The effect of polyunsaturated fatty acids on photosynthesis and the growth of the marine cyanobacterium Synechococcus sp. PCC 7002 was examined using wild-type and Delta12 fatty acid desaturase mutant strains. Under a light intensity of 250 mumol m(-2) s(-1), wild-type cells could grow exponentially in a temperature range of 20-38 degrees C, but growth was non-exponential below 20 degrees C and ceased at 12 degrees C. The Delta12 desaturase mutant cells lacking polyunsaturated fatty acids had the same growth rate as wild-type cells in a temperature range of 25-38 degrees C but grew slowly at 22 degrees C, and no cell growth took place below 18 degrees C. Under a very high-light intensity of 2.5 mmol m(-2) s(-1), wild-type cells could grow exponentially in a temperature range of 30-38 degrees C, although the high-light grown cells became chlorotic because of nitrogen limitation. The temperature sensitive phenotype in the Delta12 desaturase mutant was enhanced in cells grown under high-light illumination; the mutant cells could grow at 38 degrees C, but were killed at 30 degrees C. The decrease of oxygen evolution and nitrate consumption by whole cells as a function of temperature was similar in both wild type and the Delta12 desaturase mutant. No differences were observed in either light-induced damage of oxygen evolution or recovery from this damage. No inactivation of oxygen evolution took place at 22 degrees C under the normal light intensity of 250 mumol m(-2) s(-1). These results suggest that growth of the Delta12 desaturase mutant at low temperature is not directly limited by the inactivation of photosynthesis, and raise new questions about the functions of polyunsaturated membrane lipids on low temperature acclimation in cyanobacteria.