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Abstract:
Oxygen minimum zones are major sites of fixed nitrogen loss in the ocean. Recent studies
have highlighted the importance of anaerobic ammonium oxidation, anammox, in pelagic
nitrogen removal. Sources of ammonium for the anammox reaction, however, remain con-
troversial, as heterotrophic denitrification and alternative anaerobic pathways of organic
matter remineralization cannot account for the ammonium requirements of reported ana-
mmox rates. Here, we explore the significance of microaerobic respiration as a source of
ammonium during organic matter degradation in the oxygen-deficient waters off Namibia
and Peru. Experiments with additions of double-labelled oxygen revealed high aerobic
activity in the upper OMZs, likely controlled by surface organic matter export. Consistently
observed oxygen consumption in samples retrieved throughout the lower OMZs hints at effi-
cient exploitation of vertically and laterally advected, oxygenated waters in this zone by aer-
obic microorganisms. In accordance, metagenomic and metatranscriptomic analyses
identified genes encoding for aerobic terminal oxidases and demonstrated their expression
by diverse microbial communities, even in virtually anoxic waters. Our results suggest that
microaerobic respiration is a major mode of organic matter remineralization and source of
ammonium (~45-100%) in the upper oxygen minimum zones, and reconcile hitherto
observed mismatches between ammonium producing and consuming processes therein.
