ALBERT

Description: In the hadal zone of the ocean (6–11 km), the characteristics of sinking marine snow particles and their attached microbial communities remain elusive, despite their potential importance for benthic life thriving at extreme pressures (60–110 MPa). Here, we used simulation experiments to explore how increasing pressure levels modify the microbial degradation, organic matter composition, and microbiome of sinking diatom aggregates. Individual aggregates were incubated in rotating tanks in which pressure was incrementally increased to simulate a descent from surface to hadal depth within 20 days. Incubations at atmospheric pressure served as controls. With increasing pressure, microbial respiration and diatom degradation decreased gradually and ceased completely at 60 MPa. Dissolved organic carbon leaked substantially from the aggregates at ≥40 MPa, while diatom lipid and pigment contents decreased moderately. Bacterial abundance remained stable at 〉40 MPa, but bacterial community composition changed significantly at 60–100 MPa. Thus, pressure exposure reduces microbial degradation and transforms both organic matter composition and microbiomes of sinking particles, which may seed hadal sediments with relatively fresh particulate organic matter and putative pressure-tolerant microbes.

Description: Cold-water coral (CWC) reefs are distributed globally and form complex three-dimensional structures on the deep seafloor, providing habitat for numerous species. Here, we measured the community O2 and dissolved inorganic nitrogen (DIN) flux of CWC reef habitats with different coral cover and bare sediment (acting as reference site) in the Logachev Mound area (NE Atlantic). Two methodologies were applied: the non-invasive in situ aquatic eddy co-variance (AEC) technique, and ex situ whole box core (BC) incubations. The AEC system was deployed twice per coral mound (69 h in total), providing an integral estimate of the O2 flux from a total reef area of up to 500 m2, with mean O2 consumption rates ranging from 11.6 ± 3.9 to 45.3 ± 11.7 mmol O2 m-2 d-1 (mean ± SE). CWC reef community O2 fluxes obtained from the BC incubations ranged from 5.7 ± 0.3 to 28.4 ± 2.4 mmol O2 m-2 d-1 (mean ± SD) while the O2 flux measured by BC incubations on the bare sediment reference site reported 1.9 ± 1.3 mmol O2 m-2 d-1 (mean ± SD). Overall, O2 fluxes measured with AEC and BC showed reasonable agreement, except for one station with high habitat heterogeneity. Our results suggest O2 fluxes of CWC reef communities in the North East Atlantic are around five times higher than of sediments from comparable depths and living CWCs are driving the increased metabolism. DIN flux measurements by the BC incubations also revealed around two times higher DIN fluxes at the CWC reef (1.17 ± 0.87 mmol DIN m-2 d-1), compared to the bare sediment reference site (0.49 ± 0.32 mmol DIN m-2 d-1), due to intensified benthic release of NH4+. Our data indicate that the amount of living corals and dead coral framework largely contributes to the observed variability in O2 fluxes on CWC reefs. A conservative estimate, based on the measured O2 and DIN fluxes, indicates that CWC reefs process 20% to 35% of the total benthic respiration on the southeasterly Rockall Bank area, which demonstrates that CWC reefs are important to carbon and nitrogen mineralization at the habitat scale.

Description: To investigate sedimentation processes in hadal trenches, sediment cores were collected from Atacama, Kuril-Kamchatka, Kermadec and Mariana Trench regions. We collected 9 sediment cores from Atacama Trench (R/V Sonne SO261; from 2/Mar to 2/Apr/2018), 4 sediment cores from Kuril-Kamchatka Trench (R/V Sonne SO250; from 16/Aug to 26/Sep/2016), 3 sediment cores from Kermadec Trench (R/V Tangaroa; TAN1711 from 24/Nov to 14/Dec/2017), and 2 sediment cores from Mariana Trench (R/V Yokosuka; YK10-11 from 20/Nov to 4/Dec/2010), respectively. Using with these cores, we measured radionuclides (210Pb, 214Pb and 137Cs) and total organic carbon (TOC) profiles to calculate sedimentation, mass accumulation and TOC deposition rates. This dataset contains sapling site locations, 210Pb (and excess 210Pb), 214Pb and 137Cs concentrations and TOC contents of these cores. 14C ages of organic carbon are also measured for Mariana Trench sediment cores. Sedimentation, mass accumulation and organic carbon deposition rates calculated with excess 210Pb profiles and the surface TOC contents are also provided. Datasets about mass accumulation rates from continental shelf to the hadal environments and burial efficiency of organic carbon are compiled from our data and the previously published papers (references are shown in each file). These data are used for preparing figures, tables and the discussions in Oguri et al. (2022).

