Sponges can be considered biological engines that fuel the ecology of shallow-water to deep-sea ecosystems through the sponge loop. Here, we discuss new insights on the role of the sponge holobiont in the uptake, transformation, and transfer of resources that fuel the many food webs in which sponge engines operate. The tightly coupled animal-microbe partnership consists of many intrinsic traits (e.g., growth form, tissue architecture, and number and diversity of microbial symbionts) that depend on a huge variety of environmental factors (e.g., temperature, light, and both organic and inorganic resources). The sponge loop mediates resource cycling at organismal, ecosystem, and potentially even global scales. Linking sponge physiology and microbiome function with ecosystem budgets and refined global biomass estimates to situate sponges within Earth's elemental cycles, including the biological carbon pump, is necessary to transition from illustrative case studies to predictive theory and to forecast the response of sponge engines to future ocean conditions.
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