Class Chlorophyceae

The class Chlorophyceae, one of the three crown branches of green algae, includes morphologically and ecologically diverse organisms that occur mainly in freshwaters. The thallus morphology ranges from unicellular algae (with or without flagella) or simple colonies to branched or unbranched filaments, or even blade-like thalli (Leliaert et al. 2012). The Chlorophyceae are monophyletic and, together with the Ulvophyceae, sister to Trebouxiophyceae (Del Cortona et al. 2020). The class includes six main order-level lineages: Chlamydomonadales, Sphaeropleales, unnamed clade containing Jenufa, Treubaria and Golenkinia, and so-called ‘OCC-lineage’, consisting of Chaetophorales, Chaetopeltidales and Oedogoniales (Lemieux et al. 2015).

Sanders & Masumoto (2021) consider a total of six chlorophycean genera as lichen photobionts, even though they warn that most of them are in need for further revision. In our opinion, most of these genera lack the necessary evidence to be classified as lichen symbiotic algae. For example, Skuja (1943) mentioned a symbiotic relationship between the ascomycete Pyronema laetissimum and Chlamydomonas augustae (Chloromonas augustae; Pröschold et al. 2001). This association, which has been, to our knowledge, only reported once, however, lacks many important characteristics of a typical lichen (Sanders & Masumoto 2021) and the fungus was probably misidentified (Moore & Korf 1963).

Trochiscia was reported as a Verrucariaceae photobiont by Tschermak (1941) and Ahmadjian (1967). Its phylogenetic placement remains uncertain (del Campo et al. 2010; Fučíková et al. 2019) and the identification of this alga is doubtful (Ettl & Gärtner 2013). Genera such as Chlorsarcinopsis, Gloeocystis and Radiococcus are highly polyphyletic, share many morphological features with unrelated genera and can be easily confused for them (Neustupa 2015; Zhang et al. 2018). Yet, all existing records of these algae from lichen thalli are based solely on morphological data (Plessl 1963; Tschermak-Woess 1988; Voytsekhovich et al. 2011). For instance, algae reported as Chlorsarcinopis, symbionts of Lecidea plana and L. lapicida, could easily be confused for Trebouxia, which is known to associate with Lecidea species, including L. lapicida (Beck 1999; Ruprecht et al. 2012).