There are fungi at the bottom of the ocean

800px-Loggerhead_Sea_Turtle_(Caretta_caretta)_2

Oh noes!

Ok, I don’t actually know about the bottom of the ocean, but Jullie Sarmiento-Ramirez and colleagues have shown in a recent paper that fungal pathogens are a major problem for endangered sea turtles. Two different fungi are killing baby turtles before they even leave the nest!

The culprits are two species in the genus Fusarium.

Fusarium are ascomycetes, meaning they produce spores that are contained in sacs called asci. For Fusarium, these look a bit like pea pods, with the spores lined up like peas inside. Fusarium are filamentous fungi, meaning that they form hyphae that grow into a large mat called a mycelium.Sum_web

At least one Fusarium species, Fusarium oxysporum, can cause disease in immunocompromised humans. Other Fusarium species cause disease in soywheat, barley (including those ever important malt products), and tomatoes. Yet another Fusarium species is responsible for the Race 1 Panama disease that’s wiping out bananas worldwide. But don’t worry that Fusarium are going to eat us off the planet: one species is used as a delicious meat substitute. (No, really!)

But to get back to the sea turtles: Basically, the fungus, which lives in sand and clay, feeds on any sick, stressed, or dead eggs in a nest and forms larger and larger fungal colonies, or mycelia. Then the spores can spread to healthy eggs nearby, killing the turtles inside.

Arya-Stark-150x150

Don’t give up, Arya, you’ll find your family!

Some nests lose as many as 9 out of every 10 eggs. For the endangered Loggerhead sea turtle Caretta carettaa single nest has around 120 eggs (ranging from about 60 to 170 in one study). So, an infection with these fungi, Fusarium falciforme and Fusarium keratoplasticum, can leave the nest with only 10 live baby sea turtles to hatch, fight their way past all the other predators, and make it to the open ocean. Those odds are only slightly better than the Stark family has of surviving until Winter.  And the sea turtles don’t have direwolf companions. 

Fusarium like the heat.

The whole problem boils down to temperature and here’s why: The temperature that sea turtle eggs incubate at is exactly the same temperature that Fusarium likes: 82-86 F (28-30 Celsius). The exact temperature of a sun-warmed sandy nest on a summer night. In an interesting intersection with sea turtle biology, the temperature of the nest is also important for determining whether the eggs will produce male or female turtles – cooler eggs will be male, while warmer eggs will be females. So, by growing at 82 degrees, Fusarium is moving into a temperature niche that’s a vital part of the sea turtle life cycle. This is called adaptation to the host environment and it’s a change we often see in fungi that can cause disease in humans. For example, the yeasts Cryptococcus neoformans and Candida albicans are able to grow at human body temperature, unlike yeast that don’t cause disease.

One other factor that the scientists studying the sea turtles noticed was that some nests were more prone to severe disease than others. They tracked this down to the clay content of the sand and the amount of tidal flooding that the nest experienced. The more clay or the more tidal inundation, the higher the level of disease. These are worrying trends given the erosion of beaches in areas where these turtles nest.

So, how did the scientists figure all this out? Well, as early as the 1980’s, a number of people around the world have been reporting that something is going wrong in sea turtle communities. At first, it wasn’t clear exactly why eggs were failing to hatch -were the turtles infertile? Were they infected with bacteria? Gradually, it emerged that one problem might be infection with Fusarium fungi. There is huge diversity in the Fusarium genus, and Sarmiento-Ramirez and colleagues were able to narrow that down to a species complex called Fusarium solani. 

A species complex is a group that may contain multiple species that haven’t yet been distinguished. Imagine not being able to tell the difference between horses (Equus ferus caballus) and donkeys (Equus africanus asinus). Now imagine a huge herd of horses and donkeys all mixed together. Looking at them from a distance, you might be able to tell that they’re not all the same, but you have to look closer to figure out what exactly the mix is made of. So, until you did, you might call it the Equus species complex.

In order to solve that problem, these scientists started by looking closely at fungi that they collected from dead sea turtle eggs. (Click here to go to their paper and look at their images in Figure S3). They compared the spores and hyphae to known samples and to other samples that had been identified as Fusarium solani and confirmed that, yes, these species could be grouped in the same species complex. But in order to figure out exactly which species made up the species complex, they had to look at DNA sequences.

By looking at several sequences of DNA from the mixed groups of fungi that they isolated from lots of different sea turtle nests, Sarmiento-Ramirez and colleagues found specific DNA sequences that are unique to different species. They used this information to figure out exactly which species they were dealing with. From there, they were also able to build a family tree, known as a phylogeny, which can inform us about similarities and differences between species. (Click here to go to their paper and look at their phylogeny in Figure 3). Using this type of analysis, they found that the two species were similar to species that have been reported to cause disease in plants and mammals. They also demonstrated that isolates from turtle eggs can cause disease in potatoes (Click here to go to their paper and look at infected potatoes in figure S5).   

So, why is it so important that we know what species are responsible for the disease? First, there’s the potential for us to help save some baby sea turtles by treating infected eggs or by at least preventing erosion and changing sand content to slow fungal growth. Second, there’s the risk that these pathogens will make the transition from being a problem for sea turtles to being a problem for people by killing yet more food crops. Research like this makes it clear that when Fusarium and other fungal pathogens are left to their own devices, sea turtles may go the way of the Dodo, and the banana may soon follow.

Citation: Julie M. Sarmiento-Ramirez, Elena Aleblla-Perez, Andrea D. Phillott, Jolene Sim, Pieter van West, Maria P. Martin, Adolfo Marco, Javier Dieguez-Uribeondo. Global Distribution of Two Fungal Pathogens Threatening Endangered Sea Turtles. (2014) PLoS One DOI: 10.1371/journal.pone.0085853

This research is published in the OpenAccess journal PLoS One. You can check out this paper, including pictures of infected sea turtle eggs, for yourself, here. With thanks to Dr. Katja Schaefer for images of Fusarium microconidia, hyphae, and mycelia.

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