Excess light-induced violaxalathin cycle activity in Nannochloropsis gaditana (Eustigmatophyceae): effects of exposure time and temperature

Violaxanthin cycle activity in Nannochloropsis gaditana has been investigated using high-performance liquid chromatography (HPLC) to determine 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 changes in pigment content accompanied by a concurrent decrease in the epoxidation state (EPS). From the de-epoxidation time course, the de-epoxidation rate was found to be slower than in other plant systems. In addition, the extent of de-epoxidation was dependent on both exposure time and temperature. De-epoxidation occurred faster and more extensively at 20 degrees C (the temperature at which the experimental cultures were grown) than at 25 degrees C (optimal growth temperature) or 35 degrees C. Thus, after exposure to excess light for 240 min at these temperatures, violaxanthin decreased by about 28%, 6%, and 21% of the initial values, respectively. At 40 degrees C, up to 78% of the violaxanthin pool was de-epoxidated, which would apparently reflect the de-epoxidation limit for this alga. As a result of increasing both the temperature within the range from 20 degrees C to 35 degrees C and the period of exposure to light, there was a similar increase in both zeaxanthin and antheraxanthin contents, but zeaxanthin formation was greater at the higher temperatures. The increase in zeaxanthin was more notable at 40 degrees C than at any other temperature. It is proposed that these findings could be a result of a two-step process rather than a continuous sequence of violaxanthin de-epoxidation. In cultures exposed to 25 degrees C and 35 degrees C, zeaxanthin epoxidation to form violaxanthin in low light conditions (5-10 mu mol m(-2) s(-1)) was faster than in the culture exposed to 20 degrees C, which was consistent with the more extensive de-epoxidation observed at the latter temperature. At 40 degrees C violaxanthin did not recover, and zeaxanthin content increased during the period of recovery under low light.