Tiny algae in Earth’s oceans and lakes take in sunlight and carbon dioxide and turn them into sugars that sustain the rest of the aquatic food web, gobbling up about as much carbon as all the world’s trees and plants combined.

FungalShunt

New research shows a crucial piece has been missing from the conventional explanation for what happens between this first “fixing” of CO2 into phytoplankton and its eventual release to the atmosphere or descent to depths where it no longer contributes to global warming. The missing piece? Fungus.

“Basically, carbon moves up the food chain in aquatic environments differently than we commonly think it does,” said Anne Dekas, an assistant professor of Earth system science at Stanford University. Dekas is the senior author of a paper published June 1 in Proceedings of the National Academy of Sciences that quantifies how much carbon goes into parasitic fungi that attack microalgae.

Researchers until now have predicted that most carbon fixed into colonies of hard-shelled, single-celled algae known as diatoms then funnels directly into bacteria – or dissolves like tea in the surrounding water, where it’s largely taken up by other bacteria. Conventional thinking assumes carbon escapes from this microbial loop mainly through larger organisms that graze on the bacteria or diatoms, or through the CO2 that returns to the atmosphere as the microbes breathe.

This journey is important in the context of climate change. “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,” Dekas said. “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.”

It turns out fungus creates an underappreciated express lane for carbon, “shunting” as much as 20 percent of the carbon fixed by 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,” Dekas said.

The findings also have implications for industrial and recreational settings that deal with harmful algal blooms. “In aquaculture, in order to keep the primary crop, like fish, healthy, fungicides might be added to the water,” Dekas said. That will prevent fungal infection of the fish, but it may also eliminate a natural check on algal blooms that cost the industry some $8 billion per year. "Until we understand the dynamics between these organisms, we need to be pretty careful about the management policies we’re using.”

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