Temperature Induced Changes in the Absorption Spectra of Porphyridium cruentum and Anacystis nidulans

Cooling A. nidulans or P. cruentum from 26 °C to 4 °C results in progressive spectral changes. The most significant changes are increases in absorbance at 690 nm (photosystem II), 678 nm (antenna chlorophyll), 625 nm (phycocyanin), and 505 nm (electrochromic band). In the case of P. cruentum there are also increases in absorbance at 568 and 545 nm (B-phycoerythrin) and a decrease at 587 nm. The spectral changes in both organisms are accompanied by decreases at 703 nm (photosystem I or aggregated chlorophyll), 650 nm (allophycocyanin) and 486 nm (carotenoid). Heating A. nidulans or P. cruentum from 4°C to 44 °C results in increases in absorbance at 705 nm (photosystem I) and 486 nm (carotenoid), accompanied by decreases at 690 nm (photo­ system II), 676 nm (antenna chlorophyll), 628 nm (phycocyanin), 507 nm (electrochromic band) and 469 nm (carotenoid). In the case of P. cruentum there are also decreases at 568 and 546 nm (B-phycoerythrin) and an increase at 587 nm. The possible origin of the spectral change at 587 nm. The possible origin of the spectral change at 587 nm is discussed. The spectral changes of the chlorophyll bands (703, 690, 678 nm) and the electrochromic band (502 nm) are associated with phase changes of the lipid membrane. Lowering the temperature results in a decrease of aggregated chlorophyll or photosystem I, and vice versa. These spectral changes are also observed in green chloroplasts. The spectral changes of the phycobilins may originate from a temperature dependent change of the ion balance o f the thylakoid. A spectral change may result from the ensuring modification of the stacking or from an electrochromic effect.