Microbiome Composition in a Common Mediterranean Bryozoan Following an Unprecedented Marine Heatwave

8 pages, 2 figures, supporting information https://doi.org/10.1111/1758-2229.70185.-- Data Availability Statement: The data that support the findings of this study are openly available in SRA at https://www.ncbi.nlm.nih.gov/bioproject/PRJNA1236962, reference number BioProject: PRJNA1236962

Marine heatwaves are intensifying due to global warming and increasingly drive mass mortality events in shallow benthic ecosystems. Marine invertebrates host diverse microbial communities that contribute to their health and resilience, yet microbiome responses under thermal stress remain poorly characterised across most taxa. Here, we characterise the microbiome composition in colonies of the common Mediterranean bryozoan Myriapora truncata at two depths (13 and 17 m) following the extreme 2022 marine heatwave. Despite no visible necrosis, microbial communities at both depths exhibited shifts indicative of thermal stress, including the reduced presence of potential core microbial members. Colonies from the shallower, warmer depth showed higher alpha diversity and reduced abundance of key functional genera compared to deeper colonies, suggesting early dysbiosis. These results highlight that M. truncata—though visually unaffected—undergoes sublethal microbiome alterations under thermal stress. This study provides the first characterisation of a bryozoan microbiome after a marine heatwave and highlights the potential of host-associated microbial communities as early bioindicators of invertebrate stress in a warming ocean

B.F. acknowledges support from a Ramón y Cajal grant (RYC2022-036268-I) funded by MICIU/AEI/10.13039/501100011033 and FSE+, and a Beatriu de Pinós grant (2019 BP 00183) funded by the Catalan Government and the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 801370. This work was conducted within the framework of the projects MedCalRes (PID2021-125323OA-I00) and HOLOCHANGE (CNS2024-154224) to B.F., funded by MICIU/AEI/10.13039/501100011033 and ERDF/EU. B.F. and J.G. also acknowledge support from the Severo Ochoa Centre of Excellence accreditation (CEX2019-000928-S). B.F., C.L., P.L.S., and J.G. are part of the Marine Conservation research group funded by the Generalitat de Catalunya (2021 SGR 01073). C.L. gratefully acknowledges the financial support by ICREA under the ICREA Academia programme. J.dC. was supported by project PID2020-118836GA-I00 financed by MICIU/AEI/10.13039/501100011033 and project 2021 SGR 00420 financed by Departament de Recerca i Universitats de la Generalitat de Catalunya

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