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Important contribution of macroalgae to oceanic carbon sequestration

Abstract

The role of macroalgae in Blue Carbon assessments has been controversial, partially due to uncertainties about the fate of exported macroalgae. Available evidence suggests that macroalgae are exported to reach the open ocean and the deep sea. Nevertheless, this evidence lacks systematic assessment. Here, we provide robust evidence of macroalgal export beyond coastal habitats. We used metagenomes and metabarcodes from the global expeditions Tara Oceans and Malaspina 2010 Circumnavigation. We discovered macroalgae worldwide at up to 5,000 km from coastal areas. We found 24 orders, most of which belong to the phylum Rhodophyta. The diversity of macroalgae was similar across oceanic regions, although the assemblage composition differed. The South Atlantic Ocean presented the highest macroalgal diversity, whereas the Red Sea was the least diverse region. The abundance of macroalgae sequences attenuated exponentially with depth at a rate of 37.3% km−1, and only 24% of macroalgae available at the surface were expected to reach the seafloor at a depth of 4,000 m. Our findings indicate that macroalgae are exported across the open and the deep ocean, suggesting that macroalgae may be an important source of allochthonous carbon, and their contribution should be considered in Blue Carbon assessments.

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Fig. 1: Assemblage of macroalgae in the ocean.
Fig. 2: Export of macroalgae to the deep and open ocean.
Fig. 3: Oceanic export of macroalgal DNA relative abundance per order.

Data availability

The data that support the findings of this study can be found in ref. 20 (Tara Oceans metagenomes; https://doi.org/10.1038/sdata.2015.23), and ref. 57 (Tara Oceans 18S rDNA V9 metabarcodes; https://doi.org/10.1126/science.1261605) and Zenodo (Malaspina metagenomes; https://doi.org/10.5281/zenodo.2596829)21.

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Acknowledgements

We thank the Tara Oceans Consortium for data availability. This research was supported by the Malaspina 2010 expedition, funded by the Spanish Ministry of Economy and Competitiveness through the Consolider-Ingenio programme to C.M.D. (reference: CSD2008-00077); CARMA, funded by the Independent Research Fund Denmark to D.K.-J. (reference: 8021-00222B); and King Abdullah University of Science and Technology’s project BAS/1/1071-01-01 to C.M.D. We thank all of the scientists and crew for support during sample collection on the Malaspina 2010 cruise, and especially E. Borrull, C. Díez-Vives, E. Lara, D. Vaqué, G. Salazar and F. Cornejo-Castillo for DNA sampling. The authors are grateful to the KAUST Supercomputing Laboratory (KSL) for the resources provided.

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C.M.D. and D.K.-J. conceived the research. J.M.G., S.G.A., R.L., R.M., I.A. and A.A.K. produced and curated the data. A.O., C.M.D., N.R.G. and I.A. conducted the data analysis. A.O. and C.M.D. wrote the manuscript. All co-authors contributed to improving the manuscript and approved the submission.

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Correspondence to Carlos M. Duarte.

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Ortega, A., Geraldi, N.R., Alam, I. et al. Important contribution of macroalgae to oceanic carbon sequestration. Nat. Geosci. 12, 748–754 (2019). https://doi.org/10.1038/s41561-019-0421-8

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