Cell-supporting cytoskeletons and phagocytic acquisition of compatible solutes emerge as common strategies for high-salt adaptation in different ciliates.

Understanding the adaptation of organisms to extreme environments is a fascinating topic in biology. Ciliated eukaryotes (ciliates) that inhabit high-salinity environments exhibit remarkable diversity. We revealed various structural and molecular adaptations through a comprehensive investigation of Schmidingerothrix, a ciliate tolerant to salinity levels of up to 25%. One key finding was the presence of a unique microtubule cytoskeleton under the pellicle of Schmidingerothrix, which significantly contributed to its high-salt adaptation. Our results highlight the essential role of coexisting halophilic bacteria in supporting the thriving of ciliates in culture. Contrary to previous studies, our findings indicated an inability to synthesize glycine betaine and ectoine in Schmidingerothrix. However, Schmidingerothrix appears to have expanded its repertoire of phagocytosis-related genes, suggesting a robust mechanism for the uptake and accumulation of compatible solutes via phagocytosis of halophilic bacteria. We expanded our investigation to other high-salt ciliates from different clades and discovered that microtubule cell-shape-supporting cytoskeletons and the phagocytic acquisition of compatible solutes were common strategies for high-salt adaptation. These findings significantly enhance our understanding of how ciliates adapt to high-salt environments and provide valuable insights into the high diversity of heterotrophic protists.