Description: In a multidisciplinary ex situ experiment, benthic bacterial deep-sea communities from 2,500 m water depth at the Long-Term Ecological Research Observatory HAUSGARTEN (stationPS93/050-5 and 6), were retrieved using a TV-guided multiple corer. Surface sediments (0 - 2 cm) of 16 cores were mixed with sterile filtered deep-sea water to a final sediment dilution of 3.5 fold. The slurries were split and supplemented with five different types of habitat-related detritus: chitin, as the most abundant biopolymer in the oceans, and four different naturally occurring Arctic algae species, i.e. Thalassiosira weissflogii, Emiliania huxleyi, Bacillaria sp. and Melosira arctica. Incubations were performed in five replicates, at in situ temperature and at atmospheric pressure, as well as at in situ pressure of 250 atm. At the start of the incubation and after 23 days, changes in key community functions, i.e. extracellular enzymatic activity, oxygen respiration and secondary production of biomass (bacterial cell numbers and biomass), were assessed along with changes in the bacterial community composition based on 16S rRNA gene and 16S rRNA Illumina sequencing. In summary, differences in community structure and in the uptake and remineralization of carbon in the different treatments suggest an effect of organic matter quality on bacterial diversity as well as on carbon turnover at the seafloor.

Description: Physicochemical parameters and relative abundance of selected nutrient cycling genes were measured in sediment and pore water samples taken from a fish farming area (Site S3 16◦23.097 N, 119◦55.551 E, depth 12.6m) and a control site (Site S1 16◦22.960 N, 119◦54.723 E, depth 15m) not influenced by fish farming. From each site, five pairs of cores representing five biological replicates were collected. One core from each pair was used for physicochemical analysis while the other cores were used for microbial community analysis. Samples from S1 were collected on 2017-06-10 and samples from S3 were collected the following day. The following parameters were measured from the sediment: grain size distribution, % Carbon (%C), % Nitrogen (%N), % total organic carbon (%Corg), and stable isotope (d13C and d15N) content. The following were measured in the pore water: dissolved organic carbon (DOC), total dissolved nitrogen (TDN), ammonium, phosphate, and silicate. Nitrogen (bacterial ammonia monooxygenase/ amoA and nitrite reductase/ nirK) and sulfur (dissimilatory sulfate reductase/ dsrA and sulfur oxidation/ soxB) cycling genes were quantified and normalized against the abundance of the 16S rRNA gene.

Description: Upwelling systems are significant sources of atmospheric nitrous oxide (N₂O). The Benguela Upwelling System is one of the most productive regions worldwide and a temporally variable source of N₂O. Strong O₂ depletions above the shelf are favoring periodically OMZ formations. We aimed to assess underlying N₂O production and consumption processes on different temporal and spatial scales during austral winter in the Benguela Upwelling System, when O₂-deficiency in the water column is relatively low. The fieldwork took place during the cruise M157 (August 4th – September 16th 2019) onboard the R/V METEOR. This expedition included four close-coastal regions around Walvis Bay at 23°S, which presented the lowest O₂ concentrations near the seafloor and thus may provide hotspots of N₂O production. Seawater was collected in 10 L free-flow bottles by using a rosette system equipped with conductivity-temperature-depth (CTD) sensors (SBE 911plus, Seabird-electronics, USA). Incubation experiments were performed using stable isotope ¹⁵N-tracers. Seawater samples for ¹⁵N-tracer incubations and natural abundance N₂O analysis were collected from 10 L free-flow bottles and filled bubble-free into 125 mL serum bottles. The samples for natural abundance N₂O analysis were immediately fixed with saturated HgCl₂ and stored in the dark. To perform the incubation, we added ¹⁵N-labeled NO₂-, NO₃⁻ and NH₄⁺ to estimate the in-situ N₂O production rates and associated reactions. To determine a single rate, the bottles were sacrificed after tracer addition, and within the time interval of 12 h, 24 h and 48 h by adding HgCl₂. Rates were calculated based on a linear regression over time. Total N₂O and natural abundance isotopologues of N₂O were analyzed by using an isotope ratio mass spectrometer (IRMS, Delta V Plus, Thermo Scientific). NO₂- production was additionally analyzed by transforming ¹⁵NO₂- to ¹⁵N₂O following the azide method after McIlvin & Altabet (2005) and the nitrogen isotope ratio of N₂O was measured by an IRMS. N₂ production was determined via an IRMS (Flash-EA-ConfloIV-DELTA V Advanced, Thermo Scientific) by injecting headspace from exetainers. The N₂O yield per nitrite produced and the N₂O yield during denitrification was calculated. Samples for natural abundance N₂O was sampled and measured in triplicates and is shown as an average with standard deviation (SD). In order to estimate the contribution of different N₂O producing pathways by major biological processes and the extent of N₂O reduction to N₂, the dual-isotope mapping approach was applied to natural abundance isotopologues of N₂O, which uses the relative position of background-subtracted N₂O samples in a δ¹⁵Nˢᴾ-N₂O vs. δ¹⁸O-N₂O diagram (Yu et al., 2020; Lewicka-Szczebak et al., 2020).