Photodegradation of the brevetoxin PbTx-2 in coastal seawater

The photodegradation rate of the brevetoxin analog PbTx-2 was investigated in a variety of natural water matrices. The observed first-order photodegradation rate coefficient of the logarithmic-transformed dissolved PbTx-2 concentrations vs. irradiation time was 0.20 +/- 0.04 h(-1) in coastal seawater, corresponding to a half-life of approximately 3 h based on 10 separate photolysis experiments. No loss of PbTx-2 occurred in dark controls, indicating that this was primarily a photo-mediated process. Photodegradation rate coefficients in samples in which dissolved organic matter was removed by ultraviolet oxidation prior to photolysis resulted in significantly slower rates of PbTx-2 photodegradation (0.08 +/- 0.03 h(-1)). When trace metals were also removed prior to photolysis, no loss of PbTx-2 occurred, indicating that direct photolytic loss of PbTx-2 is insignificant in coastal seawater. The proposed mechanism of PbTx-2 decay is a photosensitized pathway involving organic matter and trace metals that is significantly accelerated by decreasing oxygen concentrations. The influence of molecular oxygen on the rate of toxin loss has important ramifications for the fate of PbTx-2 during Karenia brevis blooms, as in situ dissolved O-2 concentrations fluctuate widely during bloom development and decay. Sunlight-mediated reactions are therefore a significant, yet previously unrecognized, sink of dissolved PbTx-2 in seawater under environmentally relevant conditions.