LAUSR

Grazer-induced colony formation as a defense strategy in microalgae such as Scenedesmus species has been widely reported, but the associated costs and reversibility of the colonies are rarely studied. We experimentally showed that Scenedesmus obliquus formed chained colonies in the presence of a predator, including predators separated from the algae by a membrane, but quickly reverted to single cells after the removal of the predator—a defining characteristic of an inducible defense. We detected stress indicators—astaxanthin esters—in the algal populations in the presence of grazers but not when grazers were absent. We found significant costs associated with S. obliquus colony formation in terms of lower population growth rate, lower photosystem II efficiency and lower cellular chlorophyll a content. These results together show that colony formation as an inducible defense in S. obliquus against grazers comes at a substantial cost such that the defense must be switched off and the colonies revert to single cells when the predation risk disappears.