Photorelease of Microcystin-LR From Resuspended Sediments

A series of 12 photolysis experiments was conducted with Microcystis sp. impacted sediments to determine if this process is capable of remobilizing the biotoxin microcystin-LR (MLR) into the water column. There was a net photorelease of MLR in all cases relative to dark controls, ranging from 0.02 µg L^-1 g^-1 – 192 µg L^-1 g^-1 into the dissolved phase. This should be viewed as a minimum estimate of photorelease due to concurrent photodegradation of aqueous phase toxin. MLR concentrations increased linearly in the aqueous phase during a six-hour irradiation suggesting that longer exposure times produce greater quantities of MLR. There was a significant positive correlation between photorelease of the toxin and percent organic carbon of the resuspended material implying that organic-rich sediments yield the greatest photorelease of MLR upon exposure to full spectrum sunlight. Samples exposed to only photosynthytically active radiation produced less than 2% of the photorelease confirming abiotic processes dominate photorelease. Model calculations indicate that photochemical processing of Microcystis sp. impacted sediments could be responsible for as much as 100% of the average standing stock of MLR in a freshwater pond located in southeastern North Carolina where surface water concentrations were also measured. Photoproduction from resuspended sediments therefore represents a significant but previously unrecognized source of highly toxic MLR to sensitive aquatic ecosystems.