Due to the importance of benthic cyanobacteria in nitrogen fixation in paddyfields, the physiological responses of Fischerella ambigua FS 18 to salinity (NaCl 0, 0.5 & 1%) were investigated. Results indicated that growth rate was higher in control and that NaCl did not inhibit growth of other treatments. Similar responses were seen in the case of other physiological processes. Chlorophyll was declined with increasing salinity. Phycobiliproteins (PBP), phycocyanin (PC) and allophycocyanin (APC) were at least in 0.5% but the differences were not significant. The light-saturated photosynthetic rate was higher in control and decreased with increasing in salinity. With respect to nitrogenase activity, the highest rate was in control and the cultures with higher growth rates reached to the maximum level at a shorter time. There was no significant difference between 0.5 and 1% in relation to nitrogenase activity.

Keywords: Cyanobacteria, Fischerella sp, Nitrogenase, Photosynthetic pigments, photosynthesis, Salinity

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