Shifting alliances: exploring the impact of heat stress on soft coral microbiomes from different latitudes

Introduction Soft corals play key ecological roles in benthic communities by enhancing ecosystem health, providing structural complexity, cycling nutrients, and maintaining symbiotic relationships with diverse microbes. Although their microbiomes are highly structured and species-specific, little is known about their responses to extreme thermal anomalies such as marine heat waves. Methods We analyzed the microbiomes of three Malacalcyonaceans from polar ( Alcyonium haddoni ), temperate ( Alcyonium acaule ) and tropical ( Sinularia sp.) coastal environments, both in their natural habitats and after experimental heat exposure under controlled laboratory conditions. Using 16S rRNA amplicon sequencing, we assessed (a) bacterial taxonomic composition and structure along a latitudinal gradient and (b) the effects of thermal stress on these communities. Results Bacterial community composition differed among coral species and between corals and surrounding waters across all locations. The phylum Pseudomonadota dominated Alcyonium species and seawater samples from each environment. Alpha diversity generally increased under heat stress, though responses varied by species. Thermal stress altered bacterial communities, reducing the proportion of sequences matching natural-habitat microbiomes and making coral microbiota more similar to seawater communities. Host species and temperature treatment jointly explained 36.7% of the variation in microbial composition, highlighting a strong ecological pressure. Discussion Our findings demonstrate that rising seawater temperatures can profoundly disrupt coral–microbiota associations globally. The observed convergence of coral and seawater bacterial communities under heat stress suggests a loss of host-specific microbial signatures, potentially weakening coral resilience and affecting ecosystem functioning under future climate scenarios.