Symbiont-specific responses in foraminifera to the herbicide diuron

► Symbiotic foraminifera are highly sensitive to herbicide-induced photodamage. ► Symbiont type and ultrastructure determine herbicide kinetics and extent stress. ► Hyaline, diatom-hosting foraminifera proved most vulnerable to diuron. ► Herbicide concentrations measured in flood plumes may chronically impact photosynthesis in symbiotic foraminifera.