Elevated temperature modulates the effects of an Endozoicomonadaceae member on the coral holobiont

Endozoicomonadaceae are globally prevalent coral-associated bacteria that serve as important indicators of coral health, yet their functional roles in the holobiont remain poorly understood. Our study investigated the interactions between Endozoicomonadaceae and coral holobionts by inoculating the fluorescently labeled Endozoicomonadaceae strain SCSIO 12664 - a dominant member of Pocillopora coral's cell-associated microbial aggregates - into its native host Pocillopora damicornis under both ambient (25°C) and elevated (32°C) temperature conditions. Comparative genomic analysis revealed that SCSIO 12664 uniquely contains multiple copies of the transcriptional activator/repressor GIS1 genes that were upregulated under thermal stress in in vitro experiments. This genomic feature, combined with our amplicon sequencing data from colonization experiments showing its competitive dominance within the Endozoicomonadaceae family at high temperatures (accounting for 88.0% of total Endozoicomonadaceae reads under 32°C), suggests a potential molecular mechanism for its thermal adaptation advantage. Microscopy confirmed successful bacterial colonization at both 25°C and 32°C. Under 25°C conditions with SCSIO 12664 inoculation, coral immune pathways were downregulated while host nutrient transport systems were upregulated, accompanied by Symbiodiniaceae metabolic reprogramming that enhanced amino acid biosynthesis and protein turnover. In contrast, 32°C conditions with SCSIO 12664 inoculation led to attenuated host-symbiont interactions with fewer differentially expressed genes, though still inducing Symbiodiniaceae photosystem II related genes upregulation and improved photosynthetic efficiency that may help mitigate thermal stress. The inoculation of SCSIO 12664 under elevated temperature conditions led to a decrease in Flammeovirgaceae abundance while simultaneously enhancing microbial network connectivity. These findings reveal that temperature plays a role in shaping Endozoicomonadaceae -coral holobiont interactions. This temperature-dependent functionality challenges traditional classifications of coral associated Endozoicomonadaceae as simply mutualistic. Moreover, the study highlights that bacterial contributions to holobiont fitness cannot be properly assessed without considering the environmental context.