Microbial endolithic symbiosis in oysters enhances thermal resistance through shell corrosion

Extreme temperature events driven by climate variability are increasingly threatening biodiversity and ecosystem functioning. Intertidal ecosystems are particularly exposed to heatwaves, and face mass mortalities, local extinctions, and range contractions among keystone species, with cascading effects on biodiversity, carbon sequestration, coastal defences, and fisheries. Symbiotic interactions play a crucial role in shaping host resistance to environmental stress, particularly thermal stress intensified by climate change. This study evaluated the potential thermal buffering effects of shell corrosion by symbiotic endoliths on the Pacific oyster ( Magallana gigas ) under heat stress. Laboratory and field experiments revealed significantly higher survival rates in corroded oysters, with a significant thermal buffer as high as 9.5 °C in natural settings. Spectrophotometric analyses further showed that endolithic corrosion alters shell optical properties (pale in colour - lightness, chromatic axes and reflectance), linking shell colour shifts directly to enhanced thermal buffering. While similar patterns have been previously observed in mussels, the thermal buffer effect in oysters is substantially higher, highlighting species-specific differences in the magnitude of symbiont-mediated thermal resistance. These findings contribute to a broader framework for understanding how host-symbiont interactions modulate thermal resistance across diverse marine calcifiers, highlighting the adaptive potential of shell corrosion in enhancing resistance to rising temperatures.