Stress event alters skeletal structure but not geochemistry in a long-lived crustose coralline alga (Clathromorphum compactum)

Abstract Crustose coralline algae (CCA) are foundational species in shallow-water marine ecosystems, playing a critical role in their structural and ecological integrity. Additionally, their calcium carbonate skeletons serve as high-resolution paleoclimate archives, offering valuable insights into historical oceanic variability. Here, the effects of bleaching under controlled culture conditions on skeletal structure and geochemistry were investigated in the long-lived subarctic CCA Clathromorphum compactum . Cell wall thickness, interfilament crystal thickness, and skeletal Mg/Ca ratios derived from weight% were quantified in material formed before and after bleaching. Skeleton deposited following bleaching exhibited a distinct transition characterized by irregular cellular organization and was significantly thinner than pre-bleaching growth. Reductions in thickness occurred in individuals that later recovered as well as in those that died, indicating that calcification continued during physiological stress. In contrast, no significant difference in Mg/Ca ratio was detected between pre- and post-bleaching skeleton. These results suggest that C. compactum exerts some control over the geochemistry at its calcification site, maintaining stability despite environmental stressors that impact calcification. Thus, bleaching can leave pronounced structural irregularities in CCA skeletons, potentially affecting skeletal integrity, even when geochemical proxies remain unchanged.