# Diversity of the surface microbiome of canopy-forming brown macroalgae (Fucales) in the northern Adriatic

## TL;DR

This study explores the microbial communities on brown algae in the Adriatic Sea, revealing how environmental factors shape these communities and their potential role in coastal ecosystems.

## Contribution

The study provides a baseline for understanding algal-microbiome interactions in anthropogenically impacted marine environments.

## Key findings

- Algal microbiomes show low host specificity, with 75% of amplicon sequence variants shared with the coastal ecosystem microbiome.
- Microbiomes of algal parts differ from seawater and sediment in diversity and composition, suggesting horizontal transmission pathways.
- Deciduous algal parts show consistent microbiomes, while holdfast and axis vary between populations.

## Abstract

Canopy-forming brown macroalgae (Fucales) offer numerous key ecosystem services in Mediterranean coastal areas. However, anthropogenic pressures and climate change have significantly impacted their habitats, leading to an extensive population decline. Interactions between algae and microbiota are a major ecological aspect, yet they represent a significant knowledge gap. In our baseline study, we describe the diversity and host specificity of the microbiome of two genetically identical but morphologically distinct populations of Gongolaria barbata from anthropogenically impacted northern Adriatic Sea. Our preliminary results showed that the microbiomes of G. barbata exhibited low host specificity, with 75% of the algae-associated amplicon sequence variants (ASVs) being part of the core coastal ecosystem microbiome. However, microbiomes of specific algal parts, ambient seawater, and sediment differed significantly in terms of alpha diversity and composition. In contrast, the holdfast and axis show higher similarity with sediment microbiomes, indicating potential horizontal transmission pathways. Microbiomes associated with deciduous parts of morphologically distinct G. barbata populations showed no difference in alpha diversity and composition. In contrast, higher variation in alpha diversity and lower sequence proportion of shared ASVs were observed in the holdfast and axis of the two distinct populations. Our observational study provides valuable new insights and baseline for future hypothesis-driven research on the interactions between algae and associated microbiota—a knowledge gap that needs to be addressed in the future for better understanding of the ecological and evolutionary dynamics of coastal ecosystems.

Our study focuses on the microbiomes of canopy-forming brown macroalgae from the Fucales order, essential habitat builders in Mediterranean coastal areas. These habitats, offering key ecosystem services, face significant declines due to anthropogenic pressures and climate change. We used next-generation 16S rRNA amplicon sequencing to reveal novel insights into the diversity and host specificity of Gongolaria barbata populations in impacted ecosystems. Our findings suggest environmental factors influence the structure of the algae microbiome, with potential recruitment from adjacent sediment communities. This research enhances the understanding of marine ecosystems’ ecological and evolutionary dynamics, providing valuable insights for conservation and management efforts.