Genus and species concept in Chlorella and Micractinium (Chlorophyta, Chlorellaceae): genotype versus phenotypical variability under ecosystem conditions

摘要

According to traditional systematics, members of the genera Chlorella (spherical, solitary, without spines) and Micractinium (spherical, mostly in coenobia, with spines) belong in different families of the Chlorophyta: Chlorellaceae and Micractiniaceae (KomArek & Fott 1983). However, molecular phy-logenetic analyses have shown that classical approaches using morphological criteria for the circumscription of taxa do not adequately reflect the phylo-genetic relationships of coccoid green algae (Krienitz et al. 2003). Whereas the latest revision of the genus Chlorella based on 18S rRNA phylogeny led to a proposal to reduce the genus to a set of four 'true' spherical species (Huss et al. 1999), more recent studies found members of different morphology cluster within the clade of true Chlorella sensu stric-to species (Wolf et al. 2002, Krienitz et al. 2004). These findings raise the question in which amount these morphological criteria 'only' represent phenotypical adaptations to the ecosystem conditions. Under different environmental conditions the morphology of phytoplankton can be extremely variable. For example 'allelochemicals', such as predator kairomones from zooplankton have been shown to affect the morphology of phytoplankton species considerably. Biotests using the Daphnia-Scenedesmus model (Hessen & Van Donk 1993, Lampert et al. 1994, Luerling 1998, Von Elert & Frank 1999) demonstrated the morphological change in colony formation of Scenedesmus which can be interpreted as protection mechanisms against grazing. Population shifts in algal mass cultures of Scenedesmus and Micractinium led to the idea that the development of long spines protects Micractinium from the feeding activity of rotifers of the genus Brachionus (Schluter et al. 1987).