Grazer-induced morphological defense in Scenedesmus obliquus is affected by competition against Microcystis aeruginosa.

Xuexia Zhu,Jun Wang,Qinwen Chen,Yichun Lu,Zhou Yang

doi:10.1038/srep12743

Abstract

The green alga Scenedesmus is known for its phenotypic plasticity in response to grazing risk. However, the benefits of colony formation induced by infochemicals from zooplankton should come with costs. That is, a tradeoff in benefit-to-cost ratios is likely under complex environmental conditions. In this study, we hypothesized that the coexistence of Scenedesmus and its competitors decreases the formation of anti-grazer colonies in Scenedesmus. Results demonstrated that the presence of a competitor Microcystis aeruginosa inhibited inducible defensive colony formation of Scenedesmus obliquus, and the established defensive colonies negatively affected the competitive ability of S. obliquus. The proportion of induced defensive colonies in cultures was dependent on the relative abundance of competitors. Under low competition intensity, large amount of eight-celled colonies were formed but at the cost of decreased competitive inhibition on M. aeruginosa. By contrast, defensive colony formation of S. obliquus slacked in the presence of high competition intensity to maintain a high displacement rate (competitive ability). In conclusion, S. obliquus exhibited different responses to potential grazing pressure under different intensities of competition, i.e., Scenedesmus morphological response to grazing infochemicals was affected by competition against Microcystis.

Highlights

The green alga Scenedesmus spp. is a typical case
Scenedesmus populations that are dominated by colonies can effectively deter feeding by small grazers such as Daphnia cucullata; induced colony formation is an efficient strategy for Scenedesmus to escape grazing by predators with a diet limited by the prey size[15]
The morphology of S. obliquus monocultures changed drastically under exposure to Daphnia filtrate compared with the control

Summary

Introduction

Upon detection of infochemicals released by herbivores such as Daphnia, Scenedesmus populations become dominated by four- and eight-celled colonies instead of unicells[14]. Trade-offs potentially exist between the formation of large sinkable colonies (favoring defense) and maintenance of vulnerable unicells (favoring competition) in Scenedesmus. The disadvantages of forming colonies, including accelerated sinking rate and decreased nutrition and light absorption, may affect the competitive ability of Scenedesmus. This study aimed to examine the costs and the trade-off of forming grazer-induced defensive colonies in Scenedesmus under different levels of inter-species competition. Microcystis aeruginosa, one of the most common bloom-forming cyanobacteria, was used as the competitor of S. obliquus Both algae are found in freshwaters world-wide and often coexist in the same area[26,27]. We used filtered test water containing infochemicals from Daphnia magna to simulate S. obliquus colony formation in the presence of competitors

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