Dataset: Polysaccharide hydrolysis rates from mesocosm and bulk water incubations from waters taken aboard the R/V Endeavor in the Western North Atlantic during the research cruise EN683 in May and June 2022

Seawater was collected aboard R/V Endeavor during the research cruise EN683 (2022-05-24 to 2022-06-12). Water samples were taken at the depth of the deep chlorophyll maximum (determined via CTD; ca 35m and 152m, respectively) and at the bottom, 4092m  and 5305 m, respectively. The following stations were sampled in the Western North Atlantic:

• Station 21 at approximately 33 degrees 44  N, 75 degrees 17 W
• Station 22 at approximately 42 degrees 52  N, 53 degrees 52 W
• Station 23 at approximately 34 degrees 20 N, 69 degrees 49 W

Water was collected via Niskin bottles mounted on a rosette, equipped with a CTD. 

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. 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. This water was used to measure cell counts, bacterial productivity, and the activities of polysaccharide hydrolases, peptidases, and glucosidases. 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 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. At stations 22 and 23, carboys were filled from bottom water and deep chlorophyll maximum (DCM) water each, according to the CTD. Triplicate 20L carboys (a total of 9 carboys) were amended with high molecular weight material isolated from the diatom Thalassiosira, or the polysaccharide fucoidan, or the polysaccharide arabinogalactan; additional unamended single carboys were used for controls. 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. This water was used to measure the activities of peptidases, and glucosidases. A separate glass Duran bottle was filled with seawater from the carboy and sterilized in an autoclave to serve as a killed control for microbial activity measurements. All mesocosms were incubated in the dark at near in-situ temperatures. Mesocosms were sub-sampled for polysaccharide hydrolase activity measurements at the start of incubation (0 days), and then after at approximately 5d, 10d, 15d, and 25d.

Polysaccharide hydrolase activity was measured at each sub-sampling by filling three 50 mL falcon tubes with mesocosm incubated seawater and one 50 mL falcon tube was filled with autoclaved mesocosm incubated seawater to serve as a killed control, for each substrate.   Polysaccharide 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 processing.

The hydrolysis of high molecular weight substrate to lower molecular weight hydrolysis products was measured using gel permeation chromatography with fluorescence detection, 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. The fluorescence of the column effluent was measured at excitation and emission wavelengths of 490 and 530 nm, respectively.