Trebouxia Puymaly.

Trebouxia, the most diverse and common genus of lichen photobionts (Nash 2008), has a large axial chloroplast with at least one pyrenoid (Fig. 3). It reproduces by zoospores with two flagella of equal length, or by autospores (Archibald 1975). Only immobile stages with a reduced chloroplast can be found in the lichen thallus (Nash 2008).

Representatives of the genus were initially spread across several genera: Trebouxia, Pseudotrebouxia and Asterochloris (Nash 2008) . Pseudotrebouxia was separated because of differences in asexual reproduction (Archibald 1975) but was later rejected on the basis of morphological observations (Gärtner 1985) and, later, molecular data (Kroken & Taylor 2000). Asterochloris and Trebouxia, differing in chloroplast morphology (Škaloud et al. 2015), were separated in 2010 (Škaloud & Peksa 2010). These two genera also differ ecologically. While Asterochloris prefers mycobionts from the families Cladoniaceae and Stereocaulaceae, Trebouxia forms lichens more frequently with the families Parmeliaceae and Lecanoraceae (Muggia et al. 2018) . The 27 taxonomically accepted species (Guiry & Guiry 2022) accompany about 20 % of all lichen species (Rambold & Triebel 1992). The real species diversity of this genus is likely to be much higher, as a large proportion of the lineages discovered have not been formally described, and, in addition, new unknown species-level lineages are still being reported (Muggia et al. 2020).

As mentioned before, the existence of free-living members of the genus Trebouxia has been questioned in the past (Ahmadjian 1967, 1988, 2001) and other authors have not denied its existence but considered it very rare (Bubrick et al. 1984; Zavada & Simoes 2001). However, an overwhelming number of studies reported direct observations of free-living Trebouxia (Friedmann et al. 1967; Tschermak-Woess 1978; Bubrick et al. 1984; Sanders 2001, 2005; Sanders & Lücking 2002; Roldán & Hernández-Mariné 2009; Kharkongor & Ramanujam 2014). More recent studies report this genus as one of the most common genera of photobionts that can be encountered in nature, sometimes described as very frequent (Barberousse et al. 2006; Štifterová & Neustupa 2017) or even dominating (Ismail et al. 2019; Popović et al. 2019). Molecular genetic studies have confirmed the presence of unspecified members of this genus (related to T. jamesii and T. asymmetrica) in the fur of sloths from South and Central America (Suutari et al. 2010) and on the walls of a castle ruin in Germany (Hallmann et al. 2013). In addition to tropical rainforests (Suutari et al. 2010), this photobiont inhabits deserts (Friedmann et al. 1967; Samolov et al. 2020) and tundra environments (Elster et al. 1999; Garraza et al. 2011; Novakovskaya et al. 2020; Stewart et al. 2021), where lichen-dominated communities are often found (Novakovskaya et al. 2020) and thus the algae found may have been lichenized. Trebouxia was also detected in marine environments (Metz et al. 2019), although one cannot be certain that the detected sequences represent free-living individuals (Sanders & Masumoto 2021).

Trebouxia is often found in anthropogenic environments. It lives in coal post-mining areas (Lukešová 2001), on trees in close proximity to air polluting power plants (Ismail et al. 2019) and is often part of biofilms covering the facades of buildings (Rindi & Guiry 2004; Barberousse et al. 2006; Hallmann et al. 2013; Hofbauer & Gärtner 2021). It has also been found, for example, on historical buildings of the former concentration camp in Auschwitz (Nowicka-Krawczyk et al. 2014). Caves are also a frequent habitat (Roldán & Hernández-Mariné 2009; Vinogradova & Mikhailyuk 2009; Vinogradova et al. 2009; Popović et al. 2019; van Vuuren et al. 2019). This resilient alga tends to be one of the first organisms to colonize fire-sterilized environments (Grondin & Johansen 1993; Mukhtar et al. 1994). In addition to these substrates, unspecified Trebouxia species have also been found on trees (Wylie & Schlichting 1973; Kharkongor & Ramanujam 2014; Štifterová & Neustupa 2017), in soil (Macentee 1970; Macentee et al. 1972; Carson & Brown 1976), on rocks (Mikhailyuk et al. 2018a) and on moss (Škaloud 2009).

The most frequently observed species is T. arboricola . It has been found in soil (Andreyeva 2004, 2005, 2009; Büdel et al. 2009; Dirborne & Ramanujam 2017; Andreyeva & Chaplyginа 2007), on rocks (Gärtner & Stoyneva 2003; Škaloud 2009; Stoyneva & Gärtner 2009), tree bark (Rindi & Guiry 2003; Gupta 2008; Kharkongor & Ramanujam 2014), on dead wood (Smith & Stephenson 2010), in caves (Stoyneva & Gärtner 2009), on facades (Hofbauer & Gaertner 2021) and on unusual substrates such as basidiocarps of wood-decaying fungi ( Fomes fomentarius; Stoyneva et al. 2014), or a tombstone in a historic cemetery in Bratislava (Uher 2008). The presence of this species has been genetically confirmed on a waste container lid, however, the cells observed were not necessarily free-living (Hallmann et al. 2016). Noncultivated samples containing free-living T. arboricola have been examined in several studies (Rindi & Guiry 2003; Gupta 2008; Stoyneva & Gärtner 2009; Smith & Stephenson 2010; Kharkongor & Ramanujam 2014; Stoyneva et al. 2014).

Trebouxia aggregata has been detected on granite (Rifón-Lastra & Noguerol-Seoane 2001; Mikhailyuk 2013), in soil (Flechtner et al. 2008), in oak leaf litter (Maltsev & Maltseva 2018), and on Trametes versicolor basidiocarps (Videv et al. 2017). Zavada & Simoes (2001) isolated an unspecified representative of the genus Trebouxia from basidiocarps of the same fungal species and suggested that the two organisms might have a lichen-like relationship. However, they did not present any convincing evidence for this.

The species T. corticola is morphologically confirmed from tundra soil in north-eastern Russia (Andreyeva & Chaplygina 2006). The genetically confirmed finding of a lineage closely related to T. corticola, comes from air samples in Hawaii (Singh et al. 2018). Genetic studies have also confirmed the presence of another species, T. impressa, in the air (Genitsaris et al. 2011) and on a waste container lid (together with several other Trebouxia clones), where, however, a significant number of fungal hyphae was present in close proximity to the algae (Hallmann et al. 2016). The records of a species similar to T. gigantea comes from rock (Mikhailyuk 2013) and soil (Andreyeva 2005; Andreyeva & Chaplygina 2006). Trebouxia incrustata was found on granite (Mikhailyuk et al. 2003) and in soil (Flechtner et al. 2008). Trebouxia decolorans on façade (Vojtková 2017), T. anticipata on granite (Rifón-Lastra & Noguerol-Seoane 2001), T. potteri in moss (Škaloud 2009), T. jamesii on granite (Mikhailyuk 2013) and T. cladoniae in desert soil (Cameron 1960).