# Microbiome characterization of the sea slugs Elysia viridis and Placida dendritica: insights into potential roles in kleptoplasty

## TL;DR

This study compares the microbiomes of two sea slugs that retain chloroplasts, finding significant differences that may relate to their ability to maintain functional chloroplasts.

## Contribution

The study identifies a unique carotenoid-producing bacterial genus in Elysia viridis, potentially linked to chloroplast retention.

## Key findings

- E. viridis has a smaller, more specialized microbiome dominated by Bacteroidota and Actinomycetota.
- A ZOTU related to Fulvibacter was found exclusively in E. viridis, suggesting a role in chloroplast retention.
- P. dendritica has a more diverse microbiome dominated by Pseudomonadota.

## Abstract

Kleptoplasty is the process by which functional chloroplasts from algae food sources are sequestered and retained by a host organism. Some sacoglossan sea slugs display this ability, enabling them to survive extended periods of food shortage, as they can obtain organic compounds from photosynthesis. While research has focused on the mechanisms underlying chloroplast retention and functionality, the contribution of the microbiome to kleptoplasty in these photosynthetic sea slugs is largely unexplored. In this study, we assessed the bacterial communities of Elysia viridis and Placida dendritica, two photosynthetic sacoglossan species that share the same habitat and macroalga food source, but exhibit distinct abilities to retain chloroplasts.

High-throughput 16S rRNA gene sequencing revealed highly significant differences in bacterial community composition between E. viridis and P. dendritica. The sea slug E. viridis hosted a smaller and more specialized bacterial community, while P. dendritica supported a larger, more diverse, and generalist microbiome. Bacteroidota and Actinomycetota were the most dominant phyla in E. viridis (~ 92% of the total sequence reads), while Pseudomonadota (former Proteobacteria) was the prevalent phylum in P. dendritica (~ 66% of the total sequence reads). The dominance analysis revealed that one particularly dominant ZOTU, comprising 224,282 sequence reads (32.5% of total), was found exclusively in E. viridis that exhibits long-term retention of functional chloroplasts. According to BLAST search results, this ZOTU was related to the genus Fulvibacter (Flavobacteriaceae family), known for producing carotenoid-type pigments. These bacteria were not present in the microbiome of P. dendritica that shows short-term, non-functional, chloroplast retention.

This study provides novel insights into the microbial communities associated with photosynthetic sea slugs, highlighting significant differences in bacterial composition between E. viridis and P. dendritica. The identification of carotenoid-producing bacteria in E. viridis suggests a possible role in oxidative stress mitigation, and opens new perspectives on the relevance of the microbiome in supporting kleptoplasty. Further research is needed to characterize the functional contributions of bacterial taxa to chloroplast acquisition and maintenance in photosynthetic sea slugs.

The online version contains supplementary material available at 10.1186/s12866-025-04573-5.