LAUSR

Atmospheric CO2 level is increasing and considerably influences aquatic ecosystems. Grazer-induced defenses in phytoplankton stabilize the structures of aquatic food webs. Costs associated with defense have resulted in a situation in which the effectiveness of inducible defenses depends on environmental conditions. To investigate the grazer-induced defense responses to elevated air CO2 levels, the algae Scenedesmus obliquus was cultured under two air CO2 concentrations (390 ppm and 750 ppm) in the absence or presence of predator-derived cues (Daphnia filtrate). In response to Daphnia filtrate, colonial populations such as eight-celled colony were largely formed in S. obliquus. When air CO2 was elevated to 750 ppm, the inorganic carbon contents in media increased, thereby stimulating algal reproduction and photosynthesis. Moreover, the volumes of unicells and two-celled colonies were enlarged under elevated CO2 conditions. These physiological changes satisfied the material and energy demands for the formation of larger colonies. Based on the consistent surface area per volume of the large colony under ambient and elevated CO2 conditions, the increased inorganic carbon (by CO2 enrichment) facilitated the carbon uptake of the multi-celled colony. Thus, the morphological defense in phytoplankton was enhanced with more eight-celled colonies formed in populations grown in a high-CO2 environment. The present result verified the hypothesis that inducible defense is less costly and has higher effectiveness with increasing carbon availability. The enhancement of anti-grazer morphological defenses in phytoplankton potentially alters the trophic cascades between phytoplankton and zooplankton and contributes to the redirected energy and nutrient flow in ecosystem-level processes under a CO2-rising climate.