Physiology and chemistry integration under UV-B across green algal Klebsormidium clades (Streptophyta) reveals constitutive and inducible MAA-associated photoprotective responses

Stratospheric ozone variability continues to modulate biologically harmful UV-B radiation, exerting strong selective pressure on terrestrial green algae. These organisms have evolved protective mechanisms such as the biosynthesis of mycosporine-like amino acids (MAAs), which absorb UV radiation, thereby acting as sunscreen compounds. Here, we integrated ecophysiology and untargeted metabolomics to investigate UV-B-associated photoprotective response across six phylogenetically distinct terrestrial green algal Klebsormidium species (Streptophyta) originating from polar, alpine, and semi-arid habitats. Using a common-garden UV-B exposure approach (2 W m⁻²), we observed that UV resilience is not directly associated with total MAA content alone, but instead corresponded with contrasting patterns of MAA deployment. Alpine K. crenulatum maintained a constitutive MAA pool with klebsormidin A concentrations of approximately 12.4 mg⁻¹dw and exhibited only minimal MAA induction under UV-B exposure, while maintaining high photosynthetic performance with Y(II) values remaining > 0.4 and only minor reductions in growth rate (μ). In contrast, low-latitude species such as K. subtile and K. deserticola displayed strongly inducible MAA responses, with MAA levels increasing by approximately 120-fold under UV-B treatment, accompanied by temporary reductions in Y(II) and growth rate. UHPLC-VWD-HRMS/MS analysis followed by feature-based molecular networking identified species-specific MAA profiles beyond dominant klebsormidins, including canonical MAAs such as shinorine, asterina-330, and porphyra-334, together with additional putatively annotated UV-absorbing compounds. Polar species maintained comparatively high UV tolerance despite low MAA abundances, suggesting that additional constitutive photoprotective mechanisms might be present. Our results highlight that UV-B tolerance in Klebsormidium is more closely associated with MAA composition, timing, and deployment strategy than with total MAA content alone, suggesting contrasting UV-B response patterns among early-diverging streptophytes.