ARTICLE IN PRESS Limnologica 37 (2007) 137–145 www.elsevier.de/limno Direct effects of Daphnia-grazing, not infochemicals, mediate a shift towards large inedible colonies of the gelatinous green alga Sphaerocystis schroeteri Heike Kampea,Ã, Marie Ko¨nig-Rinkeb, Thomas Petzoldtb,Ju¨rgen Benndorfb aInstitute for Biochemistry and Biology, University of Potsdam, Maulbeerallee 2, D-14469 Potsdam, Germany bInstitute of Hydrobiology, Zellescher Weg 40, 01217 Dresden, Germany Received 30 August 2006; received in revised form 19 December 2006; accepted 5 January 2007 Abstract The influence of Daphnia galeata  hyalina grazing and of infochemicals released by the daphnids on the colony size and growth rate of the colonial gelatinous green alga Sphaerocystis schroeteri (Chlorococcales) was investigated in laboratory batch experiments run for 96 h. High zooplankton grazing pressure was exerted by a final concentration of 100 daphnids LÀ1 in the Daphnia treatments. Infochemicals were obtained by filtration (0.2 mm) of water from D. galeata  hyalina cultures (200 ind. LÀ1 exposed for 24 h). This filtrate was added to the S. schroeteri cultures in two concentrations corresponding to 7 and 50 daphnids LÀ1, respectively. The growth rate of S. schroeteri was neither affected significantly by direct Daphnia grazing nor by the presence of Daphnia infochemicals, in comparison to the control. However, the portion of inedible S. schroeteri colonies (diameter450 mm) increased under direct grazing pressure, whereas the Daphnia infochemicals did not influence the colony size significantly. We conclude that the shift in colony size by direct zooplankton grazing denotes an effective defence mechanism against size selective feeding for colonial gelatinous green algae. This effective defence in combination with unchanged growth rates of the larger colonies (under non-limiting nutrient and light conditions) falsifies the assumption of a trade-off between minimising grazing losses and maximising growth by optimising the colony size. r 2007 Elsevier GmbH. All rights reserved. Keywords: Colonial gelatinous green algae; Zooplankton grazing; Infochemicals; Algal defence mechanism Introduction 1981; Nizan, Dimentman, & Shilo, 1986), an increase in size by the formation of colonies is a common and Zooplankton grazing is an important loss factor for efficient defence mechanism. The latter mechanism is such types of phytoplankton which do not develop particularly efficient against size selective filter feeders effective defence mechanisms against grazing. In addi- which feed mainly on particles between 2 and 30 mm tion to the bizarre or filamentous shape of cells or (Lehman, 1988; Sterner, 1989). Higher sinking rates, the colonies (Gliwicz & Siedlar, 1980; Hawkins & Lampert, decreasing efficiency of nutrient uptake and utilisation of 1989) and the formation of chemical repellents (Lampert, light cause a decline of net growth rates of algal populations with increasing size (Reynolds, 1984a). ÃCorresponding author. Tel.: +49 331 977 1955. Thus, a trade-off for algal size exists between efficient E-mail address: [email protected] (H. Kampe). grazing resistance and high growth rates. Gelatinous 0075-9511/$ - see front matter r 2007 Elsevier GmbH. All rights reserved. doi:10.1016/j.limno.2007.01.001 ARTICLE IN PRESS 138 H. Kampe et al. / Limnologica 37 (2007) 137–145 green algae are a common example of the colonial Chlorella vulgaris (Boraas, Seale, & Boxhorn, 1998). As organisation of phytoplankton. Reynolds (1984a) a result of this phenotypic plasticity, these algae have a divided these algae into two functional groups: The first fundamental advantage in the phytoplankton commu- comprises the immotile genera Sphaerocystis (synonym nity due to their high competitive ability in the presence Pseudosphaerocystis), Coenochloris and Oocystis, which as well as in the absence of grazers. occur mostly during the early summer period in In the present work we investigate the influence of a mesotrophic lakes. The second group contains the genera filter feeder (Daphnia galeata  hyalina) on the growth Eudorina, Pandorina and Volvox, which are motile rate and colony size of an immotile gelatinous green alga species and also occur during the summer period but in (Sphaerocystis schroeteri). The following three hypoth- eutrophic lakes. The functional difference between these eses derived from the literature cited above were tested: two groups results from the motility which allows species (i) the presence of Daphnia increases the colony size of from the second group to regulate their position in S. schroeteri, (ii) the shift in colony size is mediated by relation to the light field (Reynolds, Huszar, Kruk, an infochemical released by Daphnia, and (iii) the Naselli-Flores, & Melo, 