1. Max Planck Institute for Marine Microbiology, Celsiusstrasse 1, D-28359 Bremen, Germany
2. National Marine Information & Research Centre Ministry of Fisheries & Marine Resources, PO Box 912, Swakopmund, Namibia
3. Baltic Sea Research Institute Warnemünde, Seestrasse 15, D-18119 Rostock, Germany
4. Department of Microbial Ecology, Vienna Ecology Centre, University of Vienna, Althanstrae 14, A-1090 Vienna, Austria
5. These authors contributed equally to this work.
6. Present address: Department of Geology and Geochemistry, Stockholm University, Svante Arrhenius väg 8C, 106 91 Stockholm, Sweden.

Correspondence to: Marcel M. M. Kuypers¹ Correspondence and requests for materials should be addressed to M.M.M.K. (Email: mkuypers@mpi-bremen.de).

Abstract

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 ecosytems^{2, 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 - and -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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