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Mayol, Eva ... et al.-- 9 pages, 4 figures, supplementary information https://dx.doi.org/10.1038/s41467-017-00110-9.-- Data availability: The original sequences of airborne microbes reported in this study are available at the Sequence Read Archive (SRA, http://www.ncbi.nlm.nih.gov/sra/SRP074223) under Bioproject ID PRJNA319484 with accession numbers SAMN04903953 to SAMN04904051, The atmosphere plays a fundamental role in the transport of microbes across the planet but it is often neglected as a microbial habitat. Although the ocean represents two thirds of the Earth’s surface, there is little information on the atmospheric microbial load over the open ocean. Here we provide a global estimate of microbial loads and air-sea exchanges over the tropical and subtropical oceans based on the data collected along the Malaspina 2010 Circumnavigation Expedition. Total loads of airborne prokaryotes and eukaryotes were estimated at 2.2 × 1021 and 2.1 × 1021 cells, respectively. Overall 33–68% of these microorganisms could be traced to a marine origin, being transported thousands of kilometres before re-entering the ocean. Moreover, our results show a substantial load of terrestrial microbes transported over the oceans, with abundances declining exponentially with distance from land and indicate that islands may act as stepping stones facilitating the transoceanic transport of terrestrial microbes, This is a contribution to the Malaspina Expedition 2010, funded by the INGENIO 2010 CONSOLIDER program (ref. CDS2008-00077) of the Spanish Ministry of Economy and Competitiveness. [...] E.M. and M.A.J. acknowledge the ‘Junta para la Ampliación de Estudios’ program (JAE-predoc and JAE-doc contracts, respectively) from CSIC, supplied by the European Social Fund. A.M.-A. acknowledges an FPI grant funded by the Spanish Ministry of Science and Innovation. B.G.-G. acknowledges a predoctoral fellowship from the BBVA Foundation