Molecular insights into symbiosis-mapping sterols in a marine flatworm-algae-system using high spatial resolution MALDI-2-MS imaging with ion mobility separation.

Waminoa sp. acoel flatworms hosting Symbiodiniaceae and the related Amphidinium dinoflagellate algae are an interesting model system for symbiosis in marine environments. While the host provides a microhabitat and safety, the algae power the system by photosynthesis and supply the worm with nutrients. Among these nutrients are sterols, including cholesterol and numerous phytosterols. While it is widely accepted that these compounds are produced by the symbiotic dinoflagellates, their transfer to and fate within the sterol-auxotrophic Waminoa worm host as well as their role in its metabolism are unknown. Here we used matrix-assisted laser desorption ionization (MALDI) mass spectrometry imaging combined with laser-induced post-ionization and trapped ion mobility spectrometry (MALDI-2-TIMS-MSI) to map the spatial distribution of over 30 different sterol species in sections of the symbiotic system. The use of laser post-ionization crucially increased ion yields and allowed the recording of images with a pixel size of 5 µm. Trapped ion mobility spectrometry (TIMS) helped with the tentative assignment of over 30 sterol species. Correlation with anatomical features of the worm, revealed by host-derived phospholipid signals, and the location of the dinoflagellates, revealed by chlorophyll a signal, disclosed peculiar differences in the distribution of different sterol species (e.g. of cholesterol versus stigmasterol) within the receiving host. These findings point to sterol species-specific roles in the metabolism of Waminoa beyond a mere source of energy. They also underline the value of the MALDI-2-TIMS-MSI method to future research in the spatially resolved analysis of sterols.