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Temperate and tropical rivers serve as a significant source of carbon dioxide to the atmosphere^,,,. However, the source of the organic matter that fuels these globally relevant emissions is uncertain. Lignin and cellulose are the most abundant macromolecules in the terrestrial biosphere, but are assumed to resist degradation on release from soils to aquatic settings^,,. Here, we present evidence for the degradation of lignin and associated macromolecules in the Amazon River. We monitored the degradation of a vast suite of terrestrially derived macromolecules and their breakdown products in water sampled from the mouth of the river throughout the course of a year, using gas chromatography time-of-flight mass spectrometry. We identified a number of lignin phenols, together with 95 phenolic compounds, largely derived from terrestrial macromolecules. Lignin, together with numerous phenolic compounds …