Temperature incubations of microbial communities from two different locations in Fram Strait

This dataset encompasses all data derived from two temperature incubation experiments aboard the RV Polarstern, using a unicellular microbial community collected from different stations in the Fram Strait during the PS126 campaign on June 1st and 13th, 2021 (Soltwedel, 2021). The initial samples were obtained from a depth of 15 m using CTD-bound Niskin bottles (SBE 32 Carousel Water Sampler attached to a Seabird SBE911+ CTD system; Seabird Scientific, Bellevue, WA, USA) and, following filtration through a 150 µm mesh, the communities were incubated on plankton wheels within three temperature-controlled containers. To simulate current and potential future temperature scenarios in the Arctic Ocean, we selected a control temperature of 2 °C, an intermediate warming scenario of 6 °C, and a higher warming scenario of 9 °C while varying several other parameters such as light, nutrients and dilution level. Parameters were sampled at several timepoints throughout the two incubations. This dataset includes measurements of chlorophyll, particulate nutrients, dissolved nutrients, carbonate chemistry, and flow cytometry at several points of the incubations. For chlorophyll a analysis, 300 ml of sample water were filtered under vacuum (<−200 mbar) onto pre-combusted glass-fiber filters (GF/F Whatman, Maidstone, UK), while 200 ml were used for particulate organic carbon and nitrogen, with equal volumes of ultrapure water used for blank corrections. The filters were stored at −80 °C in 2 ml cryovials (Sarstedt, Nümbrecht, Germany) until analysis. Chlorophyll a filters were manually shredded in 6 ml of 90% acetone and extracted for 20 hours at 8 °C, following EPA method 445.0 (Arar and Collins, 1997). After centrifugation, chlorophyll a concentration was measured using a Trilogy fluorometer (Turner Designs, San Jose, CA, USA) with corrections for phaeopigments through acidification (1 M HCl). For particulate nutrient analysis, filters were acidified with 0.5 M HCl and dried at 60 °C for 12 hours, and the elemental composition was determined via gas chromatography using a CHNS-O elemental analyzer (EURO EA 3000, HEKAtech, Wegberg, Germany). pH was measured with an EcoScan pH 5 meter (ThermoFisher Scientific, Waltham, MA, USA) and a Sentix 62 glass electrode (Mettler Toledo, Columbus, OH, USA), calibrated using a pH 7 buffer solution (Mettler Toledo). Samples for total alkalinity and dissolved nutrients were filtered through a 0.22 µm cellulose-acetate syringe filter (Nalgene, Rochester, NY, USA) and stored at 4 °C in borosilicate and polycarbonate containers. Total alkalinity was determined by potentiometric titration using a TitroLine alphaplus autosampler (Schott Instruments, Mainz, Germany), with corrections applied using certified reference materials from A. Dickson (Scripps Institution of Oceanography, San Diego, CA, USA). The complete carbonate system was modeled using CO2sys (Pierrot et al., 2011), applying the dissociation constants of carbonic acid from Mehrbach et al. (1973), as refitted by Dickson and Millero (1987). Dissolved nutrient concentrations were analyzed using a continuous-flow autoanalyzer (Evolution III, Alliance Instruments, Freilassing, Germany), employing standard methods for nitrate and nitrite (Armstrong et al., 1967), phosphate (Eberlein and Kattner, 1987), silicate (Grasshoff et al., 2009), and ammonium (Koroleff, 1970). For flow cytometry, 3.5 ml of samples were fixed with hexamine-buffered formalin (0.5% final concentration), incubated in the dark for 15 minutes and stored at −80 °C. Samples were thawed and vortexed before 2 µl of Fluoresbrite® Multifluorescent Microspheres beads (1 µm, Polysciences, Warrington, PA, USA) were added to 1 ml of sample. Each sample was measured for three minutes at high flow speed on an Accuri C6 flow cytometer (BD Sciences, Franklin Lakes, NJ, USA), using a threshold of 900 on the FL-3 channel. Subsequently, the samples were incubated for 20 minutes with SYBR® green dye (1x in TE buffer; Thermo Fisher Scientific, Waltham, MA, USA) and then measured again for two minutes at a slow flow speed with a threshold of 900 on the Fl-1 channel on the same device. The dataset provides valuable insights into the temperature responses of Arctic microbial communities. Accompanying 18S and 16S metabarcoding data from the two temperature incubation experiments have been deposited in the European Nucleotide Archive (ENA) at EMBL-EBI under accession number PRJEB81706 (https://www.ebi.ac.uk/ena/browser/view/PRJEB81706).