Temperature- and light stress adaptations in Zygnematophyceae: The challenges of a semi-terrestrial lifestyle

Abstract

Streptophyte green algae comprise the origin of land plants and therefore life on earth as we know it today. While terrestrialization opened new habitats, leaving the aquatic environment brought additional abiotic stresses. More-drastic temperature shifts and high light levels are major abiotic stresses in semi-terrestrial habitats, in addition to desiccation, which has been reviewed elsewhere. Zygnematophyceae, a species-rich class of streptophyte green algae, is considered a sister-group to embryophytes. They have developed a variety of avoidance and adaptation mechanisms to protect against temperature extremes and high radiation in the form of photosynthetically active and ultraviolet radiation (UV) radiation occurring on land. Recently, knowledge of transcriptomic and metabolomic changes as consequences of these stresses has become available. Land-plant stress-signaling pathways producing homologs of key enzymes have been described in Zygnematophyceae. An efficient adaptation strategy is their mat-like growth habit, which provides self-shading and protects lower layers from harmful radiation. Additionally, Zygnematophyceae possess phenolic compounds with UV-screening ability. Resting stages such as vegetative pre-akinetes tolerate freezing to a much higher extent than do young cells. Sexual reproduction occurs by conjugation without the formation of flagellated male gametes, which can be seen as an advantage in water-deficient habitats. The resulting zygospores possess a multilayer cell wall, contributing to their resistance to terrestrial conditions. Especially in the context of global change, understanding temperature and light tolerance is crucial.

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

“…Zygnema is noteworthy among algae for its stress resilience and can grow in extreme habitats such as the Arctic, where it is abundant (Pichrtová et al, 2018; Rippin et al, 2019). Its broad ecological amplitude is confirmed by numerous studies on temperature and light stress (summarized in Permann et al, 2022b) and desiccation stress (Becker et al, 2020). The common ancestor of all four Zygnema strains is inferred to have had 16 significantly expanded orthogroups ( Figure 2A; Table S3B ), including PP2Cs (OG 548), early light-inducible proteins (ELIPs; OG 97), and low-CO2 inducible proteins (LCICs, OG 459).…”

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

“…Their habitats often exhibit semi-terrestrial conditions, which exposes them to increased abiotic stresses. Leaving the aquatic environment entails higher levels of photosynthetic active as well as ultraviolet radiation, more drastic temperature shifts and desiccation stress (reviewed by Becker et al., 2020 ; Permann et al., 2022 ).…”

Striking differences in frost hardiness and inability to cold acclimate in two Mougeotia species (Zygnematophyceae) from alpine and lowland habitats

“…To deal with these challenges, Zygnematophyceae have evolved a variety of different adaptation strategies and avoidance mechanisms. Multiple detailed reviews on the abiotic stress tolerance of streptophyte green algae have been conducted over the past decade (Holzinger & Karsten, 2013 ; Holzinger & Pichrtová, 2016 ; Becker et al, 2020 ; Permann et al, 2022a ). Just recently, a comprehensive study on the morphological, photophysiological, and transcriptomic response of two Zygnematophyceae to desiccation stress was also performed (Rieseberg et al, 2023 ).…”