The coloration of cells of the cyanobacterium Synechococcus sp. PCC 7002 changed from normal blue-green to yellow-green when cells were grown at 15 degrees C in a medium containing
nitrate as the sole
nitrogen source. This change of coloration was similar to a general response to nutrient deprivation (
chlorosis). For the chlorotic cells at 15 degrees C, the total amounts of
phycobiliproteins and
chlorophyll a decreased, high levels of
glycogen accumulated, and growth was arithmetic rather than exponential. These changes in composition and growth occurred in cells grown at low (50 microE m-2 s-1) as well as high (250 microE m-2 s-1) light intensity. After a temperature shift-up to 38 degrees C, chlorotic cells rapidly regained their normal blue-green coloration and normal exponential growth rate within 7 h. When cells were grown at 15 degrees C in a medium containing
urea as the reduced
nitrogen source, cells grew exponentially and the symptoms of
chlorosis were not observed. The decrease in photosynthetic
oxygen evolution activity at low temperature was much smaller than the decrease in growth rate for cells grown on
nitrate as the
nitrogen source. These studies demonstrate that low-temperature-induced
chlorosis of Synechococcus sp. PCC 7002 is caused by
nitrogen limitation and is not the result of limited photosynthetic activity or photodamage to the photosynthetic apparatus, and that
nitrogen assimilation is an important aspect of the low-temperature physiology of cyanobacteria.