Mass spectrometry imaging of Antarctic Ulva and its native microbiome with AP-SMALDI-HRMS

The algal holobiont, which consists of algae and the bacteria living on it, has developed strategies to handle environmental stresses, such as changes in temperature. The green alga Ulva (Chlorophyta) hosts bacteria that promote algal growth and development, from germlings to mature plants. These associated bacteria produce bioactive compounds, including thallusin, N-acyl homoserine lactones, and ectoine, which contribute critically to the growth, development, and overall physiological fitness of the algal host. However, there is limited research on the metabolome of cold-adapted Ulva holobionts, especially those from hard-to-reach regions like Antarctica, and how they respond to environmental changes. This study uses high-resolution mass spectrometry to examine heat induced shifts of osmolytes and zwitter ion in Ulva collected in the Antarctic ecosystem of Potter Cove, King Georg Island. We identified small polar and zwitterionic metabolites, such as cysteinolic acid, ectoine, glutamine, glycerol, and proline, in Ulva tissues analyzed with Ultra-High-Performance Liquid Chromatography coupled with High-resolution Mass Spectrometry (UHPLC-HRMS). We localized these metabolites with atmospheric pressure matrix-assisted laser desorption/ionization (AP-SMALDI) imaging. Finally, we revealed that their levels change significantly in the heat-stressed Antarctic Ulva holobiont, suggesting that environmental changes substantially impact cold-adapted seaweeds from polar regions.