Reversible colony formation and the associated costs in Scenedesmus obliquus

Abstract

Grazer-induced colony formation as a defense strategy in microalgae such as Scenedesmus species has been widely reported, but the associated costs and reversibility of the colonies are rarely studied. We experimentally showed that Scenedesmus obliquus formed chained colonies in the presence of a predator, including predators separated from the algae by a membrane, but quickly reverted to single cells after the removal of the predator—a defining characteristic of an inducible defense. We detected stress indicators—astaxanthin esters—in the algal populations in the presence of grazers but not when grazers were absent. We found significant costs associated with S. obliquus colony formation in terms of lower population growth rate, lower photosystem II efficiency and lower cellular chlorophyll a content. These results together show that colony formation as an inducible defense in S. obliquus against grazers comes at a substantial cost such that the defense must be switched off and the colonies revert to single cells when the predation risk disappears.

Grazing resistance in phytoplankton

“…A trade-off between grazing loss and sinking loss in Desmodesmus/Scenedesmus has been found-larger, protected Scenedesmaceae had the lowest grazing loss, but the highest sinking loss (Verschoor et al, 2009). Metabolic costs that are associated with (induced) colony formation might be reflected in reduced growth of colonial populations (Zhu et al, 2016;Albini et al, 2019), particularly under resource limitation (Zhu et al, 2016). In diatoms, thick silicate cell walls provide protection against grazers that crush cells prior to ingestion (Pančić et al, 2019).…”

Humic Substances as Microalgal Biostimulants—Implications for Microalgal Biotechnology

“…Such mechanisms are of interest for microalgal biotechnology as a lower-cost harvesting process [ 168 ]. HS could support the initial formation of the extracellular matrix of polysaccharides and lipids required for colony formation and flocculation [ 169 ], reducing the metabolic costs of colony formation [ 170 ]. The surfactant agents, such as linear alkyl benzene (LAB) [ 171 ], benzalkonium bromide (BZK) [ 172 ], sodium dodecyl sulfate (SDS) [ 172 , 173 ], and nonionic surfactant polyoxyethylene (40) nonylphenol ether (NPE) [ 172 ] enhance microalgae colony formation in the presence of smaller amounts of grazers natural cues or even in their absence [ 171 ].…”

Inducible defensive traits of Daphnia pulex against predators are mainly determined by the offspring's environment rather than their mother's experience

“…The above studies, however, did not consider the long‐term changes in defensive traits of the offspring that obtained defensive traits from their mothers. The reversibility of inducible defence is a useful strategy which can optimize resource allocation and avoid unnecessary costs (Albini et al., 2019; Gilbert, 2012; Weiss et al., 2015). Given that Daphnia individuals can opportunely adjust their defensive status to cope with the environment during their lifetime, it can be speculated that, when the number of broods increases, the offspring may also timely adjust their defensive strategy to avoid associated costs.…”