Abstract
The algal holobiont, a symbiotic association of algae and bacteria, provides strategies to cope with environmental stresses. Bacteria associated with the green alga Ulva (Chlorophyta) are known to produce essential bioactive compounds, such as thallusin and ectoine, that promote algal growth; however, the metabolic mechanisms underlying stress adaptation in these mutualistic interactions remain poorly understood. Specifically, research is still emerging on the metabolome of cold-adapted Ulva holobionts from remote regions, such as Antarctica, and on how the metabolic profiles respond to environmental changes. This study thus examined heat-induced changes in osmolyte and zwitterion molecules bearing balanced positive and negative charges in Ulva sp. (strain FSU-UPC-109) collected from the Antarctic ecosystem of Potter Cove, South Shetland Islands, and maintained under laboratory conditions at 2°C. Using ultra-high-performance liquid chromatography coupled with high-resolution mass spectrometry (UHPLC–HRMS), we identified highly polar low molecular weight compounds, including cysteinolic acid, ectoine, glutamine, glycerol, and proline. Their spatial distribution on the thallus and rhizoids was determined with atmospheric-pressure matrix-assisted laser desorption/ionization imaging (AP–SMALDI). We demonstrated that the metabolite levels shifted significantly under heat stress, indicating that environmental temperature changes strongly influence the metabolome of Antarctic Ulva seaweed and may support their capacity for thermal acclimation. Graphical abstract Using high-resolution (imaging) mass spectrometry, we studied the heat-induced changes in abundance of the osmolytes and zwitterions, molecules bearing balanced positive and negative charges, in Ulva collected from the Antarctic ecosystem of Potter Cove, South Shetland Islands.