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Nostoc flagelliforme is a terrestrial cyanobacterium distributed in arid or semiarid area, which has the capability of nitrogen-fixing and photosynthesis. The multifunction of MAAs is of great significance for N. flagelliforme to adapt to abiotic stresses. The changes of mycosporine-like amino acids in N. flagelliforme cells with UV-B radiation and salt stress were studied. The results showed that MAAs concentrations reached 32.26 mg/g after 12 h under UV-B radiation (5 W/m(2)). After 2 days cultivation, MAAs synthesis was up to the maximum with the BG11 medium containing 50 mM NaCl and 50 mM NH4Cl, and their concentrations were 45.09 and 35.14 mg/g respectively. HPLC analysis revealed the control sample contained nine MAAs. Ten MAAs were observed after UV-B treatment, and two new MAA compounds were found but one MAA disappeared. With the treatment of NaCl and NH4Cl for 2 days, only seven MAAs were discovered and no new MAAs appeared.