Fungi living in cattail roots could improve our picture of ancient ecoystems

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Fungi living in cattail roots could improve our picture of ancient ecoystems

The paleobotanist Az Klymiuk did not seek to improve the fossil science's understanding of plant-fungal associations. She just wanted to understand the environment in which some fossil plants lived. This question led her to focus on the modern roots of cattails and the fungi that live there. She discovered that mushrooms have more difficulty growing in the roots of cattails that are underwater. And this discovery, which is published in the journal Mycologia, could change the way we interpret parts of the fossils, including a large community of ancient terrestrial plants.

"I was studying fossil plants that are 48 million years old. In these fossils, we can see plant cells and everything they contain, including fungi. In describing these fossil fungi, I realized that we know very little about the fungi that now live in wetland plants," says Klymiuk, manager of the Field Museum's palaeobotanical collections and lead author of the Mycologia study. "There has been very little work, almost nothing in terms of ecology and distribution."

Fungi, which include fungi, molds and yeasts, are not plants - they are their own type of distinct organism, more closely related to animals than anything that is green. But almost all terrestrial plants have tiny fungi that live inside their roots. These microscopic fungi produce spins that snake through the soil, where they can break down dirt and absorb phosphorus from soil and rock. The plant host then uses this phosphorus to create energy (bonus points if you remember from high school biology that the energy is in the form of a molecule called adenosinine triphosphate-ATP). Without fungi, plants have much more difficulty extracting phosphorus from the soil. All this to say that plants need the fungi that live in their roots to create the energy they need to survive. In addition to these "useful" fungi, plants also contain an enigmatic group of fungi called dark wall endophytes - the verdict is still pending as to whether these fungi are weak parasites, latent pathogens or useful partners. What we do know is that most plants shelter them, including the fossil plants that Klymiuk worked with.

Klymiuk was curious about the possible ecological roles of these fungi, but he was also intrigued by the reasons why some fossil plants contained many root fungi and others very few. Since these fossil plants came from swampy areas millions of years ago, when they were alive, she decided to look for clues in modern wetland plants, which have much in common with the old ones: cattails.

To do this, it collected cattail roots from several wetlands, sampling plants from the shoreline to the deepest point of their growth. "I was out in waders with my undergraduate assistants, and we were digging up cattails. We get in there with a shovel and pull out this giant plant as big as you are," says Klymiuk, who started working on the project at the University of Kansas with co-author Benjamin Sikes. "Memorably, in a tank, I had literally just told Abby[my undergraduate student] to be careful because there was a sudden fall, and what had I done? Ker-splash, just above. This was our last site that day, so fortunately we were near the laboratory, which meant that I could at least make a quick wardrobe change before preparing the samples."

Back in the lab, she examined the roots under a microscope and compared the fungi in the roots of cattails growing at different depths underwater. She discovered that, at least in these cattails, mushrooms do not do well with flooding at all. It didn't matter if they grew in two inches of water, or if the four-foot cattails growing in the water had very few fungi in their roots. On land, however, the cattails had many fungi that lived in their roots, comparable to the roots of the base that Klymiuk had collected along the cattails. "It turns out that any degree of flooding, whatever it may be, massively suppresses the amount of fungi in the roots of plants," explains Mr. Klymiuk.

Since the number of fungi living in plant roots suggests whether plants grow entirely in water or on shore, scientists studying fungi in the roots of fossil plants can better guess in which environment these plants lived. "It gives us a new way of understanding what we see in fossils," says Klymiuk. "I have roots where the fungi are completely absent. I can look, look and look, and there's nothing there. I have other roots that are just loaded, wrapped up. In my opinion, this indicates that we are dealing with different levels of flooding. I am quite confident that when we find a lot of fungi in a plant root, that root probably hasn't been flooded in its lifetime."

In addition to giving scientists a new tool to understand what some prehistoric ecosystems looked like, this study also suggests that part of our understanding of the fossil record of plant-fungal interactions may require recalibration. "There was this ubiquitous story, throughout biology," says Klymiuk. "The basic idea is that plants need fungi to get out of the water, to reach the earth. The oldest preserved community of terrestrial plants, Rhynie Chert, 407 million years old, is often cited as fossil evidence in this regard. This terrestrial plant community has been interpreted primarily as a wetland assemblage associated with an overflow of hot springs or land application, and many of these early terrestrial plants contained mutualist fungi, just as most living plants do today. What my research suggests is that these plants have probably been victims of intermittent flooding, instead of living in the water."

Did the plants need mushrooms to reach the mainland? "I'm not against the fact that they found the fungal partnerships extremely useful once on land, but I don't think proto-plants would have had to worry about them if they were in water, because phosphorus is easily bioavailable in water," says Klymiuk. "My personal feeling is that the first terrestrial plants were probably never in the water in the first place. There are many groups of green algae that are entirely terrestrial. I think it is very possible that terrestrial plants evolved from entirely terrestrial green algae, and that water was a secondarily invaded habitat. Stay tuned, there's a lot of really cool research emerging on this issue."

Klymiuk's work is not only about the past, but also about future life on Earth. "It is important for us to understand these relationships, because many of our cultures and forests are threatened by climate change," says Mr. Klymiuk. Not only are we facing more and longer floods (and more droughts, and more droughts, and more of everything), but it is very likely that the way plant-fungal interactions work will change as we progress in a world where CO2 is higher - we know that some groups of mushrooms are "better co-tenants" under low CO2 than under high; they pay their rent on time. Increase CO2 and suddenly, some of them fall behind phosphorus as rent, or order pizza on their host's credit card (using excess photosynthesizers at the expense of the host plant). I can make this analogy in twelve different ways, and it will always end with "we don't know enough about how these systems work to generalize or predict something with confidence". It's still very little studied."

Beyond the potential applications of this work, Klymiuk says she is excited about the project in the interest of discovery. "There's this mysterious and microscopic world that we don't usually think about. I am really fascinated by plants and their evolution, because a large part of paleobotany is detective work. I get excited by putting these puzzles back together and pruning some fundamental mysteries."