Exploring structural integrity of coralline algae in response to the environmental changes associated with the <scp>PETM</scp> : a tale of functional resistance

Abstract Coralline algae are key benthic components of shallow‐marine ecosystems globally and as habitat‐formers they support high biodiversity levels. Experiments on living coralline algae show internal growth changes in response to warming and higher CO 2 . These growth changes are leading to weakened structural integrity and increased breakage impacting their ecological function of habitat formation. Short‐term experiments, though, raise questions about long term acclimation over multiple generations. Coralline algae have an extensive fossil record across the Cenozoic. Analysing growth changes within the geological record, specifically across hyperthermals, geologically abrupt environmental changes in the Earth's history characterized by rapid ocean warming, acidification and sea level rise, can complement modern experiments. This allows us to quantify the vulnerability and response of habitat formers, such as coralline algae, to long‐term environmental change. We evaluated cellular structure and structural integrity in species of the genera Sporolithon and Lithothamnion from Meghalaya, northeast India (eastern Tethys) before and during the Paleocene–Eocene Thermal Maximum (PETM) ( c . 55.8 Ma), the most pronounced hyperthermal of the Cenozoic. Cellular structural changes were not uniform between species, some species showed increased stress and strain due to larger cell sizes during the PETM, while other species revealed negligible changes in cell sizes. Unexpectedly, stresses and strains experienced by these Palaeogene taxa are comparable to contemporary species of the study genera. These findings suggest a resilience to long term warming and lower pH conditions resulting in a resistance to breakage. However, species differences in environmental change responses potentially highlight variations in phenotypic plasticity.