Stanford scientists find that fungal infections in algae change the carbon cycle

Source: Klawonn et al. 2021, PNAS

Tiny algae in the Earth’s oceans and lakes absorb sunlight and carbon dioxide and turn them into sugar, which maintains the rest of the aquatic food web and gobbles up about as much carbon as all the trees and plants in the world put together.

New research shows that the traditional explanation for what happened between this initial “fixation” of CO. happens, a crucial piece is missing2 in phytoplankton and its eventual release into the atmosphere or descent to depths where it no longer contributes to global warming. The missing piece? Mushroom.

“Basically, carbon moves up the food chain differently in aquatic environments than we commonly think,” says Anne Dekas, Assistant Professor of Earth System Science at Stanford University. Dekas is the lead author of a June 1 in Proceedings of the National Academy of Sciences this quantifies how much carbon is in parasitic fungi that attack microalgae.

Underwater carousel

So far, researchers have predicted that most of the carbon that is trapped in colonies of hard-shelled, unicellular algae, known as diatoms, will end up in bacteria – or, like tea, dissolve in the surrounding water, where it is mostly ingested by other bacteria. Conventional thinking assumes that carbon escapes from this microbial cycle mainly through larger organisms that graze on the bacteria or diatoms, or through the CO2 which is returned to the atmosphere as the microbes breathe.

This trip is important in the context of climate change. “In order for the carbon bond to take place, carbon from CO has to be carbon2 the food chain has to rise into large chunks of biomass that can sink to the ocean floor, ”said Dekas. “So it’s really removed from the atmosphere. If it only circulates in the sea surface for a long time, it can act as CO. released back into the air2. “

It turns out that fungi create an underestimated expressway for carbon by “shifting” up to 20 percent of the carbon fixed by diatoms from the microbial loop into the fungal parasite. “Instead of going through this carousel where the carbon could eventually return to the atmosphere, you have a more direct route to the higher levels of the food web,” Dekas said.

The results also have implications for industrial and recreational facilities that deal with harmful algal blooms. “In aquaculture, fungicides could be added to the water to keep primary cultures such as fish healthy,” said Dekas. This prevents fungal infection of the fish, but can also prevent natural control of algal bloom, which costs the industry about $ 8 billion annually. “Until we understand the dynamics between these organisms, we have to be fairly careful with the management guidelines that we use.”

Microbial interactions

The authors based their estimates on experiments with populations of chytrid fungi called Rhizophydiales and its host, a species of freshwater algae or diatoms called Asterionella formosa. Co-authors in Germany worked to isolate these microbes and bacteria in and around their cells from water collected from Lake Stechlin, about 100 kilometers north of Berlin.

“Isolating a 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 real challenge,” says lead author Isabell Klawonn, who is involved in the research as Postdoctoral fellow in Dekas’ laboratory in Stanford. “Therefore, only a few model systems are available to research such parasitic interactions.”

As early as the 1940s, scientists suspected that parasites played an important role in controlling phytoplankton and observed epidemics with chytrid fungi Asterionella blooms in lake water. Technological progress has made it possible to break down these invisible worlds into fine and measurable details – and to see their influence in a much larger picture.

“As a community, we recognize that it is not just the capabilities of a single microorganism that are important to understand what is happening in the environment. This is how these microorganisms interact, ”said Dekas.

The authors measured and analyzed interactions within the Lake Stechlin pathosystem using genome sequencing; a fluorescence microscopy technique involving the attachment of fluorescent dye to RNA in microbial cells; and a highly specialized instrument at Stanford – one of only a few dozen in the world – called NanoSIMS, which creates nanoscale maps of the isotopes of elements found in tiny amounts in materials. Dekas said, “To get these single cell measurements to show how photosynthetic carbon flows between certain cells, from the diatom to the fungus to the associated bacteria, this is the only way to do it.”

The exact amount of carbon diverted to mushrooms by the microbial carousel may vary in other environments. But the discovery that it can be as high as 20 percent even in one setting is significant, Dekas said. “If you change this system more than a few percent in any direction, it can have dramatic effects on the biogeochemical cycle. That makes a big difference for our climate. “

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Stanford co-authors include Alma E. Parada and Nestor Arandia-Gorostidi, postdocs in the Department of Earth System Science at the School of Earth, Energy & Environmental Sciences (Stanford Earth). Other co-authors are affiliated with the Leibniz Institute for Freshwater Ecology and Inland Fisheries, the Swedish Museum of Natural History and the University of Potsdam. Klawonn is now affiliated with the Leibniz Institute for Baltic Sea Research.

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

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