Divergent Heat-Shock Stress Responses in Chorisodontium aciphyllum and Polytrichastrum alpinum From Maritime Antarctica.

The increasing frequency and intensity of extreme thermal events in Maritime Antarctica pose new challenges for terrestrial cryptogamic vegetation, in which bryophytes are major components. We analyzed acute heat-shock responses in two Antarctic mosses with contrasting canopy organization and habitat associations: the dense bank-forming Chorisodontium aciphyllum and the more open lawn-forming Polytrichastrum alpinum under fully hydrated conditions. Both species showed substantial short-term tissue-level heat resistance when LT50 was defined from electrolyte leakage (LT50 > 55°C), whereas Fv/Fm declined over a lower and nearly identical thermal range centered near 43°C. Heat shock induced strong antioxidant enzyme responses and coordinated transcriptional changes in heat-stress- and proteostasis-associated genes, including HSF, HSP70 isoforms, and ubiquitin-related markers. Untargeted metabolomics revealed relative metabolic reorganization in both species, including a shared LT50-associated signature characterized by relative enrichment of carbohydrate- and aromatic/phenolic-associated features and relative depletion of lipid- and terpenoid-related features. Species-related differences were most evident as differences in response amplitude and relative LC-MS feature-family weighting, without evidence of divergent pathway-level activation. Together, these results indicate that Antarctic mosses share core heat-stress modules but orchestrate them divergently through species-dependent redox regulation, membrane-associated stress responses and proteostasis-related regulation, with non-identical enzymatic and metabolomic trajectories under severe thermal challenge.