The curious link between coal and the future of energy

Clusters of fungal fruits we call mushrooms on trees in Armillaria-infected territory.
Promo image VirĂ¡g Tomity

The end of the era of coal

Coal launched the industrial revolution. The amazing black fuel burns hotter and provides more energy than the prevailing prior fuel, wood. Coal actually owes its energy to wood, compressed by geological forces for millennia. Most of the coal we burn still in these waning years of fossil fuel use derives from trees that died and could not rot, because the organisms evolved to eat the strong, tough cell walls of trees did not yet exist.

But just as microbes are now evolving an ability to eat plastics, evolution could not leave such a nutrient-dense buffet as a tree to remain uneaten. The fungi we now call "white rot fungi" perfected the evolution of organisms capable of eating trees - scientists classify fungi as white rot species when they have the ability to digest all of the components of the trees' cell walls, including the lignin. Lignin describes a class of polymers that give trees such as the giant redwood, or sequoia, the ability to grow to such towering heights.

If not for climate change, we could keep using coal until the reserves run out. The white rot fungi are now believed to have been a major influence in limiting coal reserves, as they could break dead trees down before they could be turned into coal. The evolution of fungi that eat trees was the beginning of the end for coal.

An organism that grows larger than a blue whale

Ask people to name the biggest creature on earth, and most will answer the blue whale. Curiously, the fungi that feed on trees have evolved to beat the whales, winning the prize for the largest organism ever found. Called the "humongous fungus," a growth of Armillaria ostoyae now devastating areas of Oregon's Malheur National Forest consists of one huge organism linked together by nets of underground tendrils known as rhizomorphs. By current estimates, this fungus extends over 3.4 square miles (2,200 acres; 8.8 km2) of forest floor.

Many species of fungi provide benefits to the neighboring trees, providing nutrients to the trees in trade for sugars. Other species survive by feeding on trees that are already dead. But the A. ostoyae ranks as pathogenic, killing the trees on which it feeds. By feeding on living trees, the fungus avoids competition with bacteria, other fungi, and microbes. The organisms owe their huge size and deadly effects to a wide breadth of genes, which means a lot of recipes for the little kitchen tricks that make tasty meals of the tough lignin.

Fueling the future

Other plants contain lignin too, especially in the stems and tougher parts. Too often, this biomass goes to waste because no cost-effective process to use it efficiently has been discovered. Also too often, industry is turning to parts of plants that we use for food to create new sources of energy - putting food in direct competition with energy even as human populations reach levels at which that poses ethical conflicts.

At best, we can burn this biomass. But just as burning trees could not launch an industrial revolution, burning biomass cannot sustain our current technological and economic demands. A better solution must be found. Some processes have been developed to turn the easier-to-digest bits of plant stalks, cellulose and hemicellulose, into alcohols or break them into molecules that can be reacted into better fuels or raw materials. But the hard-to-digest lignin holds 25 to 35% of the available energy.

That is why scientists are now trying to understand the tricks that fungi use to break down lignin. Just as the plastic-eating microbes are under study to find super-enzymes that can be of use in plastic recycling processes, the many evolutionary tricks of tree-eating fungi will inspire scientists looking for answers to how we can fuel the future.

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