Award: OCE-1634154

Single-celled organisms known collectively as phytoplankton play an important role in the accumulation of the most toxic form of mercury, methylmercury, in aquatic food webs. Methylmercury is concentrated by phytoplankton from water by a factor of more than 10 thousand and cells enriched in methylmercury are consumed by zooplankton, a primary food source of larval, juvenile, and some adult fish. Indeed, nearly all of the methylmercury accumulated by fish is from their diet. Sunlight drives the degradation of methylmercury in the ocean, but the effects of such photochemical processes on methylmercury inside phytoplankton cells in the sunlit upper layers of the ocean have not been studied. In addition, photochemical transformations of mercury have been shown to selectively favor certain isotopes of mercury, producing reactant and product pools of mercury with measurably different isotope ratios. The goals of this project were to 1) quantify the degradation of methylmercury inside phytoplankton cells exposed to sunlight and to 2) evaluate how such reactions affect the ratios of mercury stable isotopes in marine food webs. In laboratory experiments with a species of marine phytoplankton that was originally isolated from the Irish Sea, we examined the accumulation and photochemical transformations of cellular methylmercury exposed to various wavelengths of light. Our results showed that methylmercury inside phytoplankton cells was degraded upon exposure to both high energy ultraviolet light and moderate energy visible light. The results with visible light indicate that far more degradation of methylmercury occurs in the ocean than previously thought. We also found that the ratios of mercury isotopes left behind in the phytoplankton followed similar patterns to what is observed in marine fish, including species consumed by humans. This will allow us to better track how natural and pollution mercury enters marine food webs. Last Modified: 12/29/2019 Submitted by: John R Reinfelder