Venturing into the "Twilight Zone": where does all carbon go?

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The oceans, covering 70% of the Earth surface, are estimated to absorb nearly half of the human carbon dioxide emissions produced since 1800 playing therefore a key role in regulating global climate. The fate of carbon dioxide in the ocean is largely determined by biological processes in the water column. The mesopelagic layer, depth between 100 and 1000 m, is providing a vital planetary service by converting > 90% of organic matter exported from the sea surface back into carbon dioxide, vital nutrients, and microelements returning it to the surface over the decades thus fuelling surface primary productivity. Despite its importance, the mesopelagic realm is one of the least studied regions of the ocean and little is known about the processes that control the decomposition of organic matter or its regional variation. The importance of improving our understanding of the processes in the mesopelagic zone is becoming increasingly pressing with the proliferation of proposals for ocean fertilization programs as means reducing atmospheric carbon dioxide concentrations.