Introduction

Early assumptions about anaerobic respiration and autogenic denitrification in benthic foraminifera considered lethal effects on the long term, due to anoxia and sulfate reduction (Bernhard and Reimers 1991). Today, however, a set of ultrastructural, symbiotic, and kleptoblastic structures is under discussion for benthic foraminifera, all providing independence from dissolved O₂ (Bernhard 2003, Bernhard and Bowser 2008). Facultative but autogenic and complete denitrification was recently observed from laboratory cultures, as well as from field investigations (Risgaard-Petersen et al. 2006, Høgslund et al. 2008). Such biochemical activities of benthic foraminifera at redox boundary conditions are speculated to contribute significantly to the global nutrient cycle, rather than providing rare abnormalities of negligible impact (Høgslund et al. 2008). The diatomaceous mud belt off Namibia is one of the most inhospitable, oxygen depleted, and sulfidic open shelf environments on Earth (Baturin 2002a, 2002b, van der Plas et al. 2007). Oxygen concentrations of bottom waters are below detectability in the central area, suggesting denitrification and anaerobic ammonia oxidation (anammox) in the water column (Emeis et al. 2007). Methane and hydrogen sulfide accumulate over large areas within the sediment column (Emeis et al. 2004), and the blow outs of these toxic gases are frequently recorded at the ocean surface (Weeks et al. 2004). Sulfide diffusion from this mud is detoxified by blooms of nitrate-reducing chemolithotrophic bacteria near the sediment surface under completely anoxic conditions (Lavik et al. 2009). Our investigation aimed at recovering living benthic foraminifera from the diatomaceous mud belt in order to provide evidence for the capability of these organisms to permanently survive and thrive in this type of extreme environment.