Chemical induction of colony formation in a green alga (Scenedesmus acutus) by grazers (Daphnia)

Abstract

The green alga, Scenedesmus acutus, grows in culture in unicellular form, but it forms colonies (coenobia) when exposed for 48 h to a chemical released by the grazer Daphnia magna. The colony-forming response can be evoked only in growing cells. The Daphnia factor affects colony size but not algal growth rate. The minimum concentration of Daphnia factor that induces colony formation is equivalent to a Daphnia biomass of 0.5 mg dry wt liter-* in the culture medium. Actively feeding daphniids induce a stronger response than starved ones. Homogenized Scenedesmus, homogenized Daphnia, ammonium, and urea are not effective. The Daphnia factor is a nonvolatile, organic substance of small molecular mass (< 500 Da). It is moderately lipophilic, heat stable, pH-resistant in a range from 1 to 12, and not affected by treatment with Pronase E. The chemical activity is not lost when the substance is dried but disappears during incineration. Colony formation can be interpreted as a grazing defense mechanism. The phenotypic response may have evolved because of the tradeoff between higher sinking rates and grazing resistance of colonial forms.

Grazer-induced changes in the desmid Staurastrum

“…Obviously, when the colonies become very large or heavily spined they are a food source of lower quality for the zooplankters (Hessen & van Donk, 1993). When comparing single cells and coenobia of S. obliquus, Lampert et al (1994) found no differences in the uptake between colonies and single cells by Daphnia magna, nor did Lürling (1996) for D. pulex. However, the smaller species D. cucullata seemed to have problems with the colonies.…”

“…In this work we have shown that formation of the mucus globules was indeed induced by the physical presence of grazing daphniids. However, unlike the reaction of, for example Scenedesmus, which forms coenobia in the presence of Daphnia and Daphnia water (Hessen & van Donk, 1993;Lampert et al, 1994;Wiltshire & Lampert, 1999), Staurastrum did not react to the water in which Daphnia were previously held. We showed that gentle mechanical stirring could also induce the formation of the same sticky clumps.…”

“…Stemberger & Gilbert, 1987;Kusch, 1993;Lüning, 1995;Tollrian, 1995). Interestingly, they also have been shown between zooplankton and algae (Hessen & van Donk, 1993;Lampert et al, 1994;Wiltshire & Lampert, 1999).…”

“…To check whether the phenomenon of mucus extrusion is indeed induced by a chemical excreted by Daphnia or whether Daphnia needs to be physically present to induce the change, the strains were also incubated in water that had been previously contained a dense colony of Daphnia (see methods described by Lampert et al, 1994).The control was the algal culture on its own.…”

Importance of Nutrient Competition and Allelopathic Effects in Suppression of the Green Alga Scenedesmus obliquus by the Macrophytes Chara, Elodea and Myriophyllum

“…The Chl a content (lg l )1 ) and the efficiency of photosystem II (F PSII ) were determined using the PHYTO-PAM phytoplankton analyser (Walz, Germany; Lu¨rling & Verschoor, 2003). Since Lampert et al (1994) have shown that the MPV were highly correlated with the mean number of cells per colony, for statistical comparison of colony size the MPV were used. The MPV that were determined from the electronic particle counter were statistically compared using repeated measures ANOVA followed by Bonferroni post-hoc comparison test to distinguish the mean differences that are significantly different at p=0.05 using the statistical tool pack SPSS version 10.1.0.…”