Micromolecular Adaptations in Antarctic Echinoderms: Overlapping Roles of Organic Osmolytes in Osmoregulation and Protein Stability Under Low Temperature

For osmoconforming organisms, prolonged exposure to reduced salinity requires an adjustment to intracellular osmolyte levels to ensure an osmotic balance is maintained between the cell and external sea water. However, osmolytes—low-molecular-mass micromolecules—may also serve overlapping roles in freeze avoidance, desiccation resistance, and protein stabilisation. In Antarctic species living at or below 0 °C, multiple environmental stressors likely shape species-specific osmolyte profiles. Yet, the osmolytes utilised in osmotic acclimation and the broader micromolecular profile of Antarctic marine organisms remain poorly characterised. This study employed gas chromatography–mass spectrometry (GC–MS) to analyse the organic osmolyte composition of two endemic Antarctic echinoderms, the sea star Odontaster validus and sea urchin, Sterechinus neumayeri, following long-term acclimation (>12 weeks) to reduced salinity levels (29 ‰ and 24 ‰). Significant reductions in total tissue organic solute (osmolyte) concentrations after low salinity exposure indicated active cell volume regulation to reduce intracellular osmotic pressure. The osmolyte metabolic profiles of these Antarctic species appeared distinct from those of temperate echinoderms and other marine osmoconformers, suggesting a specialised adaptive response. Notably, the use of branched-chain amino acids (valine, leucine, and isoleucine) in cell volume regulation in O. validus, and to a lesser extent in S. neumayeri, suggests a micromolecular adaptation tailored to the extreme cold of the Antarctic environment.