# Unveiling the Microeukaryotic Landscape of the Red Coral Corallium rubrum Across the Northwestern Mediterranean Sea

## TL;DR

This study explores the microeukaryotic communities associated with red coral in the Mediterranean Sea, finding both consistent core families and high variability influenced by environmental factors.

## Contribution

The study identifies core microeukaryotic families in red coral and highlights their potential roles in coral resilience to climate stressors.

## Key findings

- Core microeukaryotic families like Licnophoridae and Dino-Group I Clade 1 were present across all sampling locations.
- Eukaryome composition varied significantly between sites, seasons, and years, indicating environmental influence.
- The study emphasizes the need to understand the ecological roles of microeukaryotes in coral resilience to climate change.

## Abstract

Octocorals such as
Corallium rubrum
are key components of temperate Marine Animal Forests (MAFs), providing three‐dimensional habitats that support diverse marine life. However,
C. rubrum
faces growing threats from overexploitation and climate stressors such as ocean warming. While the coral's bacterial microbiome is well‐documented and stable across spatial and temporal scales, the associated microeukaryotes, collectively referred to as the coral eukaryome, remain poorly characterised. In this study, we used 18S rRNA gene metabarcoding to explore the eukaryome of 46
C. rubrum
colonies collected from five sites (~44,670 km2) in the northwestern Mediterranean. We identified a limited set of core microeukaryotic families, including Licnophoridae and Dino‐Group I Clade 1, which were present at all sampling locations. Despite sharing core taxa, eukaryome composition showed high variability between sampling sites, seasons and years. This suggests the red coral eukaryome is strongly influenced by local environmental factors. Given the increasing frequency of marine heatwaves and habitat degradation, further research is needed to understand the ecological roles of key eukaryotic taxa and their contribution to coral holobiont resilience. Clarifying the function of the eukaryome is essential for predicting how
C. rubrum
and other habitat‐forming octocorals will respond to future climate scenarios.

This study demonstrates that the eukaryome of the Mediterranean octocoral
Corallium rubrum
exhibits high variability, and the dominance of certain taxa is influenced by spatiotemporal environmental factors. Despite this, core microeukaryotic families, such as Licnophoridae and Dino‐Group I Clade 1, were consistently present, suggesting key roles in the coral's holobiont.