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The inclusion Asparagopsis spp. into the diet of ruminant animals has produced compelling data regarding the mitigation of agricultural methane emissions. This reduction is achieved via the action of brominated halogenated compounds, predominantly bromoform, which act to inhibit methanogenic enzymes in ruminant digestion. As such, there is great interest in the mass cultivation of Asparagopsis for use as a dietary supplement for livestock. However, data are still lacking on the basic biology of Asparagopsis relating to factors that influence the synthesis of bromoform, the key bioactive compound of interest. One of the two precursors for bromoform biosynthesis is hydrogen peroxide, while the other is bromide, a naturally occurring ion in seawater. Hydrogen peroxide is generated internally within the alga and can be stimulated by abiotic stress. Currently, the influence of temperature and external hydrogen peroxide addition on bromoform dynamics have been explored. The aim of this study is to explore how the stimulation of hydrogen peroxide by the application of light stress influences the dynamics of bromoform precursor uptake and production, as well as how this may drive changes in bromoform concentration and the persistence of gland cells, the cellular structures where bromoform is stored. While provision of light stress significantly stimulated an increase in hydrogen peroxide production, bromide dynamics were also significantly influenced, resulting in net bromide release, rather than uptake. Further, bromoform concentrations in algal tissue immediately declined after exposure to high light, from 4.5% to 2% (dry weight), while gland cell abundance declined from 95% to around 60%. Here we present data for dramatic alterations in bromoform dynamics after exposure to moderate increases in light intensity. These findings are strongly applicable to commercial Asparagopsis cultivation and will contribute to optimising algal quality during cultivation and harvest.