When a hungry fungus anchors itself to an unsuspecting rock, it has a plan of attack. First, it unleashes acid, dissolving surface minerals to get to its food. Then, it releases chemicals that extract that food—in this case, iron. Finally, its fast-growing fungal filaments cut into the remaining rock like a knife, carving deep channels that break up iron-depleted surfaces and expose fresh layers for consumption. Step by step, the fungus Talaromyces flavus knows how to get what it wants. “These organisms, they don’t have a brain, but they’re pretty smart,” says Henry Teng, a geochemist at George Washington University in Washington, D.C.
Microbial geochemists have long known that fungi, bacteria, and other microbes are crucial to weathering, the chemical and physical breakdown of rock. But most experiments have calculated that contribution at arm’s length, mixing microbes and minerals in solution in the lab as an analogy for the watery soil and rock pore spaces in which scientists assumed microbes were doing most of their work. A new study has zoomed in to scrutinize the zone where microbes meet minerals, and showed which chemicals fungi release after they attach to mineral surfaces. It suggests that scientists have underestimated how much fungal weathering goes on at this interface, and that microbes could be more important extractors of nutrients than researchers suspected.
“No one has looked at the interface in such detailed fashion,” says Teng, who published the study with colleagues this week in the journal Geology. Outside experts are impressed. “It’s one of the most comprehensive looks at the specific fungal-mineral interaction,” says Philip Bennett, a microbial geochemist at the University of Texas, Austin.
The research team found the fungus in a serpentine mine in Donghai, China, while searching for microbes good at extracting magnesium metal from rocks. Magnesium could be useful in sequestering atmospheric carbon dioxide in a stable solid form (such as magnesium carbonate), and microbes could represent an energy-efficient and environmentally friendly way of getting it. The team cultured several dozen microbes and found that T. flavus was the best at extracting magnesium and iron from a silicate mineral called lizardite.
Back in the lab, they stuck fungus cells on top of lizardite samples and watched with special microscopic tools. They measured the pH in areas around fungal cells that hadn’t yet attached themselves to the lizardite. But once cells attached to the mineral surface, pH levels dropped sharply—evidence that the fungus was releasing mineral-dissolving acid. The researchers documented a similar surface-triggered release of siderophores, chemicals that leach iron from the mineral. Teng says previous studies have measured weak, diffuse levels of acid and siderophores when mineral and fungus are put together in the same solution, but none had measured this concentrated release at the fungus-mineral interface. “[The fungi] don’t want to use energy before they see the food,” he says. Bennett says it’s an explicit demonstration of a two-way symbiosis. “It gets to the heart of the microbe-mineral relationship,” he says. “The microbe influences mineral weathering, and the mineral influences the microbial community.”
After the iron leaching, the fungus wasn’t finished with the rock. The researchers documented the pits in the lizardite left by fungal spores and the channels left by long, filamentous fungal cells called hyphae. Pressures at hypha tips can be 100 times that of car tires, Teng says—perfect for busting up tough iron-depleted mineral crystals to expose fresh layers beneath.
The study stresses the importance of surface interactions, scientists say. Previous studies have suggested that as much as 99% of microbial bio-weathering takes place in pore space solutions, as microbes release acid near, but not right up against, mineral surfaces. The new study suggests that as much as 50% of the weathering occurs at the surface, where the microbes can more efficiently and selectively target nutrients like iron. This means that microbes may be more responsible for extracting nutrients and moving them through soil ecosystems than previously thought, says Jennifer Roberts, a microbial geochemist at the University of Kansas, Lawrence. “It has implications for the health of soils and any symbiosis with plants,” she says.