Identification of potassium transport proteins in algae and determination of their role under salt and saline-alkaline stress

K+ is an essential macronutrient in life activities of organisms. Compared with the natural environment (high Na+ and low K+), living cells contain high concentration of K+ and low concentration of Na+, which is one of the three major relics of the original birth of life. As the beginning of green plants evolution, algae are the most suitable species for understanding the efficient absorption of K+ through systematical analysis the potassium transport systems. In this study, we found that four algal species namely Chlorella variabilis (C. variabilis), Chlamydomonas reinhardtii (C. reinhardtii), Chlorella sacchrarophila (C. sacchrarophila), and Nannochloris sp. JB17 (JB17) displayed adaption to low K+ condition. Further, K+ concentrations in the growth media affected the growth of algae under the salt and saline-alkali stresses. Moreover, JB17, previously isolated from extremely saline-alkali soil, was found to maintain the high K+/Na+ ratio under the condition with high Na+ and low K+. According to bioinformatics data, there were great differences in the number, conserved motif and secondary structure of K+ transport proteins among algae, animals and plants, including the high affinity potassium transporter (HAK), Na+/K+-ATPase (NaK) and H+-ATPase (AHA). The expression of HAKs and NaK1 was significantly induced by low K+ condition, but most of them were decreased under salt or saline-alkali stress in C. reinhardtii, while the expression of those were significantly induced by saline-alkali stress in JB17. Ouabain, a specific inhibitor of Na+/K+-ATPase, impaired the saline-alkali tolerance of JB17 and C. variabilis by affecting the Na+ and K+ balance. Overexpressing JbAHA from JB17 improved the tolerance to saline-alkali stress in C. reinhardtii. All above indicating that Na+/K+-ATPase, H+-ATPase and K+/H+ high affinity transport systems in algae played important role in tolerance to high Na+ and low K+ environment.