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We report that green algae in lakes and rivers can serve as precursors of halobenzoquinone (HBQ) disinfection byproducts (DBPs) produced during chlorination.Chlorination of a common green alga,Chlorella vulgaris,produced 2,6-dichloro-1,4-benzoquinone (2,6-DCBQ),the most prevalent HBQ DBP in disinfected water.Under varying pH conditions (pH 6.0-9.0),2,6-DCBQ formation ranged from 0.3 to 2.1 μg/mg C with maximum formation at pH 8.0.To evaluate the contribution of organic components of C.vulgaris to 2,6-DCBQ formation,we separated the organics into two fractions,the protein-rich fraction of intracellular organic matter (IOM) and the polysaccharide-laden fraction of extracellular organic matter (EOM).Chlorination of IOM and EOM produced 1.4 μg/mg C and 0.7 μg/mg C of 2,6-DCBQ,respectively.The IOM generated a two-fold higher 2,6-DCBQ formation potential than the EOM fraction,suggesting that proteins are potent 2,6-DCBQ precursors.This was confirmed by the chlorination of proteins extracted from C.vulgaris:the amount of 2,6-DCBQ produced is linearly correlated with the concentration of total algal protein (R2 =0.98).These results support that proteins are the primary precursors of 2,6-DCBQ in algae,and control of green algal bloom outbreaks in source waters is important for management of HBQ DBPs.