Many marine sponges host highly diverse microbiomes that contribute to various aspects of host health. Although the putative function of individual groups of sponge symbionts has been increasingly described, the extreme diversity has generally precluded in-depth characterisation of entire microbiomes, including identification of syntrophic partnerships. The Indo-Pacific sponge Ianthella basta is emerging as a model organism for symbiosis research, hosting only three dominant symbionts: a Thaumarchaeotum, a Gammaproteobacterium, and an Alphaproteobacterium and a range of other low abundance or transitory taxa. Here, we retrieved metagenome assembled genomes (MAGs) for over 90 percent of I. bastas microbial community, facilitating the metabolic reconstruction of the near complete microbiome of the sponge, identification of metabolic complementarity between microbes, as well as describing the putative importance of symbionts present in low abundance. We also mined the metagenomes for putative viral sequences, highlighting the contribution of viruses to the overall metabolism of the sponge, and complemented this data with metaproteomics to identify active metabolic pathways in bacteria, archaea, and viruses. This data provides the metabolic framework to adopt I. basta as a model organism for studying host-microbe interactions and provides a basis for in-depth physiological experiments.