Saito, R.; Yamanobe, H.; Yabuki, K.; Suzuki, T.; Saito, T.; Hakozaki, S.; Wanner, M.; Koizumi, R.; Sakai, T.; Maribet, G.; Tanaka, T.; Ono, A.; Nguyen, H. T.; Saito, Y.; Aoyama, T.; Kojima, K.; Suizu, F.; Watanabe, K.; Sogame, Y.

Protist species are distributed worldwide. The processes that enabled this global distribution are unclear. One possible means is through the oceanic dispersal of freshwater protists, although this has not been investigated in detail to date. Here, the ability of resting cysts to tolerate saline conditions is examined as a possible mechanism that enables the oceanic dispersal of protists. Resting cysts of Colpoda cucullus, a freshwater soil protist, can tolerate saline conditions of at least 3.5% NaCl for more than one week. A transcriptome analysis showed that the relative levels of expression of genes associated with membrane function are increased in resting cysts, indicating that salinity tolerance is associated with reconstruction of the cell membrane. Additionally, the outer layer of the cyst wall, a shell-like ectocyst, includes chitin. This may function as a form of biological armor that protects the cell from physical stress during oceanic dispersal.