re: Could fungi survive in brines on Mars?
Rather than the surface, it might better to look beneath the cryosphere where water will remain liquid more often than frozen in aquifers. Salts and minerals will be leached out of the surrounding rock for a fuel source. These would be rather deep. So far MARSIS has not found any shallow aquifers. Water in any significant quantities making it to the surface from an aquifer would need to be hydrothermally sourced by contact with a lava lip. The water boiled or superheated to steam would leave salts and or acids behind to later emerge upon the surface to condense. Hense it would start out initially purer at the surface and not have the advantages that acids or brines have in cold temperatures to help form a temporary surface habitat.