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Why so much coal? The wood-rot debate
argued since 2012· Theory or idea· Plants & fungi
DebatedScientists disagree about when, how or why.
A dispute over why so much Carboniferous plant matter became coal: wood-rotting fungi that evolved late, or geography and climate?
See it on the Life map →What happened
Lignin is the tough polymer that stiffens wood, and mainly white-rot fungi can fully break it down. In 2012 David Floudas and colleagues used fungal genomes to date that ability to about 295 million years ago, near the end of the coal age, reviving a 1990 idea: dead wood piled up because nothing could rot it. In 2016 Matthew Nelsen and colleagues countered that the main coal trees were low in lignin, and that coal formed where wet tropical basins kept sinking as Pangaea assembled.
Why it matters
It tests whether one biological innovation can lock away carbon on a planetary scale, with knock-on effects for oxygen, carbon dioxide and climate.
Think of it like
Is trash piling up because no hauling company exists yet, or because the lot keeps flooding so the trucks cannot reach it?
The evidence (fossils, genes, rocks)
Fungal genome comparisons and molecular clocks (Science, 2012) versus coal-basin geology, the chemistry of fossil coal-swamp plants and the timing of coal through later periods (PNAS, 2016).
Still debated
Many researchers now favor geography and climate, but fungi may have played a smaller part. The true age of wood-rotting fungi is still uncertain.
Connections (2)
▸Why so much coal? The wood-rot debate tries to explain the coal of Carboniferous coal forestsDebated
The connection
The Carboniferous laid down some of the world's richest coal beds, and the debate asks why. One side says dead wood piled up because fungi could not yet break down lignin. The other says the main coal trees were low in lignin anyway, and coal formed because wet tropical basins kept sinking and burying plant debris as Pangaea assembled.
When and how
The coal formed about 359–299 million years ago in Europe and North America; the fungal explanation was tested in Science (2012) and challenged in PNAS (2016).
How sure are we?
Coal balls, mineral-filled lumps of peat, show what the swamp plants were made of; basin geology shows where and why coal accumulated.
Debates and open questions
How much of the coal peak was biology and how much was geography and climate remains open.
▸Why so much coal? The wood-rot debate asks when wood-rotting evolved in FungiDebated
The connection
White-rot fungi are the main organisms that can fully break down lignin. The 2012 study compared 31 fungal genomes and used a molecular clock to date their lignin-dissolving enzymes to about 295 million years ago, near the end of the coal age. If wood-rot came that late, buried wood could escape decay and become coal.
When and how
David Floudas, David Hibbett and colleagues, Science, 2012, building on a 1990 proposal by Jennifer Robinson.
How sure are we?
The enzyme dates come from molecular clocks, which have wide error bars; fossil fungi and decayed fossil wood offer only limited direct checks.
Debates and open questions
Whether wood-rot evolved earlier than the clock suggests, and whether other fungi or bacteria could already break down lignin, are open questions.