Excess light-induced violaxanthin 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 μmol m⁻² s⁻¹). Within the range 20–40°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°C (the temperature at which the experimental cultures were grown) than at 25°C (optimal growth temperature) or 35°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°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°C to 35°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°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°C and 35°C, zeaxanthin epoxidation to form violaxanthin in low light conditions (5–10 μmol m⁻² s⁻¹) was faster than in the culture exposed to 20°C, which was consistent with the more extensive de-epoxidation observed at the latter temperature. At 40°C violaxanthin did not recover, and zeaxanthin content increased during the period of recovery under low light.