Ammonia-oxidizing Bacteria of the Nitrosospira Cluster 1 Dominate over Ammonia-oxidizing Archaea in Oligotrophic Surface Sediments near the South Atlantic Gyre

Lorenzo Lagostina, Tobias Goldhammer, Hans Røy, Thomas W. Evans, Mark A. Lever, Bo B. Jørgense1, Dorthe G. Petersen, Andreas Schramm and Lars Schreiber

Section for Microbiology, Department of Bioscience, Aarhus University, Denmark

Abstract

Sediments across the Namibian continental margin feature a strong microbial activity gradient at their surface. This is reflected in ammonium concentrations of <10?µM in oligotrophic abyssal plain sediments near the South Atlantic Gyre (SAG) compared to ammonium concentrations of >700?µM in upwelling areas near the coast. Here we address changes in apparent abundance and structure of ammonia-oxidizing archaeal and bacterial communities (AOA and AOB) along a transect of seven sediment stations across the Namibian shelf by analyzing their respective ammonia monooxygenase genes (amoA). The relative abundance of archaeal and bacterial amoA (g-1 DNA) decreased with increasing ammonium concentrations, and bacterial amoA frequently outnumbered archaeal amoA at the sediment-water interface (0-1?centimeters below seafloor (cmbsf)). In contrast, AOA were apparently as abundant as AOB or dominated in several deeper (>10?cmbsf), anoxic sediment layers. Phylogenetic analyses showed a change within the AOA community along the transect, from two clusters without cultured representatives at the gyre to Nitrososphaera and Nitrosopumilus clusters in the upwelling region. AOB almost exclusively belonged to the Nitrosospira cluster 1. Our results suggest that this predominantly marine AOB-lineage without cultured representatives can thrive at low ammonium concentrations and is active in the marine nitrogen cycle.