Microbial symbionts enhance thermal tolerance in the Pacific OysterExtreme Events

Gerardo Zardi^1,2,3, Sebastien Lefebvre¹, Camille Detree⁴, Laurent Seuront¹, Katy Nicastro^1,3

¹BOREA, France - ²LOG, France - ³Rhodes University, South Africa - ⁴ OFB, France

Extreme temperature events are becoming increasingly frequent and intense, threatening biodiversity and ecosystem functioning. Intertidal zones, exposed to both air and water, are particularly at risk, with keystone species suffering mass mortalities, local extinctions, and range shifts. These losses cascade into reduced biodiversity, weakened coastal defences, diminished carbon storage, and impacts on fisheries and recreation. Symbiotic interactions may play a critical role in helping host organisms withstand environmental thermal stress. We previously demonstrated that symbiotic microbial endoliths living within mussel shells significantly enhance the thermal tolerance of their host during heat stress by inducing shell corrosion, which makes shells lighter in color and thus enhances solar reflectivity.
Building on this foundation, we explored whether this same beneficial effect extends to the Pacific oyster (Magallana gigas). Through laboratory and field experiments, we compared survival and heat tolerance across oysters with different natural shell colors and those whose shells were altered by microbial endolithic activity. Overall, our results show that endolithic shell corrosion reduced oyster body temperatures by an average of 9.5°C, leading to significantly higher survival rates during heat extremes.
This study highlights the critical role of symbiotic interactions in mitigating climate impacts and provides valuable insights for conservation strategies aimed at sustaining marine biodiversity under rising temperatures.