Bacterial degradation activity in the eastern tropical South Pacific oxygen minimum zone

Oxygen minimum zones (OMZs) show distinct biogeochemical processes that relate to microorganisms being able to thrive under low or even absent oxygen. Microbial degradation of organic matter is expected to be reduced in OMZs, although quantitative evidence is low. Here, we present heterotrophic bacterial production (3H leucine incorporation), extracellular enzyme rates (leucine aminopeptidase/β-glucosidase) and bacterial cell abundance for various in situ oxygen concentrations in the water column, including the upper and lower oxycline, of the eastern tropical South Pacific off Peru. Bacterial heterotrophic activity in the suboxic core of the OMZ (at in situ ≤ 5 µmol O2 kg−1) ranged from 0.3 to 281 µmol C m−3 d−1 and was not significantly lower than in waters of 5–60 µmol O2 kg−1. Moreover, bacterial abundance in the OMZ and leucine aminopeptidase activity were significantly higher in suboxic waters compared to waters of 5–60 µmol O2 kg−1, suggesting no impairment of bacterial organic-matter degradation in the core of the OMZ. Nevertheless, high cell-specific bacterial production was observed in samples from oxyclines, and cell-specific extracellular enzyme rates were especially high at the lower oxycline, corroborating earlier findings of highly active and distinct micro-aerobic bacterial communities. To assess the impact of bacterial degradation of dissolved organic matter (DOM) for oxygen loss in the Peruvian OMZ, we compared diapycnal fluxes of oxygen and dissolved organic carbon (DOC) and their microbial uptake within the upper 60 m of the water column. Our data indicate low bacterial growth efficiencies of 1 %–21 % at the upper oxycline, resulting in a high bacterial oxygen demand that can explain up to 33 % of the observed average oxygen loss over depth. Our study therewith shows that microbial degradation of DOM has a considerable share in sustaining the OMZ off Peru.