Nutrient-rich environments drive microbiome restructuring and mucus shedding in a coastal cnidarian.

Introduction: Anthropogenic nutrient enrichment is increasing in coastal ecosystems, yet its effects on host-microbe interactions and animal fitness remain insufficiently understood. This study investigates how elevated dissolved nutrient availability and differing nutrient compositions influence microbiome structure and host responses in the sea anemone Nematostella vectensis. Methods: We experimentally enriched seawater with either a complex organic nutrient source or a protein-rich substrate. We then assessed resulting changes in microbiome composition, host behavior (including mucus shedding), and population growth. Results: Nutrient enrichment caused substantial restructuring of the host-associated microbiome, characterized by reduced diversity and increased dominance of fast-growing bacterial taxa. Community composition differed depending on nutrient type. Enriched conditions also triggered increased ectodermal mucus shedding, which physically removed surface-associated microbes. Copiotrophic taxa that proliferated under nutrient enrichment were disproportionately represented in shed mucus relative to whole polyps, suggesting spatial structuring of microbial associations with host surfaces. While this response limited microbial overgrowth, it imposed significant physiological costs, leading to reduced or negative population growth. Discussion: These findings demonstrate that nutrient enrichment alters host-associated microbiomes and identify mucus shedding as a host-mediated mechanism for microbial regulation. Although effective, this response carries substantial fitness costs, highlighting a trade-off between microbial control and host performance. This study provides mechanistic insight into how eutrophication may drive microbiome restructuring and impact host physiology in coastal cnidarians.