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

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.