Study Highlights the Key Role of Fungus in Global Warming

Small algae present in Earth’s lakes and oceans absorb carbon dioxide (CO₂) and sunlight and convert them to sugars that preserve the rest of the aquatic food web, consuming as much carbon as all the plants and trees of the world combined together.

A new study has revealed that an important piece has been missing from the traditional explanation of what occurs between the first step of fixing CO₂ into the phytoplankton and its ultimate discharge into the air or descent to depths where it no longer plays a role in global warming. In this case, the fungus is the missing piece.

Anne Dekas, an assistant professor of Earth System Science from Stanford University stated, “Basically, carbon moves up the food chain in aquatic environments differently than we commonly think it does.”

Dekas is also the senior author of the article published in the Proceedings of the National Academy of Sciences on June 1^st, 2021, that measures the amount of carbon that goes into parasitic fungi that attack microalgae.

Underwater Merry-Go-Round

To date, it has been predicted that most of the carbon fixed into colonies of single-celled, hard-shelled algae called diatoms subsequently funnels directly into bacteria, or dissolves like tea in the surrounding water. This dissolved carbon is mostly taken up by other bacteria.

The prevalent theory is that carbon escapes from this microbial loop chiefly through larger animals grazing on diatoms or the bacteria, or via the CO₂ to returns to the air as the microorganisms breathe. This journey is significant in the context of climate change.

According to Dekas “For carbon sequestration to occur, carbon from CO₂ needs to go up the food chain into big enough pieces of biomass that it can sink down into the bottom of the ocean. That’s how it’s really removed from the atmosphere. If it just cycles for long periods in the surface of the ocean, it can be released back to the air as CO₂.”

However, the fungus produces an underappreciated express lane for carbon, 'shunting' around 20% of the carbon fixed by the diatoms out of the microbial loop and into the fungal parasite.

Instead of going through this merry-go-round, where the carbon could eventually go back to the atmosphere, you have a more direct route to the higher levels in the food web.

Anne Dekas, Assistant Professor of Earth System Science, Stanford University

The results also have implications for recreational and industrial settings dealing with harmful algal blooms. Dekas added, “In aquaculture, in order to keep the primary crop, like fish, healthy, fungicides might be added to the water.”

While that will prevent fungal infection of the fish, it may also prevent the natural check on algal blooms that cost the industry around $8 billion every year. Dekas further added “Until we understand the dynamics between these organisms, we need to be pretty careful about the management policies we’re using.”

Microbial Interactions

The researchers based their estimates on experiments performed with chytrid fungi known as Rhizophydiales and their host, a kind of freshwater diatom or algae, called Asterionella formosa. The study co-authors from Germany isolated these microbes and also the bacteria found in and around their cells, from water obtained from Lake Stechlin located around 60 miles north of Berlin.

Isolating one microorganism from nature and growing it in the laboratory is difficult, but isolating and maintaining two microorganisms as a pathosystem, in which one kills the other, is a true challenge.

Isabell Klawonn, Study Lead Author, Stanford University

Klawonn worked on the study as a postdoctoral scholar in Dekas’ laboratory at Stanford University. Klawonn added that “Only a few model systems are therefore available to research such parasitic interactions.”

As early as the 1940s, scientists had concluded the important role of parasites in controlling phytoplankton abundance and they observed that epidemics of chytrid fungus infecting Asterionella blooms in lake water.

Technological advancements have made it easier to inspect these invisible worlds in measurable and fine detail—and start to see their impact in a relatively bigger picture.

We’re realizing as a community that it’s not just the capabilities of an individual microorganism that’s important for understanding what happens in the environment. It’s how these microorganisms interact.

Anne Dekas, Assistant Professor of Earth System Science, Stanford University

The interactions within the Lake Stechlin pathosystem were analyzed and measured using genomic sequencing—a combination of fluorescence microscopy technique in which a fluorescent dye is attached to RNA inside microbial cells and a highly dedicated instrument at Stanford University called NanoSIMS, which is one of only a few dozen in the world.

This instrument produces nanoscale maps of the isotopes of elements that are present in materials in vanishingly tiny amounts.

Dekas added, “To get these single-cell measurements to show how photosynthetic carbon is flowing between specific cells, from the diatom to the fungus to the associated bacteria, it’s the only way to do it.”

The precise amount of carbon diverted to the fungus from the microbial merry-go-round may vary in other environments. However, the discovery that in one setting it can be as high as 20% carbon is significant.

Dekas remarked, “If you’re changing this system by more than a few percent in any direction, it can have dramatic implications for biogeochemical cycling. It makes a big difference for our climate.”

Stanford co-authors include Alma E. Parada and Nestor Arandia-Gorostidi, postdoctoral research fellows in the Department of Earth System Science at the School of Earth, Energy & Environmental Sciences (Stanford Earth).

Additional coauthors are affiliated with Leibniz-Institute of Freshwater Ecology and Inland Fisheries, the Swedish Museum of Natural History, and Potsdam University. Klawonn is currently affiliated with Leibniz Institute for Baltic Sea Research.

The German Academic Exchange Service, the German Research Foundation, and the Simons Foundation supported the research.

Journal Reference:

Klawonn, I., et al. (2021). Characterizing the “fungal shunt”: Parasitic fungi on diatoms affect carbon flow and bacterial communities in aquatic microbial food webs. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2102225118.