Beschreibung: Oxygen minimum zones in the ocean increased during the past 50 years and changed microbial biogeochemical cycling; thereby research was focusing on changes in the nitrogen cycle. Earlier studies suggested higher efficiency of carbon export in those regions due to reduced microbial degradation activity. However, previous findings on the effect of oxygen on microbial activity are ambiguous. Here, we present first results on bacterial biomass production (estimated by 3H leucine incorporation) and extracellular enzyme rates (leucine aminopeptidase and ß-glucosidase), for the oxygen minimum zone off Peru, which is part of one of the largest anoxic zones in the ocean. We observed no reduction in bacterial biomass production, or extracellular enzyme rates and no reduced cell abundance in anoxic and suboxic waters, compared to more oxygenated waters at the oxyclines, suggesting that microbial degradation rate does not necessarily slow down under low oxygen conditions. We estimated a mean microbial carbon uptake of 548 µmol m-3 d-1, thereby only an average of 11 % got transformed into bacterial biomass. The remaining part was respired to carbon dioxide (average: 496 µmol m-3 d-1), that was potentially released to the atmosphere and accounted on average for 32 % of the oxygen reduction in the upper 80 m. Our study therewith proposes that microbial degradation of organic matter significantly contributes to the formation of the oxygen minimum zone off Peru and can proceed at relatively high rate within anoxic waters. This indicates that carbon dioxide production by heterotrophic microbial degradation in the OMZ off Peru, is not necessarily reduced under anoxia, and driven by anaerobic heterotrophic respiration pathways like denitrification.

Beschreibung: Dissolved Organic Matter, Cell Abundance and Extracellular Enzyme Rates and Bacterial Production from several cruises and experiments from 2008-2018