Photodegradation of the algal toxin domoic acid in natural water matrices

Abstract

We investigated the photodegradation rate of the powerful marine toxin domoic acid in a variety of natural water matrices. The observed first-order photodegradation rate coefficient (k obs ), obtained by linear regression of the logarithmic-transformed domoic acid concentrations versus irradiation time in simulated sunlight, was 0.15 Ϯ 0.01 h Ϫ1 in coastal seawater. Photodegradation rate coefficients in deionized water were not significantly different than those in coastal seawater, indicating that domoic acid is mainly photodegraded through a direct photochemical pathway. Addition of 100 nmol L Ϫ1 spikes of iron III [Fe(III)] and copper II [Cu(II)] had no significant effect on domoic acid photodegradation, indicating that the formation of trace-metal chelates did not enhance photodegradation of the toxin in seawater. We observed an increase of domoic acid photodegradation rates with temperature with a corresponding energy of activation of 13 kJ mol Ϫ1 . The effect on photodegradation of pH, added humic material, and dissolved oxygen removal was also investigated. The quantum yield of domoic acid photodegradation in seawater decreased with increasing wavelength and decreasing energy of incoming radiation, with the average value ranging from 0.03 to 0.20 in the ultraviolet wavelength range (280-400 nm). Using these quantum yields together with modeled solar spectral irradiance and seawater optical properties, we estimated turnover rate coefficients for the photochemical degradation of domoic acid ranging from 0.017 to 0.035 d Ϫ1. These observations indicate that sunlight-mediated reactions are an important, yet previously unrecognized, sink of dissolved domoic acid in seawater.

Rate and apparent quantum yield of photodissolution of sedimentary organic matter

“…While direct photochemical reactions often have very low activation energies, thermally driven (non-photochemical) reactions have much higher values. We computed a moderate E a value for photodissolution, 32 6 7 kJ mol 21 , which is somewhat higher than rarely determined activation energies of dissolved-phase, marine organic photochemical reactions: 12.2 kJ mol 21 for CO photoproduction (Zhang et al 2006), 13 kJ mol 21 for domoic acid photolysis (Bouillon et al 2006), and 23 kJ mol 21 for near-surface DMS photolysis (Toole et al 2003). The phase change inherent to photodissolution (i.e., desorption or dissolution of POC) may account for the increased activation energy of this reaction.…”

Intra- and interspecies differences in growth and toxicity of Pseudo-nitzschia while using different nitrogen sources

“…One strain (Pn-12) produced DA during late exponential phase while the other (Pn-9) did not produce toxin until stationary phase. DA is a relatively stable molecule under laboratory culture conditions (Bouillon et al 2006), thus high DA content of the first sample in some strains (Pn-1, CLN47 and Pn-12) is probably carry-over from the inoculum used to start the culture and does not signify any real production of DA in those cultures at the beginning of growth.…”

Domoic acid uptake and elimination kinetics in oysters and mussels in relation to body size and anatomical distribution of toxin

“…Although Pseudonitzschia cells can release relatively large amounts of DA in the water column, especially under macronutrient- (Bates, 1998)o r iron-limitation (Maldonado et al, 2002), only minor toxin incorporation (0.3-0.6% of available DA in 5-24 h) has been reported from the dissolved phase by mussels Novaczek et al, 1991). Fast photodegradation (Bouillon et al, 2006) may further limit the availability of dissolved DA to marine organisms. Therefore, toxic diatom cells (i.e.…”