Similar to most life that has evolved on Earth, initially, fungi were aquatic organisms. New fossil evidence shows that before they gained a foothold on terrestrial habitats, the first fungi carried out their lifecycle beneath the ocean floor nearly 2.4 billion years ago! These organisms have been around for half of our planets existence, and since their genesis, have adapted to virtually every niche Earth has to offer. Now fungi didn’t just jump out of the ocean and form mycorrhizal relationships with plants, but evolved through a series of stepwise adaptations, which ultimately led to its advancement on terrestrial ecosystems. From the ocean depths, fungi adapted to freshwater rivers and streams. This leads us to the fungus featured in this week’s edition of Fungi Friday, Batrachochytrium dendrobatidis.
Batrachochytrium dendrobatidis (hereafter Bd) is from an ancient fungal lineage within the division Chytridiomycota. There are around 1000 different species of chytrid fungus but out of all of them, Bd has without a doubt, the most impact on Earth’s environments. It is the culprit of the extirpation and extinction of several amphibian species throughout the world because of its parasitic ecology. Not only are tropical frog populations being decimated by the fungal parasite, but amphibians in my own neck of the woods face chytrid adversity.
A few years back, I worked on a conservation project to protect a rare, rather large salamander called the Eastern hellbender (Cryptobranchus alleganiensis). Native throughout the northeastern United States, this amazing creature has been facing chytrid fungi for 65 million years, so why now is Bd such a threat? Well, the pet trade has vectored amphibians along with their genetically distinct strains of Bd to ecosystems all over. This has allowed the different chytrid strains that were once geographically isolated to genetically recombine. This mixing of genes has resulted in a hyper virulent strain of Bd, aptly named BdGLB, referring its now Global distribution.
Under the scope, one can easily see the adaptions present for living in an aquatic habitat. Unlike fungal spores that use wind dispersal, Bd along with other species of chytrid release flagellated zoospores. These tiny spores not only actively swim, but also have the ability to sense their surroundings, increasing their chances of finding a suitable host. With this chemotaxic response the individual spores use the chemicals, or kairomones emitted by the amphibian to locate it, where it then attaches to the skin and begins to penetrate through the skin’s surface.
The imbedded zoospore then grows germ tubes that penetrate the amphibians keratinized dermal layers. Not only does Bd utilize keratin as a carbon source, but these tubes can also penetrate the hosts skin cells, and absorb cellular cytoplasm. Subdermal structures called thalli form, which ultimately develop into zoospore bearing sporangia. As zoospores release back into the aquatic environment, many Bd spores re-encyst in the same host, causing an all-out infestation.
Amphibian skin plays an integral role in the organism’s physiology by regulating its internal water content, transporting oxygen into its body, and balancing its electrolytes. Bd thereby reduce these functions, rendering the organism completely helpless. If the infected, now sluggish amphibian doesn’t succumb to predation with its internal biology completely compromised, it soon dies either from cardiovascular failure with no electrolytes to regulate its heartbeat, or hypoxia, with little to no oxygen entering its body. Many hellbenders found dead with chytridiomycosis are stranded on land, presumptuously as a last-ditch effort to acquire oxygen.
Now it’s not all doom and gloom, as many scientists have been working together to come up with creative ways to combat the virulent strain of Bd. The most promising work has come out of the researchers analyzing the dermal communities of amphibians. Sandra Flechas and her team back in 2012 examined the skin from a population of stream-dwelling harlequin toad that was largely unaffected by Bd. Out of the 148 bacteria they isolated living on these toad’s skin, they found that 26% of them produced peptides and other anti-fungal compounds that limit Bd growth.
Amphibian skin is essentially a small-scale ecosystem. Bacterial communities living on dermal surfaces compete for nutrients and space. Bacteria that compete better, have a better chance of surviving and passing their genes to the next generation. It’s no surprise to me that this fungal disturbance has been met by bacterial communities that have anti-fungal properties. This is life on Earth. All organisms have this innate drive to compete to survive. Although amphibians might have trouble coexisting with this new strain of Bd directly, the once commensal bacteria that live on its skin are not backing down. Bacteria have a much quicker lifecycle, allowing faster adaptations to arise. Humans can utilize these rapidly evolving mutualist interactions to help reduce the impact of the detrimental fungus. Nature with its adaptive nature never ceases to amaze me. It is really up to us to identify solutions that our living environment is already solving.