Photodegradation of the algal toxin domoic acid in natural water matrices

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 (kobs), obtained by linear regression of the logarithmic-transformed domoic acid concentrations versus irradiation time in simulated sunlight, was 0.15 (plus-minus) 0.01 h^-1 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^-1 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^-1. 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^-1. These observations indicate that sunlight-mediated reactions are an important, yet previously unrecognized, sink of dissolved domoic acid in seawater.