Dataset: Measurement of Polysaccharide hydrolase activities from mesocosm incubations from water samples taken aboard R/V Endeavor cruise EN638 in the Western North Atlantic in May 2019

Samples were taken from incubation experiments of deep chlorophyll maximum (DCM), oxygen minimum zone (OMZ), and bottom waters aboard R/V Endeavor (EN638), May 2019 in the Northern Atlantic. Water was collected via Niskin bottles mounted on a rosette, equipped with a CTD.

For mesocosm (large volume) incubation experiments (referred to as “LV” incubations), seawater was transferred to 20 L carboys that were rinsed three times with water from the sampling depth and then filled with seawater from a single Niskin bottle, using silicone tubing that had been acid washed then rinsed with distilled water prior to use. Four carboys were filled at each depth from bottom water, water from the depth at which oxygen showed a minimum, and deep chlorophyll maximum (DCM) water, according to the CTD. Triplicate 20L carboys were amended with ca. 500 mg (exact mass was recorded for each addition) of High molecular weight (HMW) Thalassiosira; unamended single carboys were used for controls. All mesocosms were incubated in the dark at near in-situ temperatures. Mesocosms were sub-sampled at the start of incubation (0 days), and then after at approximately 3 d, 5 or 7 d, 10 d, 15 d, and 30 d for multiple assays including: bacterial production using 3H-Leucine, dissolved organic carbon (DOC), nutrients, bacterial cell counts, peptidase and glucosidase activity measurements in addition to the potential of the seawater microbial community to hydrolyze six high-molecular-weight polysaccharides (arabinogalactan, chondroitin sulfate, fucoidan, laminarin, pullulan, and xylan).

From each carboy, water was dispensed into smaller glass containers that were cleaned and pre-rinsed three times with water from the carboy prior to dispensing.  A separate glass Duran bottle was filled with seawater from the carboy and sterilized in an autoclave for 20-30 minutes to serve as a killed control for microbial activity measurements. For each substrate, three 50 mL falcon tubes were filled with seawater and one 50 mL falcon tube was filled with autoclaved seawater to serve as a killed control. Substrate was added at 3.5 μM monomer-equivalent concentrations, except for fucoidan, which was added at 5 μM concentrations (a higher concentration was necessary for sufficient fluorescence signal). Two 50 mL falcon tubes – one with seawater and one with autoclaved seawater – with no added substrate served as blank controls. Incubations were stored in the dark at as close to in situ temperature as possible.

Subsamples of the incubations were collected at time zero, and at a sequence of subsequent time points. At each time point, 2 mL of seawater was collected from the 50 mL falcon tube using a sterile syringe, filtered through a 0.2 μm pore size syringe filter, and stored frozen until analysis.

Gel permeation chromatography with fluorescence detection was used to measure the hydrolysis of high molecular weight substrate to lower molecular weight hydrolysis products after the method of Arnosti (1996, 2003).  In short, the subsample was injected onto a series of columns consisting of a 21 cm column of G50 and a 19 cm column of G75 Sephadex gel for adequate molecular size separation. The fluorescence of the column effluent was measured at excitation and emission wavelengths of 490 and 530 nm, respectively.