AbstractBioeroding sponges break down calcium carbonate substratum, including coral skeleton, and their capacity for reef erosion is expected to increase in warmer and more acidic oceans. However, elevated temperature can disrupt the functionally important microbial symbionts of some sponge species, often with adverse consequences for host health. Here, we provide the first detailed description of the microbial community of the bioeroding spongeCliona orientalisand assess how the community responds to seawater temperatures incrementally increasing from 23°C to 32°C. The microbiome, identified using 16SrRNAgene sequencing, was dominated byAlphaproteobacteria, including a single operational taxonomic unit (OTU;Rhodothalassiumsp.) that represented 21% of all sequences. The “core” microbial community (taxa present in >80% of samples) included putative nitrogen fixers and ammonia oxidizers, suggesting that symbiotic nitrogen metabolism may be a key function of theC. orientalisholobiont. TheC. orientalismicrobiome was generally stable at temperatures up to 27°C; however, a community shift occurred at 29°C, including changes in the relative abundance and turnover of microbialOTUs. Notably, this microbial shift occurred at a lower temperature than the 32°C threshold that induced sponge bleaching, indicating that changes in the microbiome may play a role in the destabilization of theC. orientalisholobiont.C. orientalisfailed to regainSymbiodiniumor restore its baseline microbial community following bleaching, suggesting that the sponge has limited ability to recover from extreme thermal exposure, at least under aquarium conditions.