Hoj, L. | bioRxiv Science

bioRxiv2025

Genomic prediction of symbiotic interactions between two Endozoicomonas clades and their coral host, Acropora loripes

BackgroundThe bacterial genus Endozoicomonas is a predominant member of the coral microbiome, widely recognised for its ubiquity and ability to form high-density aggregates within coral tissues. Hence, investigating its metabolic interplay with coral hosts offers critical insights into its ecological roles and contributions to coral health and resilience. ResultsUsing long- and short-read whole-genome sequencing of 11 Endozoicomonas strains from Acropora loripes, genome sizes were found to range between 5.8 and 7.1 Mbp. Phylogenomic analysis identified two distinct clades within the family Endozoicomonadaceae. Metabolic reconstruction uncovered clade-specific pathways, including the degradation of holobiont-derived carbon and lipids (e.g., galactose, starch, triacylglycerol, D-glucuronate), latter of suggest involvement of Endozoicomonas in host sex-type steroid hormone metabolism. A clade-specific type 6 Secretion System (T6SS) and predicted effector molecules were identified, potentially facilitating coral-bacterium symbiosis. Additionally, genomic analyses revealed diverse phosphorus acquisition strategies, implicating Endozoicomonas in holobiont phosphorus cycling and stress responses. ConclusionsThis study reveals clade-specific genomic signatures of Endozoicomonas supporting its mutualistic lifestyle within corals. Findings suggests possible roles in nutrient cycling, reproductive health, and stress resilience, offering novel insights into coral holobiont functioning and potential strategies for reef restoration.

genomics↗

bioRxiv2024

Differential aggregation patterns of Endozoicomonas within tissues of the coral Acropora loripes

Bacteria in the genus Endozoicomonas are well-known coral symbionts commonly found as clusters within tissues of several coral species. Mapping the spatial distribution of these microbial communities is critical to gaining a holistic understanding of the potential role they may play within the coral host. This study focuses on characterising bacterial aggregates associated with the common reef-building coral, Acropora loripes, from the central Great Barrier Reef, Australia. A conventional cultivation-based method was employed to establish a pure culture collection of 11 undescribed Endozoicomonas strains isolated from A. loripes. Subsequent 16S rRNA gene sequence analysis revealed their classification into two distinct phylogenetic clades. To resolve their spatial distribution in hospite, clade-specific fluorescence in situ hybridisation probes were designed. Aggregates were consistently observed in the gastrodermal tissue layers surrounding the upper and lower gastrovascular cavity and were predominantly formed by cells from the same phylogenetic clade, with a minor proportion of aggregates formed by Endozoicomonas from both targeted clades. Furthermore, a clear distinction in aggregation pattern was observed; one clade exhibited clusters with regular and contained growth patterns, whereas the other formed clusters lacking clear boundaries and having irregular shapes. Scanning electron microscopy revealed the presence of a membrane of unknown origin associated with bacterial aggregates in two instances, suggesting potential structural or functional differences in these aggregates. These contrasting morphological features underscore the need for comprehensive investigations into the underlying mechanisms governing bacterial aggregate formation in corals.

ecology↗