Location

Water collection sites in the northern Gulf of Mexico, particularly at the mouth of Mobile Bay and Little Lagoon, AL.

Water Collection

Briefly, water was collected from the field using a 5-gallon bucket, pre-screened with a 200 µm nitex mesh, and gently poured into 10-20 L carboys and kept in the dark until returning to the laboratory for same-day processing.

Terminology

dDA – dissolved Domoic Acid
pDAa – particulate Domoic Acid (algal fraction)
pDAOP – particulate Domoic Acid (bound to organic polymers)
cDA – Domoic Acid in copepods
POC – Particulate Organic Carbon

Organic polymer formation and sorption of DA

Seawater organic polymers were formed in controlled laboratory conditions to verify whether they could scavenge dDA (see dataset https://www.bco-dmo.org/dataset/808280). Surface seawater was collected from Dauphin Island (AL, USA) and filtered through a new 0.2 µm polycap filter (Pall Brand, USA). The freshly filtered seawater was partitioned into 1-L polycarbonate bottles and the initial measurements of dDA, pDA, cDA, pDAOP, and POC were made. POC was used as a proxy for organic polymer formation. The same filtration techniques were used for dDA and pDA as described above. Laboratory-reared adult Acartia tonsa were collected on a 200 µm screen and gently rinsed with freshly filtered artificial seawater, then transferred into 2 mL cryovials and stored in -20°C until analyzed for cDA. Twenty-five mm glass fiber filters were pre-combusted at 500°C for four hours and used to collect organic polymers. The organic polymer collection method was modified from Passow et al. (1995); loss of organic polymers via filtration was minimized by maintaining low-vacuum (<200 mbar) and filtering samples for a maximum of 15 minutes. Lastly, the treatment bottles were spiked with a DA standard to bring the final concentration to 10 µg DA L-1 and, for specific treatment bottles, 30 copepods were added to each bottle. Bottles were then placed on orbital shaker tables and gently shaken for 24 hours. Controls were not shaken. Samples for dDA, pDA, cDA, pDAOP, and POC were collected after the 24-hour time period.

Liquid chromatography-mass spectrometry method for domoic acid quantification

LC-MS sample preparation followed was modified from Wang et al. (2012) for the determination of dDA, pDA, pDAOP and cDA. The samples for DA determination were cleaned and concentrated using Bond Elut LRC - C18, 200 mg, solid-phase extraction (SPE) columns from Agilent Technologies. For dDA, 30 mL seawater samples were filtered using a 47 mm glass fiber filter; the filtrate was collected and acidified with formic acid to yield a 0.2% final solution. SPE columns were conditioned with one column volume of HPLC-grade methanol followed by one column volume of HPLC-grade water. Samples were then loaded on the SPE column and filtered at ~1 mL min-1 using a vacuum manifold, followed by 10 mL of 0.2% formic acid as a rinse for the sample tube and SPE column. The SPE column was then allowed to go dry and was eluted with 1.5 mL of 20 mM ammonium acetate in 50% methanol (pH 8) and collected in a glass tube. The tubes were centrifuged for 5 minutes at ~1300 x g, supernatant was transferred into an LC vial with a Pasteur pipette, and stored at 4°C until further analysis. For pDAa 100 mL of seawater were filtered through a 5 µm polycarbonate filter and stored in a 50 mL polypropylene tube at -20°C. Similarly, for pDAOP 150 mL of seawater was filtered through a pre-combusted 25 mm glass-fiber filter and stored at -20°C. Prior to concentration and clean-up for pDA, pDAOP, and cDA, the filters were submerged in 2 mL of 80% methanol and sonicated to ensure cells and copepods were lysed. Sonication pulses were done for a total of 45 seconds (5 seconds on/off) on a Sonics Materials Ultrasonic Processor (model - VCX 130) at 75% power. Subsequent clean-up using the SPE column is the same as for the dDA samples.

An ultra-performance liquid chromatography (UPLC) – tandem mass spectrometry (MS) system was used for the quantification of DA.The LC-MS system consisted of Acquity UPLC system (Waters, Milford, MA) coupled to a 5500 QTRAP triple quadrupole / linear ion trap mass spectrometer equipped with a TurboIonSpray interface (Sciex, Foster City, CA, USA). The analytes were separated on a Luna C18 (2), 2.0 x 100 mm column (Phenomenex, Torrance, CA, USA) with column temperature held at 40ºC. The mobile phase was water (A) and 95% aqueous acetonitrile (B) with 0.1% formic acid additive and the flow rate was 0.4 ml/min. Gradient program was: 5% B for 3 min, linear gradient to 60% B at 10 min, 95% B at 10.1 min, hold at 95% B for 2 min. MS was operated in positive ion mode. Ion spray voltage was 5 kV and declustering potential was 80 V. Gas parameter settings were: nebulizer gas, 50 psi; turbo gas, 50 psi at 500ºC; curtain gas, 20 psi; and collision gas, medium setting. The collision energy applied was 25eV. The transitions used for selected reaction monitoring were m/z 312→266, 193, 220. The transition m/z 312→266 was used for quantitation.

For field-simulation experiment results and methodology see dataset https://www.bco-dmo.org/dataset/808413