Exposure of intact cells of Spirulina to high temperature (HT) stress (40-60 °C) caused decrease in absorption spectrum and fluorescence emission spectrum. Low temperature emission spectra were altered at phycocyanin (PC) level. Room and low temperature emission spectra of intact phycobilisomes showed that PC was the main target in this cyanobacterium for the altered energy transfer under HT.Additional key words: absorption spectra; fluorescence emission spectra at room and low temperature. ___________Cyanobacteria are oxygenic photosynthetic prokaryotes whose thylakoid organization is almost similar to that of higher plants. The major pigments of Spirulina platensis are chlorophyll (Chl) a and the phycobiliproteins, phycocyanin (PC) and allophycocyanin (APC). These pigments are associated with the photosynthetic apparatus in the thylakoid membranes (Bryant 1991). Phycobiliproteins are pigmented constituents of phycobilisomes (PBSs) which contain a water soluble light-harvesting complex attached to the thylakoid membranes (Glazer 1984). The efficiency of energy transfer from PC to Chl a in intact cells is influenced by various environmental factors such as high temperature, HT (Schreiber 1979), nitrogen stress (Yamanaka et al. 1980), and heavy metals (Hg: Pecci and Fujimori 1967, Murthy and Mohanty 1991; Cu: Park and Sauer 1991). Singhal et al. (1981) made a preliminary investigation on the spectral properties of intact cells under HT stress in Synechococcus. Babu et al. (1991) studied under HT the photosynthetic electron transport and spectral properties of intact cells of Spirulina. In this work we checked the effect of HT on spectral properties of PBSs both in vivo and in vitro.Spirulina platensis was grown in a defined medium at 25±2 °C under continuous "white light" (15 W m -2 ) as described in Venkataramanaiah et al. (2003). The culture was continuously bubbled with filtered air. The midlog-phase culture was harvested by centrifugation at 12 000×g for 10 min and washed with fresh growth medium and finally suspended in the medium by maintaining 0.5 kg(Chl) m -3 . Separate aliquots of intact cells were exposed to different temperatures (35-50 °C) for 30 min in dark at Chl a concentration of 2 kg m -3 . For in vitro studies, PBSs were isolated from intact cells by following the procedure of Gantt et al. (1979). Similarly, PBSs equivalent to 30 g(protein) m -3 were taken separately and exposed to temperatures of 30-40 °C for 10 min in the dark. Chl amount was estimated according to Mackinney (1941). Absorption spectra of intact cells and PBSs were recorded using Hitachi U-2000 spectrophotometer. Fluorescence emission spectra at room temperature and 77 K were recorded on a Perkin-Elmer LS-5 spectrofluorimeter according to Murthy et al. (1991). The spectra were not corrected for the spectral efficiency of the equipment. Protein content was estimated by the procedure of Lowry et al. (1951).The Spirulina cells treated with 40 °C showed no large changes in the absorption spectrum (Fig. 1A). The increase in temperatu...