Thermostability of the Photosynthetic System of the Thermoacidophilic AlgaCyanidium caldariumin Continuous Culture

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Abstract
Ford, T. W. 1986. Thermostability of the photosynthetic system of the thermoaadophilic alga Cyanidium caldarium in continuous culture.—J. exp. Bot. 37: 1698–1707. Cyanidium caldarium, when exposed to gradual increases in temperature in continuous culture, exhibits a growth temperature maximum of 55 °C. This correlates with the thermostability of the membrane-located photosynthetic electron transport system but not with in vivo ribulose-1,5-bisphosphate (RUBP) carboxylase activity, which retained full activity after 1 h at 60 °C. Pigment content and phycocyanin: chlorophyll a ratios were relatively constant at growth temperatures up to 50 °C, but both declined in cells cultured at 55 °C. Some modification of the photosynthetic system of the alga, in response to growth temperature, was detected with both oxygen evolution and RUBP carboxylase activity showing improved thermostability in cells grown at 50 °C or 55 °C compared with those cultured at lower temperatures. However, this enhanced thermostability was at the expense of total pigment content and overall photosynthetic capacity which were considerably reduced in high temperature cells, as was the temperature spectrum for efficient RUBP carboxylase operation. The contributions of membrane and macromolecular components of the cell to the imposition of optimum and maximum growth temperatures are discussed.