Molecular evidence for anaerobic ammonium‐oxidizing (anammox) bacteria in continental shelf and slope sediments off northwest Africa

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Abstract
The presence and abundance of anaerobic ammonium‐oxidizing (anammox) bacteria was investigated in continental shelf and slope sediments (300‐3000 m water depth) off northwest Africa in a combined approach applying quantitative polymerase chain reaction (q‐PCR) analysis of anammox‐specific 16S rRNA genes and anammox‐specific ladderane biomarker lipids. We used the presence of an intact ladderane monoether lipid with a phosphocholine (PC) headgroup as a direct indicator for living anammox bacteria and compared it with the abundance of ladderane core lipids derived from both living and dead bacterial biomass. All investigated sediments contained ladderane lipids, both intact and core lipids, in agreement with the presence of anammoxspecific 16S rRNA gene copies, indicating that anammox occurs at all sites. Concentrations of ladderane core lipids in core top sediments varied between 0.3 and 97 ng g−1 sediment, with the highest concentrations detected at the sites located on the shelf at shallower water depths between 300 and 500 m. In contrast, the C20 [3]‐ ladderane monoether‐PC lipid was most abundant in a core top sediment from 1500 m water depth. Both anammox‐specific 16S rRNA gene copy numbers and the concentration of the C20 [3]‐ladderane monoether‐PC lipid increased downcore in sediments located at greater water depths, showing highest concentrations of 1.2 × 108 copies g−1 sediment and 30 pg g−1 sediment, respectively, at the deepest station of 3000 m water depth. This suggests that the relative abundance of anammox bacteria is higher in sediments at intermediate to deep water depths where carbon mineralization rates are lower but where anammox is probably more important than denitrification.