All surface seawater was collected just prior to local sunrise from a SBE-911+ Conductivity-Temperature-Depth (CTD, Sea-Bird Scientific) Niskin bottle rosette cast and then transferred or directly filtered into acid-cleaned 20-liter (L) carboys with platinum-cured silicone tubing.

Bacterial growth and dissolved organic carbon (DOC) utilization were estimated in six microbial remineralization experiments following Baetge et al (2021), in which changes in bacterioplankton cell densities, bacterial organic carbon, and TOC were measured over five day incubations. For each experiment, 3-micrometer (µm) filtered surface seawater was directly and gently gravity-filtered through a mixed cellulose ester (MCE) membrane filter (EMD Millipore, 142-millimeters (mm)) in a polycarbonate filtration cartridge (Geotech Environmental Equipment, Inc.) with platinum-cured silicone tubing. The filtrate was then filtered through a 1.2- or a 0.2-µm MCE filter (EMD Millipore, 142 mm) to generate a bacterial inoculum or DOC media, respectively. Experiments were initiated when filtrates were mixed in a ratio of 1:4 bacterial inoculum to DOC media. Mixed seawater either remained unamended or was amended with ash leachate prior to being divided into a modified acid-washed 5 L polycarbonate bottle (Biotainer, Nalgene) (Baetge et al., 2021) and a set of 18 pre-combusted 40-milliliter (ml) EPA vials. Ash leachate DOC was added to experimental treatments at a final amendment concentration of ~4 µmol C/L Bottles and vials were incubated in the dark within ± 2 degrees Celsius (C) of in situ temperatures using wine coolers (YEG-2WS24-HD, YEEGO).

For each experiment, 2 ml samples for bacterial abundance were collected daily from all incubations. Triplicate 40 ml EPA vials were preserved daily as total organic carbon (TOC )samples after acidification to a pH of < 3 with 4 N HCl. Bacterial organic carbon (BOC) samples were collected in triplicate 1 L polycarbonate bottles from the bacterial inoculum and as samples from the 5 L polycarbonate bottles on the second and fifth days of the experiment. Bacterioplankton abundance (cells/L) samples were fixed upon collection with paraformaldehyde to a final concentration of 1%, flash-frozen in liquid nitrogen, and stored at -20 degrees C. Sample volumes for BOC (micromoles C per liter (µmol C/L)) measurements were filtered through two stacked pre-combusted 0.3 µm GF/75 filters (25 mm, Advantec) in inline polypropylene cartridges (Cole-Parmer). At three experimental sites, DOC media (i.e., 0.2 µm filtrate) was also filtered to generate triplicate background correction samples to account for DOC filter adsorption (Graff et al., 2023). All filters were stored in pre-combusted 20 ml borosilicate glass vials (Wheaton) at -20 degrees C, and then later fumed for 24 hours with HCl and dried for 24 hours at 50 degrees C prior to analysis.

953702_v1_dom_remineralization_bioassays.csv