Effects of circadian rhythms of fluctuating temperature on growth and biochemical composition of Ulva pertusa

The marcoalga Ulva pertusa was cultured under (20 +/- 2)degrees C, (20 +/- 4)degrees C, (20 +/- 6)degrees C, (20 +/- 8)degrees C and (20 +/- 10)degrees C circadian rhythms of fluctuating temperature conditions, and constant temperature of 20 degrees C was used as the control. The growth rate of macroalga at (20 +/- 2)degrees C, (20 +/- 4)degrees C and (20 +/- 6)degrees C were significantly higher than that at constant temperature of 20 degrees C, while growth rate at (20 +/- 8)degrees C and (20 +/- 10)degrees C were significantly lower than that at constant temperature of 20 degrees C. The growth rate of macroalga was a quadratic function of the thermal amplitude. Such a growth model can be described by G = beta(0) + beta(1)(TA) + beta(2)(TA)(2), where G represents the relative growth rate, TA is thermal amplitude in degree Celsius, beta(0) is the intercept on the G axis, and beta(1) and beta(2) are the regression coefficients. The optimal thermal amplitude for the growth of thallus at mean temperature of 20 degrees C was estimated to be +/- 3.69 degrees C. Analysis of biochemical composition at the final stages of thaulls growth revealed that diel fluctuating temperature caused various influences (P < 0.05). The content of chlorophyll, protein and total solute carbohydrate at (20 +/- 2)degrees C and (20 +/- 4)degrees C were slightly higher than those at constant temperature of 20 degrees C, however no statistically significant differences were found among them (P > 0.05). While osmolytes (total solute carbohydrate and free proline) at (20 +/- 10)degrees C were significantly higher than that at 20 degrees C (P < 0.05). Therefore, more chlorophyll and carbohydrate production might account for the enhancement in the growth of macroalga at the diel fluctuating temperatures in the present study.