The highly abundant microalga Emiliania huxleyi forms blooms, and although E. huxleyi blooms harbour a rich community of bacteria, these bacteria are not considered to be a factor that influences algal physiology and bloom dynamics. Segev et al. showed that E. huxleyi associates with the bacterium Phaeobacter inhibens when co-cultured. During the initial 10 days of culturing, the number of algae co-cultured with P. inhibens increased compared with algal cultures alone, which suggests that P. inhibens promotes algal growth. However, after 17 days, most algal cells in the co-culture were dead. These findings suggest that following the mutualistic phase in the algal–bacterial interaction, P. inhibens causes the bleaching and death of its algal partner. Interestingly, the bacterial compound indole-3-acetic acid promotes algal growth at low concentrations, but becomes harmful at higher concentrations and causes algal death.