The role of particles on the biogeochemical cycling of domoic acid and its isomers in natural water matrices

The adsorption of dissolved domoic acid (DA) and its geometrical isomers was assessed in aqueous solutions containing various types of particles. In one series of experiments carried out in coastal seawater, detectable net adsorption of 100 nM DA occurred only onto natural seawater particles (unfiltered seawater) and 0.5 g L−1 chromatographic silica (18%) in 0.2 μm-filtered seawater. Some net adsorption (<5%) also occurred in the 0.5 g L−1 suspension of estuarine sediment and 0.5 g L−1 solution of humic acid in filtered seawater. No losses were seen in 0.5 g L−1 suspensions of illite, kaolinite, montmorillonite, and silica sand. Biological degradation accounted for small losses (8–10%) in filtered seawater without particles. A second series of experiments using organic-free, <5 μm fractions of kaolinite and montmorillonite in deionized water (DIW) demonstrated that 70% of DA adsorbed onto kaolinite, but only 5% onto montmorillonite. Geometrical isomers of DA (iso-DA D, E, and F) showed negligible adsorption (0–8%) onto a variety of particles in filtered seawater, suggesting that major ions in seawater neutralize electrostatic attractions between particles and DA isomers. These results suggest that DA and its isomers are relatively hydrophilic and not particle reactive. Our data suggest that photochemical and biological degradation of dissolved DA and its isomers appears to occur in bulk surface seawater and its transport to bottom sediments must be mainly biologically driven.