Bromide elevation in surface freshwater is unavoidable due to climate change and anthropogenic activities. Freshwater impaired by algal activity and bromide elevation has become problematic worldwide. However, studies of their impact on water quality and treatment practices are limited. This study is the first to demonstrate that genera of freshwater algae (e.g., cyanobacterium Microcystis aeruginosa and green alga Chlorella sp.) can accumulate bromine within their intracellular organic matter (IOM) when bromide occurs in water (i.e., 0, 0.5, 1, and 4 mg/L), which substantially enhances brominated disinfection byproduct (Br-DBP) formation upon chlorination. Using energy-dispersive and X-ray photoelectron spectrometers, we discovered clear bromine signals on algal surfaces in both cultures and natural water. Bromide elevation in source water particularly increased bromine accumulation inside algae cells and intracellularly, with greater than 33%-85% found in IOM. This phenomenon dramatically increased Br-DBP formation from Br-rich IOM precursors to ∼80% of total DBPs at an elevated Br- level of 4 mg/L, resulting in the exponential rise of toxic potency of chlorinated water. These findings are significant to understand the origins of natural organic bromine in freshwater environments and provide further insights into how the co-impairment of algae and bromide elevation may influence finished water quality upon chlorination.