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The Lignin Enigma


What would you guess the most abundant untapped resource on earth is? And when I say untapped, I don't mean by humans. Nothing is able to tap this resource -- and there may be a high level design reason why that is so.

Think wood.

Wood is composed of a number of complex organic biopolymers that give it its strength, lightness, and flexibility. The most abundant of these, cellulose, is completely digestible by fungi, bacteria and protozoa. Higher organisms like insects and ruminants that get their food from plants rich in cellulose rely on symbiotic relationships with microorganisms in order to digest it. More interesting, though, is the fact that wood's second most abundant biopolymer, lignin, cannot serve as an energy source for any organism. In fact, the only organisms known to break it down require energy to do so.

This is the case even though, as the authors of a new BIO-Complexity article state,

Lignin is the most abundant aromatic polymer on earth and the second most abundant organic polymer of any kind, exceeded only by cellulose. It is estimated that 30% of the earth's non-fossil organic carbon is in the form of lignin. Considering its massive abundance and its high energy content (40% higher than cellulose, gram for gram), it is striking that no organism seems to have tapped it as an energy source.
Why should such an abundant resource go unexploited? Darwinian evolution has apparently failed to evolve "a relatively modest innovation -- growth on lignin" -- over 400 million years, even though many other spectacular innovations -- nodulation (a symbiotic relationship between plants and bacteria that permits the fixation of nitrogen), symbiotic pollination systems (between plants, hummingbirds, and bees), and the appearance of carnivorous plants -- all appeared during the same time period, and complex biochemical pathways such as C4 photosynthesis have apparently evolved independently many times.

The authors continue,

How can one mechanism [Darwinism] have been at the same time so effective and so ineffective? That tension vanishes completely when the design perspective is adopted. Terrestrial animal life is crucially dependent on terrestrial plant life, which is crucially dependent on soil, which is crucially dependent on the gradual photo- and biodegradation of lignin. Fungi accomplish the biodegradation, and the surprising fact that it costs them energy to do so keeps the process gradual. The peculiar properties of lignin therefore make perfect sense when seen as part of a coherent design for the entire ecosystem.
Sometimes you have to step back to see the forest and the trees with greater clarity.

Cross-posted at Biologic Perspectives.