EXCESS LIGHT-INDUCED VIOLAXALATHIN CYCLE ACTIVITY IN NANNOCHLOROPSIS-GADITANA (EUSTIGMATOPHYCEAE) - EFFECTS OF EXPOSURE TIME AND TEMPERATURE

Violaxanthin cycle activity in Nannochloropsis gaditana has been inves tigated using high-performance liquid chromatography (HPLC) to determi ne the changes in pigment content on exposure to high irradiance (2000 mu mol m(-2) s(-1)). Within the range 20-40 degrees C, the effects of exposure time and temperature on the activity of the cycle have also been ascertained. Violaxanthin cycle activity was clearly shown by cha nges in pigment content accompanied by a concurrent decrease in the ep oxidation state (EPS). From the de-epoxidation time course, the de-epo xidation rate was found to be slower than in other plant systems. In a ddition, the extent of de-epoxidation was dependent on both exposure t ime and temperature. De-epoxidation occurred faster and more extensive ly at 20 degrees C (the temperature at which the experimental cultures were grown) than at 25 degrees C (optimal growth temperature) or 35 d egrees C. Thus, after exposure to excess light for 240 min at these te mperatures, violaxanthin decreased by about 28%, 6%, and 21% of the in itial values, respectively. At 40 degrees C, up to 78% of the violaxan thin pool was de-epoxidated, which would apparently reflect the de-epo xidation limit for this alga. As a result of increasing both the tempe rature within the range from 20 degrees C to 35 degrees C and the peri od of exposure to light, there was a similar increase in both zeaxanth in and antheraxanthin contents, but zeaxanthin formation was greater a t the higher temperatures. The increase in zeaxanthin was more notable at 40 degrees C than at any other temperature. It is proposed that th ese findings could be a result of a two-step process rather than a con tinuous sequence of violaxanthin de-epoxidation. In cultures exposed t o 25 degrees C and 35 degrees C, zeaxanthin epoxidation to form violax anthin in low light conditions (5-10 mu mol m(-2) s(-1)) was faster th an in the culture exposed to 20 degrees C, which was consistent with t he more extensive de-epoxidation observed at the latter temperature. A t 40 degrees C violaxanthin did not recover, and zeaxanthin content in creased during the period of recovery under low light.