For want of a nail, the nursery rhyme goes, a kingdom was lost. A similar, seemingly innocuous change—the evolution of a lineage of mushrooms—may have had a massive impact on the carbon cycle, bringing an end to the 60-million year period during which coal deposits were formed.
Coal generated nearly half of the roughly four trillion kilowatt-hours of electricity consumed in the United States in 2010, according to the U.S. Energy Information Administration. This fuel is actually the fossilized remains of plants that lived from around 360 to 300 million years ago. An international team of scientists, including researchers at the U.S. Department of Energy Joint Genome Institute (DOE JGI), has proposed a new factor that may have contributed to the end of the Carboniferous period—named after the large stores of what became coal deposits. The evidence, presented online in the June 29 edition of the journal Science, suggests that the evolution of fungi capable of breaking down the polymer lignin, which helps keep plant cell walls rigid, may have played a key role in ending the development of coal deposits. With the arrival of the new fungi, dead plant matter could be completely broken down into its basic chemical components. Instead of accumulating as peat, which eventually was transformed into coal, the great bulk of plant biomass decayed and was released into the atmosphere as carbon dioxide.
“We’re hoping this will get into the biology and geology textbooks,” said Clark University biologist David Hibbett, senior author of the comprehensive study comparing the complete genomes of dozens of species of fungi, most of which were sequenced at the DOE JGI. “When you read about coal formation it’s usually explained in terms of physical processes, and that the rate of coal deposition just crashed at the end of the Permo-Carboniferous. Why was that? There are various explanations. The evolution of white rot fungi could’ve been a factor – perhaps a major factor. Once you have white rot you can break down lignin, the major precursor of coal. So the evolution of white rot is a very important event in the evolution of carbon cycle.”
“The concept of the invention of an enzyme that can break down the ‘unbreakable’ is really great,” said Kenneth Nealson, Wrigley Chair in Environmental Studies and Professor of Earth Sciences and Biological Sciences at the University of Southern California. “The idea that a stable (inedible) form of organic carbon can become edible (and thus more difficult to bury over time), changes our perspective not only on global energy storage in the past, but on what it means for present day carbon sequestration and storage, in that sense this idea will have a big impact on our thinking about the past and the present.”
For their study, Hibbett and his colleagues focused on Basidiomycetes, which include mushroom species with the familiar cap-and-stem look that most people associate with fungi. Basidiomycetes also include brown rot fungi such as the dry rot that can destroy houses by breaking down the cellulose in the construction wood but leave the lignin untouched and white rot fungi of interest to the pulp and paper industries that can break down both types of polymers. Of the 31 brown rot and white rot fungal genomes that were compared for the study, 26 were sequenced at the DOE JGI, including a dozen that were done specifically for the study to flesh out representation of the fungal orders.