Aquatic Fungi
I have recently taken up searching for mushrooms in our lovely Missouri forests. Like everyone else it seems, the obsession began with morels. However, it didn’t take long to realize there are seemingly countless types of mushrooms in the woods. Fungi have an important job to do, and they aren’t confined to just the forests. At the Lakes of Missouri Volunteer Program we most frequently talk about bacteria, phytoplankon (see Venn diagram below), zooplankton (small crustaceans that eat phytoplankton), and sometimes even fish. However, to my knowledge, we haven’t yet addressed a large group of aquatic life, the fungi.
Terrestrial fungi perform many ecologically important functions including decomposing organic matter, transforming nutrients into forms usable by non-fungal organisms, forming symbiotic relationships with other organisms (e.g., lichens), and as a direct food source for other organisms (like me!). Aquatic fungi are not nearly as well studied but perform the same ecological services.
Perhaps the most studied is the role of aquatic fungi in breaking down organic matter. Fungi can break apart large pieces of plant matter, making them easier for invertebrates to consume. Small leaf parts colonized by fungi provide invertebrates with a better food source than leaf parts without fungi. Think of the leaf parts as plain white bread and the fungi as a layer of peanut butter. Additionally, fungi transform organic matter that is otherwise unusable to other organisms into an easily consumed form. One group of aquatic fungi have been observed living in the guts of larval aquatic flies, mayflies, and stoneflies and may aid in digestion. The spores of some aquatic fungi, called zoospores, can swim (yes, SWIM!), and are grazed upon by zooplankton.
Two sporangia attached to a phytoplankton (diatom) cell. Image souce: Creative Commons
Fungi Controlling Phytoplankton Blooms
Some members of one group of fungi, the chytrids, have been found to control phytoplankton blooms. In short, chytrid zoospores swim through the water column, seeking out phytoplankton cells. Once they find a cell, the zoospores attach and begin consuming the organism. The nutrients harvested from the cell are used to grow the fungus and eventually create a “pod” of spores, called a sporangium (plural=sporangia), that will open to release another 4-28 zoospores. Those zoospores swim around looking for more phytoplankon, and the cycle continues.
Many phytoplankton cells are too large to be eaten by zooplankton. When these cells are infected by chytrid fungi, some of the nutrients within are converted to zoospores which can be eaten by zooplankton. The zooplankton may be eaten by fish, moving those nutrients up through the food web. In this way, chytrids transform otherwise unavailable phytoplankton nutrients into something usable by animal life. This nutrient pathway even has a neat name, the "Mycoloop.”
Chytrids are found all over the world. In some aquatic environments, there may be nearly 300,000,000 zoospores in a single gallon of water. But aquatic fungi are even more prevalent than that, as chytrids are just one of the many groups of fungi in our waters. Most terrestrial fungi are still unidentified, and the aquatic fungi are not nearly as well studied as the terrestrial. As recently as 2010, scientists described the first underwater mushroom. This mushroom was found in the Rogue River of Oregon and its stipe (or stem) was rooted over a foot and a half deep to help it withstand the river current.
The underwater mushroom, Psathyrella aquatica. Photo by Robert A Coffan. "Psathyrella 1-headwaters-of-the-rogue-river-august-3-2007" by OggiScienza is licensed under CC BY-ND 2.0
It’s not as if we need another environmental issue to worry about, but there are concerns about the effects of fungicides in aquatic habitats. In Switzerland and Norway, antifungal pesticides were second in prevalence to herbicides detected in rivers flowing through agricultural areas. Because so little is known about the ecological role of fungi and particularly aquatic fungi (compared to phytoplankton or fish, for example), we don’t know what the ramifications of fungicide pollution are. Some studies have shown reduced leaf litter breakdown, shifting invertebrate feeding preference, changes in the fungal community, and reduced fungal biomass in aquatic systems with exposure to varying levels of water-borne fungicide.
There is much we don’t know about aquatic fungi and its role in our streams, lakes, and oceans. From my research for this article, it appears the science community has recently issued a call to action for more research into these fascinating organisms. Hopefully scientists grab the torch and carry it so we learn more about this fascinating and important kingdom of life in our water.
Tony Thorpe, LMVP Project Manager
Grossart, HP., Van den Wyngaert, S., Kagami, M. et al. 2019. Fungi in aquatic ecosystems. Nat Rev Microbiol 17, 339–354. https://doi.org/10.1038/s41579-019-0175-8
Ittner, L.D., Junghans, M. Werner, I.. 2018. Aquatic Fungi: A disregarded trophic level in ecological risk assessment of organic fungicides. Front. Environ. Sci. 25. https://doi.org/10.3389/fenvs.2018.00105
Frank, J.L., Coffan, R. A., Southworth, D. Aquatic gilled mushrooms: Psathyrella fruiting in the Rogue River in southern Oregon. 2010. Mycologia 102, 93-107. https://doi.org/10.3852/07-190 - Full Paper
Kagami, M., de Bruin, A., Ibelings, B. W., Van Donk, E.. 2007. Parasitic chytrids: their effects on phytoplankton communities and food-web dynamics. Hydrobiologia 578, 113–129. doi: 10.1007/s10750-006-0438-z
Phytoplankton, Algae, Cyanobacteria?
Science changes all the time. That's part of its charm. When we learn more about something, the science gets updated. Admittedly, it can be confusing keeping up with the changes.
Cyanobacteria used to be called "bluegreen algae." It was handy to be able to use the term "algae" and mean both algae and cyanobacteria. While handy, it was also problematic because cyanobacteria aren't algae at all, they are bacteria. There's a big difference between the two!
From now on, the LMVP will be using the term "phytoplankton" instead of "algae" when referring to photosynthesizing organisms out in the lake. Hopefully the Venn diagram below will help make the distinction.
Algae and cyanobacteria (formerly "bluegreen algae") are part of the larger phytoplankton community, as shown in this Venn diagram. When we discuss phytoplankton, we are referring to both algae and cyanobacteria.
Glossary
Algae: A group of photosynthesizing aquatic organisms that have a nucleus and other organelles. A subgroup of the phytoplankton.
Chytrid: A phylum of aquatic fungus that produces motile spores.
Cyanobacteria: A group of aquatic photosynthesizing bacteria. A subgroup of the phytoplankton.
Mycoloop: A pathway involving the movement of nutrients from phytoplankton to fungi and then to zooplankton in the water column.
Phytoplankton: Term used to describe photosynthesizing organisms. Includes algae and cyanobacteria.
Sporangium: A structure in which asexual spores are formed.
Symbiotic relationship (or symbiosis): A biological living arrangement between two different species of organisms that may benefit either or both organisms. There are many subcategories of symbiotic relationships.
Zoospore: A spore that can swim using a flagellum.