Table 1.
Overview of sampling locations and times for the various types of data considered in this study.
Fig 1.
Physicochemical zonation and rates of microbial respiration in the OMZs off Namibia and Peru.
(a-c) Namibian shelf (station 252, 111m). (d-f) Peruvian coastal OMZ (station 807, 115 m). (g-i) Offshore Peruvian OMZ (station 3, 4697 m). Dashed lines indicate the upper OMZ boundary (O2 ≤15 μmol l-1). Previously determined rates of aerobic and anaerobic NH4+ oxidation [14,24,25] are tenfold magnified. Please note the differences in scale between stations. *Chlorophyll a concentrations in panel b in relative units.
Fig 2.
Abundance of genes and transcripts encoding for terminal respiratory oxidases in the ETSP OMZ.
(a, b) Abundance of low-affinity (cytochrome c oxidase) and high-affinity (cytochrome bd and cbb3 oxidase) aerobic oxidases in the Peruvian OMZ (station 3). (c-f) Abundance and expression of cytochrome oxidase genes in the OMZ off Chile during cruise MOOMZ-1 [34]. Taxonomic affiliations of cytochrome oxidases are shown on domain, phylum or class level if represented by at least 5% of oxidase-coding sequences. Exact abundance and expression levels as well as taxonomic assignments of the individual types of cytochrome oxidases are given in S3 Table.
Fig 3.
Oxygen sensitivity of aerobic respiration and OMZ particle size distributions.
(a) O2 sensitivity assays in the Namibian (station 225) and Peruvian OMZ (stations 13 and 28) during cruises M76 and M77-3, respectively. Oxygen consumption rates are given as percentages of the highest rate observed (= 100%) among all O2 treatments (see S2 Table for absolute rates). Error bars for O2 consumption rates are standard errors calculated from linear regression. Isolines (grey) indicate diffusion-limited respiration rates inside aggregates of 0.01–25 mm in diameter. A detailed description of how aggregate-size-dependent rates were calculated is included in the S1 File. (b) Vertical distribution of particle volumes (20 m bins) for six size classes between 0.06 and 5.32 mm (ESD) in the central Peruvian OMZ (12.62°S/77.55°W) during cruise M93. Color shading indicates diffusion limitation of aerobic respiration inside particles. For clarity, particles >5.32 mm are not depicted here. A more general overview of particle size distributions in the ETSP OMZ is given in S2 Fig.
Table 2.
Ammonium budget for the upper Namibian and Peruvian OMZ considering aerobic and anaerobic NH4+-producing and consuming processes.
For the sake of clarity, standard errors for the individual processes determined at each station are not listed here (typically ~10% of the measured rate). Directly measured rates are in italics, the remainder were inferred from idealized stoichiometries (see S1 File for further details). Liberation of NH4+ from organic matter via oxic as well as NO3- respiration accounts for bacterial N-uptake assuming a growth efficiency of 0.15 [66] and a C/N ratio of 6.6 for the heterotrophic community [67,68].