Physiological and proteomic analysis of salinity tolerance of the halotolerant cyanobacterium Anabaena sp

The halotolerant cyanobacterium Anabaena sp was grown under NaCl concentration of 0, 170 and 515 mM and physiological and proteomic analysis was performed. At 515 mM NaCl the cyanobacterium showed reduced photosynthetic activities and significant increase in soluble sugar content, proline and SOD activity. On the other hand Anabaena sp grown at 170 mM NaCl showed optimal growth, photosynthetic activities and comparatively low soluble sugar content, proline accumulation and SOD activity. The intracellular Na⁺ content of the cells increased both at 170 and 515 mM NaCl. In contrast, the K⁺ content of the cyanobacterium Anabaena sp remained stable in response to growth at identical concentration of NaCl. While cells grown at 170 mM NaCl showed highest intracellular K⁺/Na⁺ ratio, salinity level of 515 mM NaCl resulted in reduced ratio of K⁺/Na⁺. Proteomic analysis revealed 50 salt-responsive proteins in the cyanobacterium Anabaena sp under salt treatment compared with control. Ten protein spots were subjected to MALDI-TOF–MS/MS analysis and the identified proteins are involved in photosynthesis, protein folding, cell organization and energy metabolism. Differential expression of proteins related to photosynthesis, energy metabolism was observed in Anabaena sp grown at 170 mM NaCl. At 170 mM NaCl increased expression of photosynthesis related proteins and effective osmotic adjustment through increased antioxidant enzymes and modulation of intracellular ions contributed to better salinity tolerance and optimal growth. On the contrary, increased intracellular Na⁺ content coupled with down regulation of photosynthetic and energy related proteins resulted in reduced growth at 515 mM NaCl. Therefore reduced growth at 515 mM NaCl could be due to accumulation of Na⁺ ions and requirement to maintain higher organic osmolytes and antioxidants which is energy intensive. The results thus show that the basis of salt tolerance is different when the halotolerant cyanobacterium Anabaena sp is grown under low and high salinity levels.