Characterization of NaCl-tolerant Mutant Strain of the Cyanobacterium Spirulina platensis Overproducing Phycocyanin

Background: Spirulina spp., a promising source of phycocyanin, is commercially grown in open ponds and raceways photoautotrophically. However, the economic exploitation in an open system seems to have been limited because of lack of multiple stress-tolerant strains. Methods: In order to have an improved production of phycocyanin, a NaCl tolerant mutant of the cyanobacterium Spirulina platensis was isolated using a chemical mutagen, N-methyl-N'-nitro-Nnitrosoguanidine (NTG) and selection for NaCl tolerant strain. Results: The mutant strain exhibited improved growth and overproduced phycocyanin at 0.8 M NaCl. NaCl-tolerant mutant also exhibited higher rate of photosynthesis and reduced rate of respiration in response to 0.8 M NaCl. The mutant strain took up more nitrate and reduced it more effectively in presence of NaCl which explains its enhanced phycocyanin accumulatory behavior. The mutant strain also protected its cells from NaCl stress by ccumulating low amounts of trehalose, glucosyl-glycerol and proline, and by synthesizing anti-oxidative enzymes in comparison to wild type strain which further supports the subdued effects of NaCl and halotolerant behavior of mutant strain. Conclusion: The NaCl tolerant mutant synthesized 1.5 folds higher phycocyanin as compared to wild type in response to 0.8 M NaCl. The mutant strain clearly demonstrates unique haloprotective strategies that mitigate the deleterious effects of high NaCl concentration as reflected by its efficient photosynthesis and nitrogen metabolism. Moreover, the subdued defense response of mutant in relation to osmolytes accumulation and antioxidant enzymes confirms its adaptive and tolerant behavior against NaCl.