K+ and Na+ transport contribute to K+/Na+ homeostasis in Pyropia haitanensis under hypersaline stress

Intertidal macroalgae have a strong capacity to resist hypersaline stress; however, the underlying mechanism is still unclear. In the present study, we investigated the role of K+/Na+ homeostasis in the salinity tolerance of Pyropia haitanensis for the first time. We found that P. haitanensis had a higher K+/Na+ ratio (2.91) compared to many halophyte plants and seagrasses in the control seawater. High K+ levels and lower Na+ levels caused the K+/Na+ ratio to remain high (average 2.04 and 1.62 under 100 parts per thousand and 110 parts per thousand treatments, respectively), rather than be disrupted by severe hypersaline stress (120 parts per thousand). The thallus did not accumulate Na+ under hypersaline stress, indicating that P. haitanensis avoided Na+ toxicity by excluding Na+ into the apoplast rather than compartmentalizing it into vacuoles. Since there was no passive leakage from the vacuoles back to the cytosol, there was no secondary toxicity. Further analysis suggested that the activation of plasma membrane H+-ATPase occurs to drive the K+/H+ antiporter to exclude Na+ while also reducing K+ loss through depolarization-activated channels in P. haitanensis, which help maintain K+/Na+ homeostasis under hypersaline stress. Therefore, the present study can improve our understanding of the salt tolerance mechanism underlying intertidal macroalgae.