internet to find how may spores are released from certain fungi per minute. In fact, I did just that in an earlier post. I reported that Ganoderma applanatum releases 30 billion spores a day, but is spore release really this consistent? The paper I feature in this post, along with some personal experiences tell me otherwise. While hiking in a creek valley chock full of fallen trees years ago, I witnessed a moment in time of mass spore dispersal.
It was a really dry period in July, and the sun was just beaming. In the creek valley, I suddenly heard the green vegetation in the canopy above getting splattered with summer rain. It was one of those welcoming, sunny rains to occur with little notice. It poured for ten minutes, only for the incredibly hot rays to reclaim the valley. It was at this time where I came across several fallen trees engulfed in perennial fungi. There were several species of Ganoderma, and Trametes, as well as Trichaptum biforme and Schizophyllum commune. The angle of the sun hitting these fungi and their associated logs allowed me to see the mass of spores being released into the air. Now I realize, with the newly acquired water that these fruiting bodies absorbed, their temperature as well as the air surrounding them was lower than the surrounding air. The conditions that these species were experiencing triggered them to release their spores with the convective air currents. I will never forget the acrid, earthy smell of billions of spores being released into the air. This one experience taught me that fungi are opportunistic organisms that are triggered by external forces. The paper I just read however taught me that fungi can manipulate their own microclimate.
Spore dispersal in most gilled mushrooms was thought to be a passive mechanism up until about a year ago. We tend to miss what millions of years of evolutionary adaptations can physiologically ‘come up with,’ and both simplify and overcomplicate processes throughout nature. This study showed how vital water is for not only basic physiological developments in fungi, but for their active spore dispersal. It also shows why it might be advantageous for gilled mushrooms to form in groups, as more fruiting bodies can manipulate their microclimate more efficiently. By releasing copious amounts of water, the temperature surrounding these fungi plummets, creating convective cells that carry spores away from the stagnant air just above the forest floor.