N losses and nutrient regeneration in oxygen minimum zone waters

The importance of N-loss by denitrification and anammox in oxygen minimum zone (OMZ) waters for regulating the availability of nutrients for primary production and as source of nitrous oxide (N2O) in the ocean is well established. Although molecular evidence indicates that heterotrophic denitrification can occur, our incubation experiments show that the major quantitatively important N loss process in the OMZ waters off Peru is the autotrophic anammox process. New results from subproject B4 (Phases I and II) clearly indicate that primary production via organic matter export is the controlling factor of the fixed nitrogen loss and other nutrient cycling processes in OMZ waters. More surprisingly, we found that anaerobic processes can prevail under apparently high oxygen concentrations (> 20 μM) and both anaerobic and aerobic processes, as well as the corresponding key genes, coexist throughout a large part of the OMZ. Our combined results show that oxygen plays an important part in the N-cycling (N-loss and N-gain) of the OMZs and that micro-niches, e.g. in aggregated particles may allow the co-existence of aerobic/anaerobic activity.

Moreover, viral lyses may cause a shunt in the classic microbial loop of nutrient regeneration by fast decay of particles with potential implications for aggregate formation. Consequently, the next step to better define the regulation of nutrient regeneration and N-loss and N-gain in OMZ waters will be to focus on the regulation of microbial activity under micro-aerobic conditions and substrate availability related to micro-niches i.e. aggregated particles. Rates of N-loss (anammox and denitrification), nutrient regeneration (including nitrification and N2 fixation) as well as trace gas production and consumption will be determined after size fractionation of particles. The microorganisms and viruses involved will be identified using molecular biological techniques. In a highly dynamic environment non-measurable concentrations of oxygen (> 20 nM) can still support considerable microaerobic activity. Therefore, independent indicators for the presence of oxygen are required. The combination of N2O and nitric oxide (NO) can potentially be used as indicators for microaerobic and truly anoxic environments. To study the controlling factors such as sub-micromolar oxygen levels on N-cycling processes including N2O and NO accumulation, without the influence of aggregates and co-occurring processes, we aim to cultivate and study key species for N- cycling in OMZ waters. By better defining the regulating factors like oxygen availability, viral lysis, organic matter load and the potential micro-niches on N-loss and nutrient regeneration, we aim to provide further knowledge that can be used to improve integrated biogeochemical models for prediction of future change.


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Prof. Ruth Schmitz-Streit                  Prof. Marcel Kuypers                        PD Dr. Hermann Bange
Tel.: +49 431 880 4334                     Tel.: +49 421 2028 647                     Tel.: +49 431 600 4204
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