What is the primary reason that fungi are important in the carbon cycle?

a. Fungi are able to reach incredibly large sizes, storing carbon while they are alive, and contributing carbon back to the atmosphere when they die and decompose.

b. Fungi are able to reach incredibly large sizes. Their high ratio of surface area to volume allows them to decompose large amounts of dead/decaying material, thereby returning carbon to the atmosphere. Fungi are able to reach incredibly large sizes.

c. They have a high ratio of surface area to volume and are able to fix more carbon through photosynthesis than other plants of comparable size. Fungi are able to reach incredibly large sizes.

d. Their high ratio of surface area to volume allows them to store more carbon (in the form of sugar) obtained through mycorrhizal associations than their symbiotic hosts.

Fungi don’t get the respect they deserve. Maybe that’s because they do most of their work in the dark, beneath the ground or on dead matter, or because there’s something essentially alien and bacterial about their appearance and the way they grow. But fungi are so plentiful and basic to life that they’re recognized as their own phylogenic kingdom. There may be more than 5 million separate species of fungi, and the largest single organism on the planet is a fungus: the four sq. mi. (10 sq. km) Armillaria ostoya fungus, which lives in the soil of Oregon’s Blue Mountains and which may be more than 8,000 yeas old. Without fungi we wouldn’t have antibiotics, blue cheeses and most importantly, beer. And we won’t even get into the magic kind.

Fungi also play an important role in the carbon cycle, the biogeochemical process by which carbon—the essential element of life on Earth—moves between the air, soils and water. Plants sequester carbon dioxide, but when they die, that carbon enters the soil—a lot of it. Globally, soil is the biggest single terrestrial reservoir of carbon, far more than the amount of carbon contained in living things and in the atmosphere combined. (On a planetary scale, the oceans hold by far the most carbon.) As the dead plant matter is broken down by microbes in the oil, that carbon is released back into the air. The rate at which that carbon leaves the soil can obviously have a major impact on the amount of carbon in the atmosphere, which in turn helps drive climate change.

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One of the limits to the growth of those decomposing microbes is the availability of nitrogen in the soil. Living plants and soil microbes compete for nitrogen, and the less nitrogen is available to the microbes, the slower decomposition is—and the more carbon remains in the soil, instead of outgassing into the atmosphere. This is where the fungi come in. Most plants have a symbiotic relationship with mycorrhizal fungi: the fungi extract the nitrogen from the soil, and make it available to the plants through their roots. But according to a new study in Nature, one major type of the symbiotic fungi can extract nitrogen much more quickly than other types—and that in turn slows the growth of the competing microbes and leaves much more carbon locked away in the soil.