Four diatom strains obtained from different culture collections were grown to stationary phase in L1 medium plus silicate (Guillard & Hargraves 1993) at 15°C under a light/dark regime of 14/10 h. For experimentation, the diatom strains were incubated at either atmospheric pressure (0.1 MPa = control treatment) or at gradually increasing hydrostatic pressure (0.1-100 MPa = pressure treatment) throughout 21 days in darkness and at 3°C. The pressure treatment simulated the sinking of particle-associated diatoms from the ocean surface down into a hadal trench of 10 km depth. Pressure-induced leakage of dissolved organic matter (DOM) was followed throughout the incubations. Both ambient concentrations and intracellular concentrations of different DOM components were measured: Dissolved organic carbon (DOC), total dissolved nitrogen (TDN), protein-like and humic-like DOM fluorescence, total proteins, total polysaccharides, laminarin, and dimethylsulfoniopropionate (DMSP). The cell abundance of the different diatom strains was followed throughout the incubations.

Information about the used cultured organisms:Skeletonema marinoi, Bigelow National Center for Marine Algae and Microbiota, Strain No. CCMP1332, isolated from Long Island Sound (41.2264 N, -73.0639 W) (isolation date is unknown); https://ncma.bigelow.org/CCMP1332Thalassiosira weissflogii, Bigelow National Center for Marine Algae and Microbiota, Strain No. CCMP1336, isolated from Long Island Sound (41.11 N, -72.1 W) (isolation date is unknown); https://ncma.bigelow.org/CCMP1336Chaetoceros socialis, Roscoff Culture Collection, Strain No. RCC6904, isolated from Gulf of Naples (40.7344 N, 14.2718 E) in 2016; https://roscoff-culture-collection.org/rcc-strain-details/6904Phaeodactylum tricornutum, Algal Bank GUMACC (University of Gothenburg), Strain No. 2, isolated from Gullmarsfjorden (58.2934 N, 11.5096 E) in 2004; https://fmprod27.it.gu.se/fmi/webd/GUMACC