High bacterial organic carbon uptake in the Eastern Tropical South Pacific oxygen minimum zone

Abstract

<p><strong>Abstract.</strong> 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 (<sup>3</sup>H leucine-incorporation), extracellular enzyme rates (leucine aminopeptidase/ß-glucosidase) and bacterial cell abundance for various in situ oxygen concentrations in the water column of the Eastern Tropical South Pacific off Peru. Bacterial heterotrophic activity in the suboxic core of the OMZ (at in situ ≤ 5 µmol O<sub>2</sub> kg<sup>−1</sup>) ranged from 0.6 to 160 µmol C m<sup>−3</sup> d<sup>−1</sup> and was not significantly lower than in waters of 5–60 µmol O<sub>2</sub> kg<sup>−1</sup>. Moreover, bacterial abundance in the OMZ was slightly and leucine aminopeptidase activity even significantly higher in suboxic waters compared to the upper oxycline suggesting no impairment of bacterial organic matter degradation in the core of the OMZ. Nevertheless, high cell-specific bacterial production and extracellular enzyme rates were observed in samples from the upper or lower oxyclines corroborating earlier findings of highly active and distinct micro-aerobic bacterial communities. To assess the impact of bacterial degradation of dissolved organic matter 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 bacterial growth efficiencies of 0.5–8.6 % 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.</b>

Sediment release of dissolved organic matter to the oxygen minimum zone off Peru

“…Therefore, a non-conservative behaviour of DOC and DON in the BIGO chambers during the sediment enclosure might be a result of sediment release/microbial DOM consumption and reworking in the near bottom waters or the sediment-water column interface. In turn, DOM released by the sediment could potentially support an enhanced microbial abundance and carbon oxidation rates reported near the sediment on the 12 o S transect (Maßmig et al, 2019a) and influence the activity of microbial mats that cover up to 100 % of the sediment surface at the inner shelf stations (Sommer et al, 2016). Furthermore, the DIC fluxes, evaluated in benthic lander systems (Dale et al, 2015) may include sediment release of DIC and the in situ DIC production by DOM remineralization.…”

“…High productivity, followed by intense organic matter remineralisation (e.g. Loginova et al, 2019;Maßmig et al, 2019a) in combination with sluggish ventilation (Stramma et al, 2005;Keeling et al, 2010) leads to a formation of pronounced oxygen minimum zone (OMZ) (e.g. Stramma et al, 2008).…”