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

Abstract

Abstract. 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.

Reactivity, fate and functional roles of dissolved organic matter in anoxic inland waters

“…hydroxyl radicals) [49]. Such abiotically processed DOM promotes or withstands further microbial processing, [17] from studies that compare rates of sediment release [18][19][20][21][22][23][24] and bacterial production [13,16,[25][26][27] in oxic and anoxic waters. Circles indicate singular observations, while lines connect averages from similar systems (electronic supplementary material, table S1).…”

“…Microbial respiration and production initiate a range of biological and chemical reactions that control the composition and fate of DOM. Diagrams show data[17] from studies that compare rates of sediment release[18][19][20][21][22][23][24] and bacterial production[13,16,[25][26][27] in oxic and anoxic waters. Circles indicate singular observations, while lines connect averages from similar systems (electronic supplementary material, tableS1).…”

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

“…The sediment release of DOM could potentially serve as an important carbon and N source (e.g. Moran and Zepp, 1997) and reduce the penetration depth of light in the water column (e.g. Belzile et al, 2002), potentially affecting phototrophic pelagic microbial communities and hence influencing biogeochemical processes of the water column.…”

Climate-Biogeochemistry Interactions in the Tropical Ocean: Data collection and legacy

“…CC BY 4.0 License.chromatography (HPAEC) (DIONEX ICS3000DC). More detail on molecular DOM composition may be found inLoginova et al (2019) andMaßmig et al (2020).Bacterial abundance was determined by flow cytometry on a FACS Calibur (Becton Dickinson) after Gasol and Del Giorgio (2000) from 1.6 ml sample, fixed with 0.75 μl 25 % glutaraldehyde on board and stored at -80°C until analyses. To 400 µl sample 10 μl Flouresbrite® fluorescent beads (Polyscience, Inc.) and 10 μl Sybr Green (Invitrogen) were added.For the extracellular enzymes leucine aminopeptidase and ß-glucosidase, potential hydrolytic rates were determined afterHoppe (1983).…”

“…Afterwards 200 μl sample were added and fluorescence was measured with a plate reader fluorometer (FLUOstar Optima, BMG labtech) (excitation: 355 nm; emission: 460 nm) after 0 and 12 h of incubation. For details about incubation conditions and subsequent calculations seeMaßmig et al (2020).…”