Zygospores of the green alga Spirogyra: new insights from structural and chemical imaging

Abstract

Zygnematophyceae, a class of streptophyte green algae and sister group to land plants (Embryophytes) live in aquatic to semi-terrestrial habitats. The transition from aquatic to terrestrial environments requires adaptations in the physiology of vegetative cells and in the structural properties of their cell walls. Sexual reproduction occurs in Zygnematophyceae by conjugation and results in the formation of zygospores, possessing unique multi-layered cell walls, which might have been crucial in terrestrialization. We investigated the structure and chemical composition of field sampled Spirogyra sp. zygospore cell walls by multiple microscopical and spectral imaging techniques: light microscopy, confocal laser scanning microscopy, transmission electron microscopy following high pressure freeze fixation/freeze substitution, Raman spectroscopy and atomic force microscopy. This comprehensive analysis allowed the detection of the subcellular organization and showed three main layers of the zygospore wall, termed endo-, meso- and exospore. The endo- and exospore are composed of polysaccharides with different ultrastructural appearance, whereas the electron dense middle layer contains aromatic compounds as further characterized by Raman spectroscopy. The possible chemical composition remains elusive, but algaenan or a sporopollenin-like material is suggested. Similar compounds with a non-hydrolysable character can be found in moss spores and pollen of higher plants, suggesting a protective function against desiccation stress and high irradiation. While the tripartite differentiation of the zygospore wall is well established in Zygnematopyhceae, Spirogyra showed cellulose fibrils arranged in a helicoidal pattern in the endo- and exospore. Initial incorporation of lipid bodies during early zygospore wall formation was also observed, suggesting a key role of lipids in zygospore wall synthesis. Multimodal imaging revealed that the cell wall of the sexually formed zygospores possess a highly complex internal structure as well as aromatics, likely acting as protective compounds and leading to impregnation. Both, the newly discovered special three-dimensional arrangement of microfibrils and the integration of highly resistant components in the cell wall are not found in the vegetative state. The variety of methods gave a comprehensive view on the intricate zygospore cell wall and its potential key role in the terrestrial colonization and plant evolution is discussed.

Chromosome-level genomes of multicellular algal sisters to land plants illuminate signaling network evolution

“…Overall, these findings agree with an earlier origin of xyloglucan backbone enzymes and a later origin of side chain modification enzymes in the common ancestor of Zygnematophyceae and Embryophyta (Del-Bem, 2018; Mikkelsen et al, 2021). Recent reports have investigated the occurrence of homogalacturonans (e.g., Herbruger et al 2019) and AGPs in Zygnema (Palacio-Lopez et al 2019) and the filamentous Zygnematophyceae Spirogyra , where recently a rhamnogalactan protein has been described (Pfeifer et al, 2022); further, the specific cell wall composition of zygospores of the filamentous Zygnematophyceae Mougeotia and Spirogyra have been described (Permann et al, 2021; 2022a). Cell wall modifications by endotransgylcoylases and novel transglycosylation activities between xyloglucan and xylan, xyloglucan and galactomannan were described previously (Herburger et al, 2018).…”

“…Zygnematophyceae is a class of freshwater and semi-terrestrial algae with more than 4,000 described species (Guiry 2021). Their unifying feature is sexual reproduction by conjugation and lack of motile stages; zygospore formation can often be observed in field samples e.g., in Mougeotia (Permann et al, 2021a) and Spirogyra (Permann et al, 2021b, 2022a). Zygnematophyceae have been recently rearranged into five orders: Spirogloeales, Serritaeniales, Spirogyrales, Desmidiales, and Zygnematales (Hess et al, 2022).…”

3D‐reconstructions of zygospores in Zygnema vaginatum (Charophyta) reveal details of cell wall formation, suggesting adaptations to extreme habitats

“…In pollen grains, the degradation of the tapetum, a tissue rich in lipidic compounds, is directly linked to the development of the pollen coat (Levesque-Lemay et al, 2016;Parish & Li, 2010;Zienkiewicz & Zienkiewicz, 2020). As lipids have been suggested as components of the mesospore (Permann, Gierlinger, & Holzinger, 2022;Permann, Herburger, Niedermeier, et al, 2021), lipid reallocation from degrading paternal organelles might also play a vital role in the development of the zygospore wall.…”

“…The zygospore cell wall is much more complex and composed of three main layers, termed endo‐, meso‐, and exospore (Kadlubowska, 1984 ). The inner (endospore) and outer layer (exospore) are both colorless and contain mainly polysaccharides (Permann, Gierlinger, & Holzinger, 2022 ; Permann, Herburger, Felhofer, et al, 2021 ; Permann, Herburger, Niedermeier, et al, 2021 ; Poulícková et al, 2007 ). In Spirogyra and Zygnemopsis zygospores, an internal helicoidal order of the microfibrils was observed in these two layers (Permann, Gierlinger, & Holzinger, 2022 ; Pichrtová et al, 2018 ).…”

Proteome plasticity during Physcomitrium patens spore germination – from the desiccated phase to heterotrophic growth and reconstitution of photoautotrophy

“…Already before terrestrialization, streptophyte algae, the paraphylum that nowadays includes the closest relatives to the land plants, were likely able to form desiccation-tolerant cells, including akinetes, pre-akinetes, and zygospores (Herburger et al, 2015;McLean & Pessoney, 1971;Pichrtov a et al, 2014). Furthermore, zygospores of filamentous Zygnematophyceae have resilient coating rich in aromatics (resembling sporopollenin) and accumulate lipids and starch (Permann et al, 2021(Permann et al, , 2022. It is, therefore, likely that such structures occurred in the algal progenitors of land plants.…”