Grazer density-dependent response of induced colony formation of Scenedesmus obliquus to grazing-associated infochemicals

Highlights

• Induced colony formation of Scenedesmus obliquus increased with increasing grazer density.

• Grazer density-dependent response followed a hyperbolic function at early stage.

• Grazer density-dependent response followed a linear function at late stage.

• Relation between induced colony size and grazer density depend on induction time.

Abstract

To investigate the effects of infochemicals from Daphnia magna reared at different densities on the growth and morphological development of Scenedesmus obliquus, we cultured S. obliquus with filtrates from D. magna with a density of 0, 6, 30, 150, 300, and 600 ind L⁻¹, respectively. Results showed no significant differences in S. obliquus growth rates among the treatments with different Daphnia densities. On days 3 and 5, the proportion of eight-celled colonies in the treatments constituted 30% of the S. obliquus populations. Importantly, the number of cells per particle increased significantly with increasing density of Daphnia, indicating a grazer density-dependent response. Specifically, the density-response relationship followed a rectangular hyperbolic model in the first five days of treatment, and then switched to a linear model, which implies that the relationship between the inducible colony size of S. obliquus and Daphnia density may also be dependent upon incubation time.

Introduction

Many algal species have developed multiple defenses, including constitutive defenses and inducible defenses, to avoid predation by species at higher trophic levels (van Donk et al., 2011). Inducible defenses are a ubiquitous form of phenotypic plasticity that can be an important adaptive strategy in aquatic environments, especially when herbivory is variable (Tollrian and Harvell, 1999; Verschoor et al., 2004). There are extensive studies about inducible defenses in algae (Hessen and van Donk, 1993; Lampert et al., 1994; Lürling, 2003; Yang et al., 2006; Toth and Pavia, 2000), and van Donk et al. (2011) have excellently reviewed the induced defenses in marine and freshwater phytoplankton. Among these, the family of Scenedesmaceae is by far the best studied phytoplankton taxon with respect to grazer-induced defenses (Hessen and van Donk, 1993; Lampert et al., 1994; Lürling, 2003; Verschoor et al., 2004; Yang et al., 2007). Most of these studies have discovered that the presence of herbivorous zooplankton in the filtered culture media increases the formation of colonies in species of the genus Scenedesmus (Hessen and van Donk, 1993; Lampert et al., 1994; Lürling, 2003; Lürling and van Donk, 2000; van Holthoon et al., 2003). After exposure to chemicals released from grazers such as Daphnia, unicellular Scenedesmus rapidly produced populations with eight-celled colonies (Hessen and van Donk, 1993; Lampert et al., 1994; Lürling, 1998). Grazer induced colony formation was interpreted as a defense mechanism against grazing because colonies can experience a lower grazing losses than single cells (Hessen and van Donk, 1993; Lürling and van Donk, 1997; Lürling, 1999).

In aquatic systems, algae experience rapid variations in abundance of grazers (Sommer et al., 1986), which means the grazing risk that the algae are facing is not constant and the grazing-associated infochemical is also variable, thus the degree of inducible colony formation in Scenedesmus obliquus maybe depend on the grazer's density. Some studies have indicated a positive relationship between the number of zooplankton in culture and the degree of induced colony formation in S. obliquus (e.g. Lampert et al., 1994; Lürling and van Donk, 1997; Lürling, 2003; Verschoor et al., 2004), however, a few studies did not find any correlation between induced colony formation and Daphnia biomass in the laboratory (e.g. von Elert and Franck, 1999). To eliminate such uncertainty, we conducted an experiment to test the effects of exudates released from Daphnia reared at different densities on growth and morphology in Scenedesmus. Specifically, we focused on testing the grazer density-dependent response of inducible colony formation in S. obliquus and distinguished the responses at different stages of incubation. As models can provide estimates of biological meaningful parameters that allow us to make quantitative predictions about the change tendency (Xiang et al., 2012), in this study, thus, grazer density- response functions of induced colony formation in S. obliquus were fitted where appropriate.