Spatial metabolomics for symbiotic marine invertebrates

Microbial symbionts frequently localize within specific body structures or cell types of their multicellular hosts. This spa-tiotemporal niche is critical to host health, nutrient exchange, and fitness. Measuring host-microbe metabolite exchange has conventionally relied on tissue homogenates, eliminating di-mensionality and dampening analytical sensitivity. We have developed a mass spectrometry imaging workflow for a soft-and hard-bodied cnidarian animal capable of revealing the host and symbiont metabolome in situ, without the need for a priori isotopic labelling or skeleton decalcification. The mass spec-trometry imaging method provides critical functional insights that cannot be gleaned from bulk tissue analyses or other presently available spatial methods. We show that cnidarian hosts may regulate microalgal symbiont acquisition and re-jection through specific ceramides distributed throughout the tissue lining the gastrovascular cavity. The distribution pattern of betaine lipids showed that once resident, symbionts pri-marily reside in light-exposed tentacles to generate photo-synthate. Spatial patterns of these metabolites also revealed that symbiont identity can drive host metabolism.