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Heatwaves are increasingly severe and frequent, posing significant threats to ecosystems and human well-being. Characterised by high thermal variability, intertidal communities are particularly vulnerable to heat stress. Microbial endolithic communities that are found in marine calcifying organisms have been shown to induce shell erosion that alters shell surface colour, lowering body temperatures and increasing survival rates. Here, we investigate how the symbiotic relationship between endolithic microbes and the blue intertidal mussel Mytilus edulis mitigates thermal stress during the unprecedented 2022 atmospheric heatwave in the English Channel. Microbial infestation of the shell significantly enhanced mussel survival, particularly higher on the shore where thermal stress was greater. Using data from biomimetic temperature loggers, we predicted the expected thermal buffer and observed differences up to 3.2 °C between individuals with and without symbionts under the known conditions of the heat wave-induced mortality event. The ecological implications extend beyond individual mussels, affecting the reef-building capacity of mussels, with potential cascading effects for local biodiversity, carbon sequestration, and coastal defence. These findings emphasize the importance of understanding small-scale biotic interactions during extreme climate events and provide insights into the dynamic nature of the endolith-mussel symbiosis along a parasitic-mutualistic continuum influenced by abiotic factors., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Sea turtles are vulnerable to climate change since their reproductive output is influenced by incubating temperatures, with warmer temperatures causing lower hatching success and increased feminization of embryos. Their ability to cope with projected increases in ambient temperatures will depend on their capacity to adapt to shifts in climatic regimes. Here, we assessed the extent to which phenological shifts could mitigate impacts from increases in ambient temperatures (from 1.5 to 3°C in air temperatures and from 1.4 to 2.3°C in sea surface temperatures by 2100 at our sites) on four species of sea turtles, under a "middle of the road" scenario (SSP2-4.5). Sand temperatures at sea turtle nesting sites are projected to increase from 0.58 to 4.17°C by 2100 and expected shifts in nesting of 26-43 days earlier will not be sufficient to maintain current incubation temperatures at 7 (29%) of our sites, hatching success rates at 10 (42%) of our sites, with current trends in hatchling sex ratio being able to be maintained at half of the sites. We also calculated the phenological shifts that would be required (both backward for an earlier shift in nesting and forward for a later shift) to keep up with present-day incubation temperatures, hatching success rates, and sex ratios. The required shifts backward in nesting for incubation temperatures ranged from -20 to -191 days, whereas the required shifts forward ranged from +54 to +180 days. However, for half of the sites, no matter the shift the median incubation temperature will always be warmer than the 75th percentile of current ranges. Given that phenological shifts will not be able to ameliorate predicted changes in temperature, hatching success and sex ratio at most sites, turtles may need to use other adaptive responses and/or there is the need to enhance sea turtle resilience to climate warming., (© 2023 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)