# Unravelling Diatom–Microbiome Dynamics in the Red Alga Gelidium Corneum (Florideophyceae, Rhodophyta)

## TL;DR

This study explores how the diatom communities on the red alga Gelidium corneum change with seasons and locations, showing that environmental factors strongly influence their diversity and structure.

## Contribution

The study is the first to examine diatom–microbiome dynamics in Gelidium corneum, revealing seasonal and environmental influences on community variation.

## Key findings

- Diatom community structure was more strongly shaped by seasonal shifts than by location-specific factors.
- Diatom diversity was higher in autumn, while spring samples showed lower diversity but more distinct community structures across populations.
- Environmental variables, except irradiance, significantly shaped diatom distribution.

## Abstract

The microbiome plays a crucial role in host health, a recognition that has grown in recent years across many organisms. These host–microbiome associations may not be uniform at the host-species level; instead, they could differ among geographically separated populations, potentially reflecting adaptation to local conditions. In the Cantabrian Sea, rising seawater temperatures are shifting the distribution of many macroalgae species. Among them, Gelidium corneum, a key habitat-forming red alga, is showing a reduction in biomass, yet remains present in this coastal region. Given the wide geographic separation of its populations, it is plausible that G. corneum harbours distinct diatom-dominated microbial communities signalling local adaptation. In this study, we characterized the epibiotic diatom communities associated with G. corneum across three populations spanning 340 km along the northern Iberian Peninsula. Sampling was conducted at two depths, 5 and 12 m, in autumn 2022 and spring 2023. Using an integrated morphological and molecular approach, we identified 268 diatom taxa across 94 genera. Our results revealed a marked seasonal effect on community composition, and all environmental variables, except irradiance, significantly shaped diatom distribution. Certain taxa were consistently associated with seasons and specific locations, indicating site-specific associations. Taken together, these findings show that G. corneum supports a highly diverse and dynamic epibiotic diatom community. As the first study to examine these associations in this species, our results suggest that environmental factors are the main drivers of diatom community variation. However, favourable spring conditions may enhance differentiation among G. corneum populations, allowing the development of more distinct microbial assemblages that may include dominant taxa providing potential mutual benefits.

The online version contains supplementary material available at 10.1007/s00248-026-02723-4.

Gelidium corneum biofilms hosted a diverse community of benthic diatom species.Diatom community structure was more strongly shaped by seasonal shifts than by location-specific factors.Depth variation within the studied range did not affect the composition of benthic diatom assemblages.Diatom diversity was higher in autumn, while spring samples showed lower diversity but more distinct diatom community structures across G. corneum populations.

Gelidium corneum biofilms hosted a diverse community of benthic diatom species.

Diatom community structure was more strongly shaped by seasonal shifts than by location-specific factors.

Depth variation within the studied range did not affect the composition of benthic diatom assemblages.

Diatom diversity was higher in autumn, while spring samples showed lower diversity but more distinct diatom community structures across G. corneum populations.

The online version contains supplementary material available at 10.1007/s00248-026-02723-4.