The interaction of sunlight and dissolved chromophoric matter produces reactive chemical species that are significant in the removal of dimethyl sulfide (DMS) in the surface ocean. Using artificial solar radiation, we examined the role of several inorganic components of seawater on the kinetics of NO3- -photolysis-induced DMS removal in aqueous solution. This study strongly suggests that NO3- photolysis products react significantly with DMS in aqueous solution possibly via an electrophilic attack on the electron-rich sulfur atom. This supports previous field observations that indicate that NO3- photolysis has a substantial control on DMS photochemistry in nutrient-rich waters. A key finding of this research is that the oxidation rate of DMS induced by NO3- photolysis is dramatically enhanced in the presence of bromide ion. Moreover, our results suggest that bicarbonate/carbonate ions are involved in free radical production/scavenging processes important for DMS photochemistry. These reactions are pH dependent. We propose that DMS removal by some selective free radicals derived from bromide and bicarbonate/carbonate ion oxidation is a potentially important and previously unrecognized pathway for DMS photodegradation in marine waters.

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[1] The wavelength dependence of the photochemical removal efficiency for DMS was studied for samples from an iron-induced bloom in the Northeastern Pacific Ocean. In July 2002, a 64 km 2 patch of ocean was iron-fertilized near Ocean Station Papa (50°12 0 N; 144°45 0 W). Only small changes in pseudo-first-order apparent quantum yield (AQY * DMS (l)) were observed outside the iron-patch. However, inside the patch, AQY * DMS (l) decreased considerably over the two weeks following the initial iron injection. A positive strong correlation was found between pseudo-first-order apparent quantum yield determined at 330 nm (AQY * DMS (330 nm)) and NO À 3 concentrations. We propose that NO À 3 -photolysis has a substantial influence on DMS photo-degradation rates in oceanic waters. This finding demonstrates that in addition to control DMS production, marine phytoplankton could indirectly influence the DMS photochemical loss rate via its control on NO À 3 distribution.

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.