Description: Water samples were collected using 7.5-Liter Niskin bottles and fixed with 25% electron microscopy graded glutaraldehyde (1% final concentration) and stored at -80°C until quantification by flow cytometry. Samples were measured in triplicates using a BD FACSCanto™ II flow cytometer, after staining with SYBR Green I (Brussaard 2004). The flow rate was 5–7 μl/min, as determined by BD Trucount™ Beads. The laser settings and gating examples can be found in the Supporting Information of Schauberger et al. (2021).

Description: The effect of increasing hydrostatic pressure on the microbial degradation, the organic matter composition, and the microbiome of 'marine snow' particles was studied in laboratory incubation experiments. Model aggregates were produced from the diatom Skeletonema marinoi and the natural microbial community of surface seawater collected in the Kattegat. The aggregates were incubated individually in rotating pressure and control tanks to keep them suspended during 20-day incubations in the dark and at 3°C. In the pressure tanks, hydrostatic pressure was increased at increments of 5 MPa per day to finally reach 100 MPa. This pressure scheme simulates the descent of diatom aggregates from the surface ocean down into a 10-km deep hadal trench. In the control tanks, pressure always remained at atmospheric level. Aerobic respiration was continuously measured as a proxy for oxidative carbon mineralization in the aggregates (Stief et al. 2021, https://doi.org/10.1002/lno.11791). Leakage of dissolved organic carbon was monitored as an additional carbon loss term. The contents of different diatom lipids and photopigments were measured throughout the incubation. The succession of microbial (mainly bacterial) communities associated with the sinking diatom aggregates was followed by 16S rRNA gene amplicon sequencing throughout the incubation; the corresponding data are deposited in the NCBI short-read archive under the accession number PRJNA976707.

Description: Sediment cores were collected using a multicorer (MUC). Microbes and virus-like particles were extracted from sediments in a 3°C room using a modified version of the washing protocol of Danovaro and Middelboe, 2010 (see Schauberger et al., 2021). After the washing procedure, the extracted microbial cells and virus-like particles were fixed with 25% glutaraldehyde (1% final concentration) and stored at −80°C prior to flow cytometry. These samples were measured in triplicates using a BD FACSCanto™ II flow cytometer, after staining with SYBR Green I. Sediment extracts were diluted 1 : 10 in 0.02 μm-filtered TE Buffer prior to all measurements. The flow rate was 5–7 μl/min, as determined by BD Trucount™ Beads. The laser settings and gating examples can be found in the Supporting Information of Schauberger et al. (2021).

Description: Water samples were collected using 7.5-Liter Niskin bottles and fixed with 25% electron microscopy graded glutaraldehyde (1% final concentration) and stored at -80°C until quantification by flow cytometry. Samples were measured in triplicates using a BD FACSCanto™ II flow cytometer, after staining with SYBR Green I (Brussaard 2004). The flow rate was 5–7 μl/min, as determined by BD Trucount™ Beads. The laser settings and gating examples can be found in the Supporting Information of Schauberger et al. (2021).

Description: Sediment cores were recovered by multiple corers. Upon recovery, sediment cores were brought to a thermoregulated room kept at in situ temperature, here sediment cores were sectioned in predefinded sediment horizons and frozen for later analysis. On land, the frozen sediment was thawn and homogenized. Water contents of the respective samples were determined as the weight loss after 24 h at 105°C and wet density from the weight of a given sample volume. Porosity was calculated from the two measured parameters.

Description: Oxygen microprofiles were measured using an autonomous benthic lander system. Once the lander had stabilized at the seabed, an array of O2 microelectrodes was vertically moved across the sediment-water interface at a predefined resolution - measurements were recorded at each depth after a delay of a few seconds. When the array had reached the maximum measuring depth, sensors were retracted to the start position, and the array was moved horizontally before the measuring routine was repeated (Glud et al. 2021). Sensor signals were converted into O2 concentrations using a linear calibration curve that was based on measurements in the bottom water of known O2 concentration and measurements in the anoxic sediment layers.

Description: Oxygen microprofiles were measured using an autonomous benthic lander system. Once the lander had stabilized at the seabed, an array of O2 microelectrodes was vertically moved across the sediment-water interface at a predefined resolution - measurements were recorded at each depth after a delay of a few seconds. When the array had reached the maximum measuring depth, sensors were retracted to the start position, and the array was moved horizontally before the measuring routine was repeated (Glud et al. 2021). Sensor signals were converted into O2 concentrations using a linear calibration curve that was based on measurements in the bottom water of known O2 concentration and measurements in the anoxic sediment layers.