An isolated polynucleotide is provided. The isolated polynucleotide comprising a nucleic acid sequence encoding a polypeptide of a Type II reaction center of a photosynthetic organism, the nucleic acid sequence being capable of imparting the type II reaction center with an activity under a temperature range different than that of the type II reaction center endogenous to the photosynthetic organism. Also provided are methods of using the sequences for generating photosynthetic organisms or tailor-made thermotolerance.