Distribution and Activity of Anammox and Sulfide-Oxidizing Nitrate-Reducing Bacteria in Oxygen Minimum Zones

Highly productive marine upwelling regions that are located along continental boundaries play a critical role in the global ocean nitrogen budget. The high organic matter input in combination with the poor regional ventilation result in extremely low water column oxygen concentrations, or so-called oxygen minimum zones (OMZs). OMZs harbor a diverse community of anaerobic microorganisms including an enhanced nitrogen cycle. For instance, the microbial processes anammox and denitrification thrive in these waters, catalyzing the reduction of fixed nitrogen species (NO3-, NO2- and NH4 ) to the more inert form, N2 gas, thereby contributing to the loss of fixed nitrogen. Together the five major OMZs (which represent a small 1% of the global ocean volume) contribute to 30-50% of oceanic nitrogen loss. However, the biological and physical forces that regulate anammox and denitrification activity in OMZs remain poorly constrained. This thesis work aims to elucidate the dynamics that govern the activity and distribution of anammox and denitrifying (e.g. SUP05 and Arcobacter) bacteria in OMZs. In this work we employed a comprehensive range of techniques including cultivation, metagenomics, 15N-labeled stable isotope incubation experiments, and single-cell techniques in combination with oceanographic tools.