This past weekend, I traveled down south to Nashville Tennessee for a much-needed vacation. After just three days of experiencing the boisterous Music City, it was time to come home. We were in desperation mode driving back to Western New York, as we all didn’t want the road trip to end. Just an hour on the road, it had dawned on us that we had passed through a cave system in central Kentucky en route to Nashville. We googled and realized that this was not just any cave system, this was Mammoth Cave National Park! The GPS destination was changed and excitement spread throughout the car like a pathogenic fungus as we officially extended our vacation.
Paying a five-dollar admission fee to see the longest cave system known on Earth seemed like a bargain. We followed the trail and made our way to the main cave. One of my favorite parts was actually climbing down into the main entrance of the cave. I was presented with a clear cross section of the forest floor, with trees growing from a 2-meter layer of soil, which then turned into layers and layers of broken up and solid rock. Water from the above ecosystem cascaded over the cave entrance as a forest waterfall. This kept the surrounding rocks moist, permitting dense mats of mosses and ferns.
I grew eager entering the cave, wondering if I would encounter any fungi well beneath the forest floor. Understandably, there was nothing from the fungal realm that I found. Although it is a moist and dark place with a very suitable temperature for countless fungal genera, there is no light for plants to grow. This idea highlights the importance of bottom up control in ecosystems everywhere. Systems without organisms that create their own carbohydrates depend on nutrients that trickle in from surrounding ecosystems (allochthonous sources). These cave systems don’t receive many allochthonous resources so they remain relatively depauperate. Without a steady supply of large amounts of woody substrate, not many saprophytes are found in systems like the Mammoth caverns that are largely cut off to the forest floor above.
I have seen pictures of fruiting bodies in caves, with some of them being mycorrhizal species growing from penetrating roots through cave ceilings. In some other pictures I found, mushrooms grow from woody debris that has made its way into these unique habitats. Most other fungi in these systems exist as parasites, infecting the different animals that make caves their home. Cordyceps species infect and consume unlucky cave crickets, but these fungal species are not the focus of today’s Fungi Friday.
The animal that goes hand in hand with caves is the bat. Bat species in North America are vital to the health of many deciduous forest ecosystems as they play roles of plant pollinator and keep insect populations in check through predation. Unfortunately, an invasive fungus has made its way to North America most likely vectored by us humans, and these flying mammals have been hit hard. This destructive fungus is aptly named Pseudogymnoascus destructans and causes the detrimental disease, white-nose syndrome. When infected, fungal hyphae and conidia cover bat’s noses, ears and wings appearing as a white fuzz. These hyphae penetrate and break down the bat’s tissue, and greatly reduce the function of many physiological processes.
There was a time not too long ago when a person entering Mammoth Cave would encounter millions of bats. It was estimated that just over one decade ago, there were 9-12 million bat individuals inhabiting world’s longest cave system. Unfortunately, this fungus is not a bat specialist and can infect a wide range of bat species. The five different bat species endemic to Mammoth Cave have been nearly extirpated. Now, this huge cave system is occupied by just a few thousand bats.
In 2006, Pseudogymnoascus destructans was unknowingly brought to New York from Europe. Each year after its introduction, its distribution spread, now infecting several bat species in 28 northeastern American states. This species of fungus is categorized as psychrophilic, a cold-loving organism with its temperature optimum being between 4-20°C, perfect for infecting bats residing in caves. Studies that physically separated infected and healthy bats, showed that the fungus doesn’t become airborne, and only spreads through physical contact. This clearly has not perturbed its transmission. A side effect of white-nose syndrome is hypothermia, so, in attempt of keeping warm, infected bats nestle in with other bats, thereby spreading the fungus to knew hosts.
Another feature that has aided the spread of Pseudogymnoascus destructans is its ability to break down other deceased cave organisms. This dual ecology being a bat pathogen and cave saprophyte broadens its niche width, allowing it to persist in caves for much longer periods of time. A paper published in 2014 by Hannah Reynolds and Hazel Barton tested the enzyme activity of Pseudogymnoascus destructans and phylogenetically similar species within the same genera. These researchers found that P. destructans still produces enzymes involved in a saprophytic life strategy, including lipases, hemolysins, urease, chitinase and cellulases. Only, the bat pathogen produces significantly less urease and endoglucanase than its closely related cousins that are solely saprophytic.
It is looking rather bleak for bats in the eastern United States, but it is all not doom and gloom. A relatively new paper by Hoyt et al. 2015 showed a potential way we could combat this fungus. These researchers isolated several strains of bacteria growing on the skin of living bats. The goal of their study was to identify possible bacteria that could enhance disease resistance. They grew these different strains with cultures of P. destructans and found six strains of Pseudomonas bacteria that significantly inhibited fungal growth. Two of these Pseudomonas strains performed better than the other four, suppressing the fungus for 34 days!
Many times, I reveal the positive influence fungi have on the ecosystems they inhabit. I always highlight how they enhance ecosystem functioning and species diversity, but when they show up where they have been evolutionarily absent, they can have an insanely detrimental effect. Unlike other aesthetically pleasing species of fungi usually featured in these weekly Fungi Fridays, P. destructans is a serious threat to the forest ecosystems of the United States. Evolutionarily, the fungus has moved away from a saprophytic ecology, yet has still held on to enzymes that allow it to persist in caves without bats. We are doing our best to come up with solutions to aid these incredibly important mammals, and the development of a Pseudomonas probiotic looks promising, but there is still much more research to be done. Bats that have co-evolved with the fungus in Europe might hold onto the answers we a are looking for. Let this be a reminder of how fragile forest ecosystems are, and that threats well beneath the forest floor lurk, and have the potential to alter the natural communities we love.