Gut microbiome plasticity and host resistance in response to ocean warming in sub-Antarctic sea urchins.

BACKGROUND: Sub-Antarctic marine ecosystems are highly vulnerable to climate change, with rising ocean temperatures threatening key benthic species. Abatus cordatus, an endemic sea urchin of the Kerguelen Islands with limited dispersal capacity, has been hypothesised to possess a narrow thermal niche, which would render it particularly susceptible to environmental shifts. However, microbiome-mediated acclimation may provide a potential mechanism of resilience to ocean warming. To test these hypotheses, this study evaluates host survival and gut microbiome responses of A. cordatus to medium-term seawater warming under near-future temperature scenarios using 16S rRNA gene sequencing to compare these changes with those observed in sediment microbiomes. RESULTS: Host mortality remained relatively low across all temperatures, showing no association with warming intensity and thereby suggesting thermal tolerance. While gut microbiome alpha-diversity remained stable, its composition shifted and variability increased with experiment duration and temperature, leading to greater inter-individual divergence and a decline in both the richness and abundance of core taxa. In contrast, sediment microbiomes remained more stable, exhibiting more deterministic assembly and increased core stability over time. At the taxonomic level, specific gut bacterial ASVs showed temperature-dependent abundance shifts, with greater flexibility at moderate thermal stress. Notably, the depleted and enriched ASVs were affiliated to known sulphate-reducing and fermentative taxa, respectively, suggesting a possible functional shift. CONCLUSIONS: Overall, our findings suggest that A. cordatus can tolerate medium-term warming, with gut microbiome plasticity representing a potential mechanism supporting host resilience.