Nitrification produces the most abundant form of bioavailable nitrogen in the ocean, which is also a major electron acceptor in the oxidation of organic matter. The latter role of nitrate becomes crucial in the absence of oxygen. One major aim of this thesis is to investigate the role of nitrite oxidation in oxygen minimum zone (OMZ) N-cycling. Nitrite oxidation was detected throughout the Namibian OMZ and appears unaffected even by non-detectable oxygen levels. It could recycle up to 100% of reduced nitrate and thereby potentially reduce N-loss. The high abundance of nitrite oxidizing bacteria of the genus Nitrococcus can likely be explained by their versatile metabolism. While Nitrococcus grows chemolithoautotrophically in the presence of oxygen, it gains energy via organoheterotrophic nitrate reduction to nitrite and nitrous oxide in the absence of oxygen. In the oxygenated ocean of the Mauritanian upwelling ammonia- and nitrite oxidizers appeared to be associated with marine aggregates. An alternative source of nitrite was provided by nitrate reduction, which is likely facilitated by oxygen reduced microniches within marine aggregates
