Asterochloris Tschermak-Woess.

Veselá, Veronika, Malavasi, Veronica & Škaloud, Pavel, 2024, A synopsis of green-algal lichen symbionts with an emphasis on their free-living lifestyle, Phycologia 63 (3), pp. 317-338 : 318-320

Asterochloris Tschermak-Woess.

Asterochloris is one of the most common, widespread and diversified genus of lichen photobionts ( Škaloud et al. 2015). The morphology of the cell is simple, mostly spherical, sometimes oval or pear-shaped with a thin cell wall that may be thickened locally. The genus was named after the single star-shaped chloroplast with differently shaped lobes, extending to the edges of the cell ( Fig. 1 ). The deeply lobed chloroplast is one of the main morphological features that distinguish this genus from Trebouxia . The presence of one or more pyrenoids is also another important feature. Asexual reproduction is by aplanospores and zoospores. Sexual reproduction is very rare and has been observed only in a single species A. woessiae ( Škaloud et al. 2015) .

Currently, the genus includes 19 species ( Guiry & Guiry 2022), a large proportion of which have only recently been discovered or separated from the genus Trebouxia . However, a number of cryptic species still remain to be formally described ( Škaloud & Peksa 2010; Škaloud et al. 2015; Kosecka et al. 2021). Škaloud & Peksa (2010) formally delineated the genus Asterochloris and established new combinations for former Trebouxia species. Asterochloris associates mainly with mycobionts from the families Cladoniaceae and Stereocaulaceae ( Muggia et al. 2018) . All known members of this genus are capable of lichenization ( Škaloud & Peksa 2010; Škaloud et al. 2015; Kosecka et al. 2021).

The presence of free-living Asterochloris species was genetically confirmed in two studies: the alga was detected on the hair of sloths in South and Central America ( Suutari et al. 2010) and on the bark of trees growing in the sub-Mediterranean region (Fiesa, Piran, Slovenia and Cernizza, Duino, Italy; Kulichová et al. 2014). The authors of the second paper emphasize that they avoided habitats with lichens during the sampling and carefully removed all isolated pieces of lichen thalli. The species A. excentrica was isolated from soil in the mountain tundra of the Northern Urals. However, the sample came from a community dominated by the lichens Cladonia rangiferina and Flavocetraria nivalis , and thus it cannot be excluded that the alga was lichenized ( Novakovskaya et al. 2020). Two other records of the occurrence of this species come from soil samples ( Andreyeva & Chaplygina 2006, 2007). The species A. italiana (= Trebouxia italiana ) was found on a granite rock outcrop of the left bank of the Ukrainian river Pivdennyi Bug. However, the collection sites showed a high percentage of lichen cover (40–90 %; Mikhailyuk et al. 2003). The presence of A. magna in soil was reported by Andreyeva (2005) and Andreyeva & Chaplygina (2006). Non-lichenized cells of Asterochloris were also isolated from the tree bark samples ( Neustupa & Štifterová 2013; Štifterová & Neustupa, 2015). However, in both cases it could not be excluded that the observed cells belong to the genus Trebouxia .

Figures

Figs 1–18. Lichen photobiont genera (including nonlichenized species) belonging to classes Trebouxiophyceae and Chlorophyceae. Fig. 1. Asterochloris lobophora. Fig. 10. Prasiola crispa. Fig. 2. Myrmecia incisa. Fig. 11. Pseudochlorella pringsheimii. Fig. 3. Trebouxia impressa. Fig. 12. Stichococcus sp. Fig. 4. Vulcanochloris symbiontica. Fig. 13. Auxenochlorella protothecoides. Fig. 5. Chloroidium saccharophilum. Fig. 14. Apatococcus sp. Fig. 6. Jaagichlorella luteoviridis. Fig. 15. Leptosira sp. Fig. 7. Coccomyxa sp. Fig. 16. Symbiochloris tschermakiae. Fig. 8. Elliptochloris subsphaerica. Fig. 17. Uvulifera verrucariae (Lorenz 2016). Fig. 9. Diplosphaera chodatii. Fig. 18. Bracteacoccus. sp. Scale bars: 10 μm (Figs 1-9, 11-18), 200 μm (Fig. 10).