Coccomyxa Schmidle.

Coccomyxa Schmidle. View in CoL

Coccomyxa View in CoL cells are ellipsoid or nearly spherical, and sometimes dorsoventrally flattened ( Fig. 7 View Figs 1–18 ). They form microscopic and macroscopic colonies that are connected by mucilage. In older colonies, mucilage forms concentric envelopes around the periphery of cells. Reproduction is usually by 2–4 autospores (Ettl & Gärtner 2013). The genus currently includes 34 recognized species ( Guiry & Guiry 2022) and while described as early as 1901 ( Schmidle 1901), it is still taxonomically poorly understood ( Muggia et al. 2011; Malavasi et al. 2016).

It is a relatively abundant and widely distributed genus that inhabits almost all environments except the sea ( Darienko et al. 2015). However, it can occur as a parasite of mussels in marine environments ( Rodríguez et al. 2008). Some species survive even in extreme environments such as those highly polluted by heavy metals ( Malavasi et al. 2016), acidic water ( Fuentes et al. 2016), and high-ionizing radiation habitats ( Rivasseau et al. 2013; Malavasi et al. 2020). Even macroscopic mass development on grassland has been described ( Gärtner & Ernet 1993). It occurs in lichens (Asco- and Basidiomycota) and has also been found inside of the cells of Gingko biloba ( Trémouillax-Guiller & Huss 2007) and in association with carnivorous plants of the genus Drosera ( Sciuto et al. 2019) View in CoL . In the lichenized state, mycobiont haustoria do not penetrate cells, but are only attached to the cell surface ( Muggia et al. 2011). Many species ( C. glaronensis , C. icmadophilae , C. mucigena , C. solorinae , C. ovalis , C. thallosa , C. arvernensis , C. dispar , C. viridis , and C. subellipsoidea ) are recognized as lichen photobionts (Ettl & Gärtner 2013; Malavasi et al. 2016). However, lichenized and strictly free-living species are closely related and it can be very difficult to distinguish them from each other ( Malavasi et al. 2016).

A genetic study confirmed the presence of free-living algae closely related to C. mucigena and C. glaronensis on a polyethylene lid of a container in Göttingen, Germany ( Hallmann et al. 2016). Another genetic study clearly shows that C. subellipsoidea and C. arvernensis clades include both free-living and lichenized strains. Interestingly, the C. subellipsoidea clade includes only free-living epiphytic strains and photobionts of terricolous lichens but does not occur in the soil ( Malavasi et al. 2016). Additionally, C. dispar was detected during environmental sequencing of soil in the French Alps ( Stewart et al. 2021). Among morphology-based studies, free-living cells of the species C. cf. solorinae-saccatae and C. cf. solorinae-croceae have been detected in soil samples from an oak forest in the Ardennes, Belgium ( Hoffmann et al. 2007). These species, however, represent one monophyletic lineage described as C. solorinae ( Malavasi et al. 2016) and here we treat them as synonyms for simplicity. Other records of C. solorinae come from soil samples collected at various sites in Russia ( Getsen et al. 1994; Andreyeva 2009). Coccomyxa cf. thallosa was recorded on the soil surface in Czech Republic ( Neustupa et al. 2002).