Chloroidium ellipsoideum (= Chlorella ellipsoidea)

is probably the most common free-living photobiont species. It is often one of the most abundant species of aerophytic algae (Lukešová 2001; Fathi & Zaki 2003; Hoffmann & Darienko 2005; Štifterová & Neustupa 2015) and forms macroscopic growths (Mikhailyuk 2008). The species was genetically confirmed from waste container biofilm in Germany (Hallmann et al. 2016) and from the bark of pine and oak from two different sub-Mediterranean sites (Kulichová et al. 2014). Several more molecular findings of C. ellipsoideum on different substrates are published in Darienko et al. (2010). Chloroidium ellipsoideum can adapt to freezing (Broady 1984; Elster et al. 1999) as well as heat and drought (Flechtner et al. 1998; Fathi & Zaki 2003). It is very common in substrates affected by anthropogenic activities (Neustupa & Škaloud 2005; Škaloud et al. 2008a) and, possibly due to its ability to produce osmotically active substances (Darienko et al. 2010), it can tolerate hypersaline soils (Sommer et al. 2020). Sometimes it even dominates in similar extreme anthropogenic habitats (Lukešová & Hoffmann 1996; Lukešová 2001). However, this alga cannot withstand severe air pollution caused by dust particles of a diameter of 10 µm or smaller (PM 10), whereas it seems to be very resistant to elevated ozone concentrations (Freystein et al. 2008).

Chloroidium ellipsoideum cells can attach to a wide variety of substrates. Indeed, they are frequently found on building facades and walls (Schlichting 1975; Rifón-Lastra & Noguerol-Seoane 2001; Rindi & Guiry 2004; Barberousse et al. 2006; Wasserbauer et al. 2014; Hofbauer & Gärtner 2021), on various tree species (Czerwik & Mrozinska 2000; Johansen et al. 2007; Khaybullina et al. 2010; Štifterová & Neustupa 2017), on granite (Rifón-Lastra & Noguerol-Seoane 2001; Mikhailyuk et al. 2003; Mikhailyuk 2008), sandstone (Hoffmann & Darienko 2005), and sand (Schulz et al. 2016; Mikhailyuk et al. 2018a). The presence in soil is also quite common (Durrell 1964; Zancan et al. 2006; Škaloud et al. 2008a; Stoyneva & Gärtner 2009; Bakieva et al. 2012; Glaser et al. 2018). This species has also been recorded in air samples (North & Davis 1988; Chu et al. 2013) and in caves (Vinogradova & Mikhailyuk 2009; Vinogradova et al. 2009).

Molecular sequences confirm the presence of C. saccharophilum (= Chlorella saccharophila) on various hard substrates (Darienko et al. 2010) and in soil (Vishnivetskaya 2009). Similarly to the first-mentioned species, it is often isolated from soils (Zancan et al. 2006; Andreyeva 2009; Dirborne & Ramanujam 2017), including those heavily anthropogenically impacted (Lukešová & Komárek 1987; Lukešová 2001; Škaloud et al. 2008a), from tree bark (Freystein et al. 2008; Neustupa & Škaloud 2010; Štifterová & Neustupa 2017), and from the air (Parrando & Davis 1972; North & Davis 1988). It also cannot tolerate high concentrations of airborne dust particles (Freystein et al. 2008). Although C. saccharophilum occurs frequently in the subtropics and tropics (Neustupa & Škaloud 2010; Kharkongor & Ramanujam 2014; Dirborne & Ramanujam 2017), many records of its presence also come from Antarctica (Broady 1984; Mataloni et al. 2000; Cavacini 2001), the Arctic permafrost (Vishnivetskaya 2009) and other cold regions (Elster et al. 1999). However, no records of the organism have been found in deserts.

Genetically confirmed records of C. lichinum (= C. lichenum, C. angusto-ellipsoideum, Chlorella angustoellipsoidea) come from the lid of a container (Hallmann et al. 2016) and a variety of other substrates (Darienko et al. 2010). This species occurs in very small numbers in some localities (Neustupa & Albrechtová 2003; Štifterová & Neustupa 2017), elsewhere (rock) it can form visible growths (Mikhailyuk et al. 2003). It grows epiphytically on tree bark (Neustupa & Škaloud 2010; Štifterová & Neustupa 2017) and on spruce needles (Neustupa & Albrechtová 2003). A different species, C. viscosum (= Chlorella viscosa), was isolated from a biofilm collected from the bark (Darienko et al. 2018).