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Dissolved organic matter molecular analyses were performed on a Solarix FT-ICR-MS equipped with a 15 Tesla superconducting magnet (Bruker Daltonic) using a an electrospray ionization source (Bruker Apollo II) in negative ion mode. Molecular formula calculation for all samples was performed using an Matlab (2010) routine that searches, with an error of < 0.5 ppm, for all potential combinations of elements including including the elements C∞, O∞, H∞, N = 4; S = 2 and P = 1. Combination of elements NSP, N2S, N3S, N4S, N2P, N3P, N4P, NS2, N2S2, N3S2, N4S2, S2P was not allowed. Mass peak intensities are normalized relative to the total molecular formulas in each sample according to previously published rules (Rossel et al., 2015; doi:10.1016/j.marchem.2015.07.002). The final data contained 7400 molecular formulae.

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Sampling was conducted with a TV-guided multiple corer (MUC), in order to retrieve undisturbed sediment cores. Upon retrieval of the MUC, the overlying water was carefully removed from each core and the core was cut into the following sediment horizons using a steel plate and a custom-made plastic ring: 0-1 cm, 1-5 cm, 5-10 cm. At each station, sediment samples from three replicate cores were pooled and transferred to a cooling container (0°C), where subsamples for extracellular enzymatic activity analyses were taken. The potential extracellular enzymatic activities of the hydrolases beta-glucosidase, chitobiase, aminopeptidase and extracellular enzymes acting as esterases were determined in alignment with Boetius and Lochte, 1994 (https://doi.org/10.3354/MEPS104299) and Boetius & Damm 1998 (https://doi.org/10.1016/S0967-0637(97)00052-6). The method uses small substrate proxies linked to a fluorophore. When the fluorophore is cleaved from the substrate, enzymatic hydrolysis is measured as an increase in fluorescence with time. Determinations of extracellular enzymatic activities were performed in triplicate for all samples with a Hitachi F-2000 spectrofluorometer. The substrates and final concentrations used in the assays were the following:100 µM 4-Methylumbelliferyl ß-D-glucopyranoside to measure the activity of beta-glucosidases.100 µM 4-Methylumbelliferyl N-acetyl-ß-D-glucosaminide to measure the activity of chitobiases .500 µM L-Leucine-7-amino-4- methylcoumarin hydrochloride to measure the activity of aminopeptidases.100 µM Fluorescein diacetate to measure the activity of esterases.

Prokaryotic cell abundances and biomass were determined in deep-sea sediment samples collected at the deep-sea LTER HAUSGARTEN, located in the Fram Strait. Sampling was performed during two cruises, i.e. RV Maria S. Merian MSM 29 in summer 2013 (23 June-12 July) and RV Polarstern PS85 in summer 2014 (6 June-3 July). Undisturbed sediment cores were collected with a TV-guided multicorer. Prokaryotic cell abundances were determined using acridine orange direct counts, following the procedure described in (Hoffmann et al., 2017; doi:10.3389/fmicb.2017.00266). For each sample two replicate filters were counted. Cell volumes were determined using a stage micrometer and converted to biomass using a conversion factor of 3 x 10^-13 g C /µm³ (Børsheim, K. Y., Bratbak, G., and Heldal, M. (1990). Enumeration and biomass estimation of planktonic bacteria and viruses by transmission electron microscopy. Applied and Environmental Microbiology 56, 352-356).

Net Primary Production was measured using the 14**C uptake method with minor modifications. Melt pond samples were spiked with 0.1µCi ml**-1 of 14**C labelled sodium bicarbonate (Moravek Biochemicals, Brea, USA) and distributed in 10 clear bottles (20 ml each). Subsequently they were incubated for 12 h at -1.3°C under different scalar irradiances (0-420 µmol photons m**-2 s**-1) measured with a spherical sensor (Spherical Micro Quantum Sensor US-SQS/L, Heinz Walz, Effeltrich, Germany). At the end of the incubation, samples were filtered onto 0.2 µm nitrocellulose filters and the particulate radioactive carbon uptake was determined by liquid scintillation counting using Filter count scintillation cocktail (Perkin Elmer, Waltham, USA). The carbon uptake values in the dark were subtracted from the carbon uptake values measured in the light incubations. Dissolved inorganic carbon (DIC) was measured for each sample using the flow injection system (Hall and Aller, 1992). The DIC concentration was taken into account to calculate the amount of labeled bicarbonate incorporated into the cell. Carbon fixation rates were normalized volumetrically and by chlorophyll a. Photosynthesis-irradiance curves (PI curves) were fitted using MATLAB® according to the equation proposed by Platt et al. (1980) including a photoinhibition parameter (beta) and providing the main photosynthetic parameters: maximum Chla normalized carbon fixation rate if there were no photoinhibition (Pb) and the initial slope of the saturation curve (alpha). The derived parameters: light intensity at which photosynthesis is maximal (Im), the carbon fixation rate at that maximal irradiance (Pbm) and the adaptation parameter or photoacclimation index (Ik) were calculated according to Platt et al. (1982).

Contribution of different molecular categories, H/Cwa and O/Cwa elemental ratios, aromaticity index (Aimodwa) and double bond equivalents (DBEwa) and indices of degradation (Ideg) and lability (MLBwl; Ibioprod). The contribution of each molecular category is expressed as a percentage of the total formulas in each sample obtained after normalization. Molecular formulas were additionally associated with different molecular categories because less than 6% of the DOM has been identified on a molecular level. Molecular categories are defined as follow: 1) condensed aromatics, with an AImod ≥ 0.67 (Koch and Dittmar, 2006, doi:10.1002/rcm.2386) was separated in 3 different subgroups, either <15 (condensed aromatics <15C), or ≥15 (condensed aromatics ≥15C) C atoms both without heteroatoms, or containing heteroelements (condensed aromatics-CHOx); 2) polyphenols, with an AImod >0.5 but <0.67; 3) highly unsaturated formulas, with an AImod <0.5, H/C <1.5; 4) formulas of unsaturated aliphatics, with an 1.5< H/C <2, O/C <0.9, N = 0; 5) saturated fatty acids, with an H/C >2, O/C <0.9 and with either no heteroatoms (sat. fatty acids) or with heteroatoms (sat. fatty acids-CHOx); 6) formulas of sugars, with an O/C >0.9 and with either no (sugars) or with heteroatoms (sugars-CHOx) and 7)unsaturated with N (unsaturated-N) molecular formulas, with 1.5 < H/C <2, O/C < 0.9 and N >0. Additionally, polyphenols, highly unsaturated and unsaturated aliphatics were further separated in oxygen rich (-O-rich) and oxygen poor (-O-poor) if the element ratio O/C was > 0.5 or < 0.5 respectively.