 monomethylmercury is a neurotoxin that poses significant risks to human health1 due to its bioaccumulation in food webs 
 . sunlight degradation to inorganic mercury is an important component of the mercury cycle that maintains methylmercury at low concentrations in natural waters . 
 rates of photodecomposition , however , can vary drastically between surface waters25 for reasons that are largely unknown . here , we show that photodegradation occurs through singlet oxygen , a highly reactive form of dissolved oxygen generated by sunlight irradiation of dissolved natural organic matter . 
 the kinetics of degradation , however , depended on water constituents that bind methylmercury cations . relatively fast degradation rates ( similar to observations in freshwater lakes ) applied only to methylmercury species bound to organic sulfur - containing thiol ligands such as glutathione , mercaptoacetate , and humics . 
 in contrast , methylmercury - chloride complexes , which are dominant in marine systems , were unreactive . 
 binding by thiols lowered the excitation energy of the carbon - mercury bond on the methylmercury molecule67 and subsequently increased reactivity towards bond breakage and decomposition . 
 our results explain methylmercury photodecomposition rates that are relatively rapid in freshwater lakes24 and slow in marine waters5 . 
Supplementary Material