December 25th, 2017
(written by lawrence krubner, however indented passages are often quotes). You can contact lawrence at: email@example.com
Here is the crux of our problem: lignin made the lycopod trees a little too successful. Because their leaves were lofted above many herbivores and their trunks were made inedible by lignin, lycopods were virtually impervious to harm. They grew and died in vast quantities, and their trunks piled up in swamps, eventually becoming submerged and locking huge quantities of carbon dioxide out of the atmosphere for good in the form of coal. Without any decomposition to recycle this carbon, atmospheric carbon dioxide levels crashed, leading to global cooling and making it much harder for plants to grow. Atmospheric oxygen concentration, in turn, soared to an estimated 35%, much higher than the 20% of modern times.
But why was all this lignin laying around in the first place? Plenty of organisms had found a way to make use of cellulose, so why didn’t they jump on this new source of energy that was laying around free for the taking? The are several reasons: first, whereas cellulose was made of glucose, which can be readily converted to energy, lignin was based on phenol, a derivative of benzene, which is only a good energy source when it’s on fire. This isn’t a solution for your average bacterium. Digesting lignin was so difficult that lycopods had free reign over the planet for over 40 million years, leading to the world’s first and only wood pollution crisis. Finally, however, a fungus belonging to the class Agaricomycetes – making it a distant cousin of button mushrooms – did find a crude way to break down lignin. Rather than devise an enzyme to unstitch the lignin molecule, however, it was forced to adapt a more direct strategy. Using a class of enyzmes called peroxidases, the fungus bombarded the wood with highly reactive oxygen molecules, in much the same way one might untie a knot using a flamethrower. This strategy reduced the wood to a carbohydrate-rich slurry from which the fungus could slurp up the edible cellulose.
This was the one and only time in the last 300 million years that the wood-rotting ability evolved. All the fungi today that can digest wood (and a few that can’t) are the descendants of that enterprising fungus. Its strategy may have been inelegant, but wood decay played a crucial role in reversing the loss of carbon dioxide in the atmosphere and bringing about the end of the Carboniferous period.