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Over 90% of introduced marine species are seaweeds. Seaweeds rely on their microbiome for host settlement, nutrition, development and health. As such, it is likely that microbiomes are involved in seaweed invasions. Sargassum muticum, indigenous to Southeast Asia, inhabits the North-east Atlantic from Norway to Morocco. This is the only known successful case of a non-clonal marine invader with almost no genetic variation over its large NE Atlantic introduced range. This makes it a very interesting model to study an invasive seaweeds microbiome, as it practically uncouples host genetic variation from microbiome variation. Associated bacteria potentially contribute to the plasticity and acclimation of S. muticum leading to its success over the last 50 years. Dispersing host organisms can either bring their acquired microbes along or obtain new ones locally, following the mutualist and generalist host hypothesis, respectively. We used partial 16S rRNA gene amplicon sequencing to characterize the total and core microbiome across S. muticum structures/tissues from five NE Atlantic regions, from Norway to Morocco, covering over 30° of latitude. In contrast to host genotypes, highly diverse, regional, total and core microbiomes, with differentiation levels depending on tissue, bacterial community structure were detected. Atlantic S. muticum follows the generalist host hypothesis, possibly recruiting a new microbiome in each new region. This host promiscuity may promote the invasiveness of S. muticum. Diversity was lower in the young/annual tissues compared with the older tissues for the total bacterial community, suggesting that these are mostly transitory bacteria accumulating over time in the older parts. The total core microbiome included 10 OTUs, representing dominant community members commonly found in other seaweeds' cores. The core Granulosicoccus OTU followed a clear regional pattern where northern and southern regions clustered differentially, suggesting a regional signature even at an OTU level. North-east (NE) Atlantic Sargassum muticum microbiome differentiation does not match host genetic background. NE Atlantic S. muticum shows a geographic region and tissue-specific total and core bacterial community matching bestthe generalist host hypothesis. Core OTU – the bacterium Granulosicoccus – shows a strong regional specialization. [ABSTRACT FROM AUTHOR]