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Degradation of terrestrially derived macromolecules in the Amazon River

Nature Geoscience volume 6, pages 530–533 (2013)Cite this article

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Abstract

Temperate and tropical rivers serve as a significant source of carbon dioxide to the atmosphere1,2,3,4. 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 biosphere5, but are assumed to resist degradation on release from soils to aquatic settings6,7,8. 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, disappeared from our analytical window following several days of incubation at ambient river temperatures, indicative of biological degradation. We estimate that the net rate of degradation observed corresponds to 30–50% of bulk river respiration. Assuming that a significant fraction of these compounds is eventually remineralized to carbon dioxide, we suggest that lignin and other terrestrially derived macromolecules contribute significantly to carbon dioxide outgassing from inland waters.

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Figure 1: Van Krevelen diagrams of incubated Amazon River DOM.
Figure 2: Bulk and specific respiration rates in the Amazon River.
Figure 3: Fate of lignin in the Amazon River watershed.

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Acknowledgements

Financial support was provided by the Gordon and Betty Moore Foundation Marine Microbial Initiative for the River-Ocean Continuum of the Amazon project (Lead PI: P.L.Y.), NSF, and FAPESP Grant #08/58089-9. Lignin analyses were performed in the UW Aquatic Organic Geochemistry Laboratory with assistance from B. Kimball, J. Neibauer and A. Townsend. Sampling in Macapá was assisted by students J. E. Melo Diniz, K. dos Santos, E. Santos, B. Zelinsky, C. Smith, B. Satinsky and C, Fortunato. Bacterial abundance counts were made by A. MacDougall at the University of Georgia. Sampling in Óbidos was assisted by J. Mauro, T. Beldini and R. da Silva.

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Authors and Affiliations

  1. School of Oceanography, University of Washington, Box 355351, Washington, 98195-5351, Seattle, USA

    Nicholas D. Ward, Richard G. Keil & Jeffrey E. Richey

  2. Department of Marine Sciences, University of Georgia, Athens, Georgia 30602-3636, USA

    Patricia M. Medeiros & Patricia L. Yager

  3. Universidade Federal do Amapá, Rodovia Juscelino Kubitschek, KM 02, Macapá, AP, 68902-280, Brazil

    Daimio C. Brito & Alan C. Cunha

  4. Max Planck Research Group for Marine Geochemistry, Carl von Ossietzky University, Institute for Chemistry and Biology of the Marine Environment, Carl-von-Ossietzky-Straße 9-11, D-26129 Oldenburg, Germany

    Thorsten Dittmar

  5. Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Av. Centenário, 303, Piracicaba, SP, 13400-970, Brazil

    Alex V. Krusche

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  1. Nicholas D. Ward
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Contributions

P.M.M. and T.D. performed ESI–FT–ICR–MS analyses and data interpretation. D.C.B. and A.C.C. measured bulk respiration rates and organized Macapá sampling logistics. A.V.K. measured DOC concentrations and oversaw and organized all Amazon River sampling logistics. P.L.Y. measured bacterial abundance and coordinated the River-Ocean Continuum of the Amazon project. J.E.R. calculated discharge rates and managed field logistics. N.D.W. and R.G.K. designed the study, performed all lignin and GC–ToF–MS analyses, and wrote the manuscript. All authors discussed the results and commented on the manuscript.

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Correspondence to Nicholas D. Ward.

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The authors declare no competing financial interests.

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Ward, N., Keil, R., Medeiros, P. et al. Degradation of terrestrially derived macromolecules in the Amazon River. Nature Geosci 6, 530–533 (2013). https://doi.org/10.1038/ngeo1817

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