Coastal waters support ∼90 per cent of global fisheries and are therefore an important food reserve for our planet1. Eutrophication of these waters, due to human activity, leads to severe oxygen depletion and the episodic occurrence of hydrogen sulphide—toxic to multi-cellular life—with disastrous consequences for coastal ecosytems2,3,4,5. Here we show that an area of ∼7,000 km2 of African shelf, covered by sulphidic water, was detoxified by blooming bacteria that oxidized the biologically harmful sulphide to environmentally harmless colloidal sulphur and sulphate. Combined chemical analyses, stoichiometric modelling, isotopic incubations, comparative 16S ribosomal RNA, functional gene sequence analyses and fluorescence in situ hybridization indicate that the detoxification proceeded by chemolithotrophic oxidation of sulphide with nitrate and was mainly catalysed by two discrete populations of γ- and ε-proteobacteria. Chemolithotrophic bacteria, accounting for ∼20 per cent of the bacterioplankton in sulphidic waters, created a buffer zone between the toxic sulphidic subsurface waters and the oxic surface waters, where fish and other nekton live. This is the first time that large-scale detoxification of sulphidic waters by chemolithotrophs has been observed in an open-ocean system. The data suggest that sulphide can be completely consumed by bacteria in the subsurface waters and, thus, can be overlooked by remote sensing or monitoring of shallow coastal waters. Consequently, sulphidic bottom waters on continental shelves may be more common than previously believed, and could therefore have an important but as yet neglected effect on benthic communities.
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Sequences for the 16S rRNA, aprA and rdsrA genes obtained in this study have been submitted to GenBank under the accession numbers FM246507–FM246787, FM246819–FM246832 and FM246788–FM246818, respectively.
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We thank B. Barker Jørgensen, F. Inagaki, C. Hubert, T. Ferdelman and G. Collins for discussions; the Namibian authorities for access to their national waters; the crew of RV Alexander von Humboldt for assistance onboard; S. Krüger for operating the pump-CTD and oceanographic equipment; T. Heene for assistance with the collection of oceanographic data and generating oceanographic plots; and G. Klockgether, J. Sawicka, J. Wulf, S. Lenk, D. Franzke and K. Nkandi for assistance with the analysis. The investigations were supported by the MPG, the BMBF programme Geotechnologien and the project NAMIBGAS, the DFG, BENEFIT and the Namibian Ministry of Fisheries and Natural Resources.
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Lavik, G., Stührmann, T., Brüchert, V. et al. Detoxification of sulphidic African shelf waters by blooming chemolithotrophs. Nature 457, 581–584 (2009). https://doi.org/10.1038/nature07588
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