Nutrient availability influences kairomone-induced defenses in Scenedesmus acutus (Chlorophyceae)

Abstract

Many phytoplankton species have evolved induced defenses against herbivore grazers, but the effectiveness of, and costs associated with, these defenses may depend on environmental conditions. For example, grazer chemical cues (kairomones) induce colony formation in many phytoplankton, but temperature, light and nutrient availability can also affect colony size. Little is know about how kairomones and abiotic variables interact to induce algal defenses. Here, we investigate this interaction in the freshwater chlorophyte Scenedesmus acutus, which responds to kairomones from zooplankton herbivores and changes in the abiotic environment by forming colonies (coenobia) of up to eight cells. We tested the effects of ambient phosphorus concentration (0.5, 1.0 and 5.0 μM) and exposure to kairomones from the zooplankton herbivore Daphnia magna on S. acutus growth and coenobia formation. Daphnia magna kairomones did not affect S. acutus population growth during the exponential phase. While kairomones induced coenobia formation in all three P treatments, more coenobia were formed in the absence of kairomones in high-P treatments than in low-P treatments. Thus, coenobia formation appears to be a primarily constitutive defense when P is abundant, but an induced defense when P is scarce, suggesting a trade-off between coenobia formation and phosphorus availability in S. acutus. © 2012 The Author 2012. Published by Oxford University Press. All rights reserved.