CHLORELLALES

Auxenochlorella (I. Shihira & R. W. Krauss) T. Kalina & M. Punčochářová. The genus Auxenochlorella is a unicellular coccoid alga of minute cell size (Fig. 13), differentiated from other Chlorella -like coccoid algae by molecular genetic data (Huss et al. 1999). The cells are ellipsoidal to globose, bounded by a double-layered cell wall without mucilage cover. Chloroplasts are parietal without pyrenoids. The cells reproduce by autospores (Kalina & Punčochářová 1987). The type species A. protothecoides was previously considered closely related to the non-photosynthetic genus Prototheca and placed within the ‘APH lineage’ (Ueno et al. 2005). Thüs et al. (2011) places this genus in the Chlorellales .

An unspecified species-level lineage Auxenochlorella sp. related to free-living A. prototheciodes was isolated from the lichen Psoroglaena stigonemoides (Nyati et al. 2007; Thüs et al. 2011) but has not yet been recorded free-living.

Chlorella- like algae. Members of the genus Chlorella are sometimes considered as photobionts of lichens (Tschermak-Woess 1988; Muggia et al. 2013). Nearly 100 different ‘green balls’ were previously included in this highly paraphyletic genus (Guiry & Guiry 2022). However, most of them were later reassigned to other and often unrelated genera (apart from the aforementioned genera Chloroidium and Auxenochlorella, for example, the genera Watanabea, Mychonastes, Muriella and Scenedesmus; Huss et al. 1999). These shifts highlight how difficult it is to identify the genus Chlorella and similar genera on the basis of morphological characters alone. Therefore, the genus Chlorella was not considered when searching for articles focused on the diversity of free-living lichen symbionts.