Taxonomically different symbiotic communities of sympatric Arctic sponge species show functional similarity with specialization at species level.

Marine sponges harbor diverse communities of associated organisms, including eukaryotes, viruses, and bacteria. Sponge-associated microbiomes contribute to the health of host organisms by defending them against invading bacteria and providing them with essential metabolites. Here, we describe the microbiomes of three sympatric species of cold-water marine sponges IMPORTANCE: Sponges are regarded among the earliest multicellular organisms and the most ancient examples of animal-bacterial symbiosis. The study of host-microbe interactions in sponges has advanced rapidly due to the application of next-generation sequencing (NGS) technologies that help overcome the challenges of investigating their communities. However, many sponge species, particularly those from polar ecosystems, remain poorly characterized. Here, we demonstrate that three sympatric cold-water sponge species, including two analyzed for the first time, harbor distinct sets of bacterial symbionts, stably associated over 6 years. Using CORe contigs ITerative Expansion and Scaffolding, an algorithm developed in this study, we reconstructed high-quality symbiont genomes and revealed shared features indicative of convergent evolution toward symbiosis. Notably, we identified a potentially novel symbiotic feature-a gene cluster likely involved in sulfoacetate uptake and dissimilation. We also observed shifts in microbiome composition, associated with increasing water temperatures, raising concerns about the impact of global warming on cold-water ecosystems.