A new glance at the chemosphere of macroalgal-bacterial interactions: In situ profiling of metabolites in symbiosis by mass spectrometry

Therefore, we used high sensitivity, spatial resolution mass spectrometry and imaging to gain a new perspective on the mutualistic interactions between bacteria and macroalgae. Using atmospheric pressure scanning microprobe matrix-assisted laser desorption/ionisation high-resolution mass spectrometry (AP-SMALDI-HRMS), low- molecular-weight polar compounds were identified by comparative metabolomics in the chemosphere of Ulva.

Choline (2-hydroxy-N,N,N-trimethylethan-1-aminium) was only determined in the alga grown under axenic conditions, whereas ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidinecarboxylic acid) was found in bacterial presence. Ectoine was used as a metabolic marker for localisation studies of Roseovarius sp. within the tripartite community because it was produced exclusively by these bacteria. By combining confocal laser scanning microscopy (cLSM) and AP-SMALDI-HRMS, we proved that Roseovarius sp.

MS2 settled mainly in the rhizoidal zone (holdfast) of U. mutabilis. Our findings provide the fundament to decipher bacterial symbioses with multicellular hosts in aquatic ecosystems in an ecologically relevant context. As a versatile tool for microbiome research, the combined analysis of AP-SMALDI and cLSM imaging with resolution down to a single bacterial cell level can be easily applied to other microbial consortia and their hosts.

The novelty of this contribution is the use of an in situ setup to resolve spatial distributions of metabolites and the identification of specific symbiotic bacteria, which is designed to avoid all types of external contamination.