Relationship between growth irradiance and the xanthophyll cycle pool in the diatom Nitzschia palea

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Abstract
The size and de‐epoxidation state of the xanthophyll cycle pool was measured in cultures of Nitzschia palea grown at six fluence rates in continuous light or with a 12 h photoperiod. In both series the size of the pool increased with increasing irradiance. The de‐epoxidized form, diatoxanthin, was only present at fluence rates saturating for growth. The portion of diadinoxanthin, which was not readily de‐epoxidized in saturating light, was constant and not related to the size of the pool. In the culture grown in a light‐dark cycle at 300 μmol photons m‐2 s‐1 (PAR) increasing de‐epoxidation took place in the latter half of the photoperiod, when the rate of photosynthesis was decreasing. A rapid, spectrophotometric method for measuring the extent of de‐epoxidation of the xanthophyll cycle pool in a culture of diatoms is described. Upon addition of a small volume of hydrochloric acid to an extract of pigments in 90% acetone, the absorbance at 480 nm is reduced. The size of the reduction is a measure of the state of the xanthophyll cycle pool, since the absorbance of diatoxanthin is reduced by 5%, but the absorbance of diadinoxanthin by 87% due to an epoxide‐furanoid rearrangement, which causes the absorption spectrum to be shifted by ca 20 nm towards shorter wavelengths.