2002). Species of the first group growth rate of S. schroeteri is not significantly reduced react sensitively to phosphorus and nitrogen enrichment in Daphnia-induced large colonies when light and because of their lower competitive ability for light and nutrients are available at saturation levels. carbon (Reynolds et al., 2002). Gelatinous green algae have been reported to benefit from high zooplankton biomass overall (Porter, 1975; Sommer, Sommer, Santer, Material and methods &Zo¨llner, 2003; Vanni, 1987) and, because of zooplank- ton induced nutrient recycling, at unsaturated nutrient levels in particular (Kagami, Yoshida, Gurung, & Starter cultures Urabe, 2002). The functional role of the gelatinous AstrainofSphaerocystis schroeteri (217-1b) from the sheath has been discussed and is controversial. The culture collection of algae (SAG) of the University of secretion of a gelatinous sheath has long been postulated Gottingen was used for the experiments. Light for the as a mechanism for reducing sinking rates (Reynolds, ¨ starter cultures was provided with a 16:8 h light–dark 1984b). Furthermore, the increase in size using gelatine cycle and at 125 (75) mEmÀ2 sÀ1. Cultures were kept in can prevent ingestion by filter-feeding zooplankton (Burns, 1968) with concomitant improved intercellular membrane-filtered (0.2 mm, Schleicher & Schuell) lake water from the mesotrophic Saidenbach Reservoir diffusion in comparison to non-gelatinous colonial forms (Saxony, Germany) enriched with phosphorus at non- (Reynolds, 1984b). Otherwise, the production of gelatine limiting concentrations. Before use for cultivation, the is energy-consuming and also leads to an increase of water was stored for 2 weeks to ensure the decline of sinking velocity with increasing size (Stoke’s law). Porter naturally occurring infochemicals. The experimental (1973, 1975, 1976) reported the viable gut passage of animals were taken from a laboratory culture of Daphnia gelatinous green algae in Daphnia with an additional galeata  hyalina reared in the same lake water as was nutrient uptake and enhanced algal growth after the passage. She concluded that the gelatinous sheath used for the experiments. The inoculum of this culture consisted of animals caught in April 2003 in the eutrophic constitutes an effective protection against digestion and Bautzen Reservoir (Saxony, Germany). The culture zooplankton grazing enhances the growth of gelatinous animals were fed thrice weekly with Scenedesmus sp. and green algae. In contrast, Stutzman (1995) found neither additionally once weekly with Sphaerocystis schroeteri. significant increase in mortality nor decrease in the number of moults and clutches of Daphnia feeding on monocultures of Sphaerocystis or Pandorina. However, Experimental conditions the number of offspring per clutch was significantly reduced. In comparison to Diaptomus, Daphnia utilised Tests were run in batch cultures in 300 mL Erlen- the gelatinous green algae more effectively in Stutzman’s meyer flasks with 150 mL culture volume and non- (1995) study. limiting nutrient concentrations (4100 mgPO4- A grazer-mediated unicell-to-colony transformation PLÀ1,44.2 mg inorganic N LÀ1). The initial algal cell and formation of spines, connected with an improved concentration was 23007250 cells mLÀ1. Flasks were protection against grazing, has been reported in some incubated on a rotating table at 18 1C and a light strains of the non-gelatinous green alga Scenedesmus intensity of 125 (75) mEmÀ2 sÀ1, the light–dark cycle (Hessen & Van Donk, 1993; Lampert, Rothaupt, & von was 16:8 h for experiment 1 and 12:12 h for experiment Elert, 1994; Lu¨rling & Van Donk, 1996). After exposure 2. Flasks were shaken every 10 min for 1 min. The to an infochemical released by Daphnia the formerly daphnids used for the grazing experiments were replaced unicellular algae formed colonies. Grazer-induced col- by new animals every 2 days. We used similar sized adult ony formation has also been reported in the green alga animals for all experiments (1.870.2 mm). ARTICLE IN PRESS H. Kampe et al. / Limnologica 37 (2007) 137–145 139 Experiment 1 (low infochemical concentration) solution. The growth rates (dÀ1) were calculated during the exponential period of growth according to A first experiment was intended to differentiate the m ¼ðln N2 À ln N1Þ=ðt2 À t1Þ, (1) effects of direct Daphnia grazing and of Daphnia infochemicals on the size distribution of Sphaerocystis where N1, N2 are the fluorescence intensities at time t1, schroeteri. The experimental design included the control, t2, respectively. The maximum size of colonies that Daphnia treatment and infochemical treatment, each could be ingested by the daphnids was determined comprising 3 replicates. Control flasks contained neither according