Magnesium depletion suppresses the anti-grazer colony formation in Scenedesmus obliquus.

In aquatic ecosystems, many phytoplankton species have evolved various inducible defense mechanisms against the predation. The expression of these defenses is affected by environmental conditions such as nutrient availability. Here, we investigated the anti-grazer colony formation in Scenedesmus obliquus at different magnesium concentrations (0-7.3 mg L-1  Mg2+ ) in the presence of zooplankton (Daphnia)-derived infochemicals. Results showed that at adequate Mg2+ , S. obliquus formed high proportions of multi-celled (e.g., four- and eight-celled) colonies, resulting in significantly increased number of cells per colony in response to Daphnia filtrate. On the other hand, in Mg2+ -deficient treatment, the proportion of multi-celled colonies decreased, together with reduced algal growth rate and photosynthetic efficiency. Finally, the treatment without Mg2+ strongly suppressed the formation of large colony (mainly eight-celled colonies), whereas the algal growth rate was comparable to that in Mg2+ sufficient treatment. Despite the inhibition of colony formation, the time reaching the maximum number of cells per colony was not affected by the Mg2+ concentration, which generally took three days in all groups. Our results indicate that Mg2+  deficient/absent environments significantly reduced anti-grazing colony formation but not the algal growth, suggesting strong dependability of this morphological defensive trait to magnesium fluctuation in S. obliquus.