Epidaphic and endaphic cyanobacteria of oil polluted regions of southern Iran live under extreme environmental conditions including extremely low irradiance, limited carbon dioxide concentration and high degrees of salinity. Calothrix sp. ISC 65 is an unexplored strain which seems common in southern Iran and near the Persian-Gulf. Until now, we have no report about the basic and applied aspects of such a strain. In this research Calothrix sp. ISC 65 has been characterized physiologically by the combination of extremely low irradiance (2 µE·m-2·s-1), different salinities (0%, 0.5%, and 1%), and extremely limited carbon dioxide concentration (no aeration, no carbon dioxide enrichment). Spectroscopically, an analysis showed that 1% salinity after 96 hours caused a significant increase in growth rate, chlorophyll, and phycobilisome production. Lower salinity caused about a 40% decrease of phycobilisome production even after 24 hours. Phycocyanin seemed the main part of phycobilisome but salinity at 1% caused phycoerythrin and allophycocyanin production excitation as the outer part of the photosynthetic antenna as well. Results of long-time oxygen production supported the above. Increasing salinity to 0.5% and 1% caused a significant increase of photosynthetic long time activity. Differences between 0.5% and 1% were not significant. Increasing salinity affected the CO2 concentration mechanism and caused more activation. Activation of the CO2 concentration mechanism appeared as the basic strategy supporting the high rate of photosynthesis providing enough material necessary for photosynthetic processes. Results of a spectrofluorometric analysis showed that the PSI : PSII ratio increased by increasing salinity and reached the highest level after 96 hours. A surface response plot analysis showed that low salinity was able to increased the ratio even more but only for a very short time after inoculation. Salinity at the level of 0.5% caused increases in nitrogenase activity and the excitation of heterocyst production. This was also true for salinity levels of 1% though the heterocyst production declined only a little. Fourier transform infrared analysis (FTIR) analysis showed that 1% salinity caused the most outstanding configuration changes of the functional groups. The differences in functional group patterns between culture media with no additional salinity and 1% were completely obvious. Differences around asymmetric carbon vibration, lipid stretching and OH bending of the polysaccharides occurred at 1% salinity treatments. Collectively, 0.5 to 1% salinity caused the most physiological activities in Calothrix sp. ISC 65 at extremely limited irradiance and carbon dioxide concentration.
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