IN-VIVO MANIPULATION OF THE XANTHOPHYLL CYCLE AND THE ROLE OF ZEAXANTHIN IN THE PROTECTION AGAINST PHOTODAMAGE IN THE GREEN-ALGA CHLORELLA-PYRENOIDOSA

Chlorella pyrenoidosa was grown in steady state continuous cultures in either high or low light. Samples of these cultures were incubated in darkness (violaxanthin state) or in saturating light (zeaxanthin stat e). These samples were kept in the respective preadapted states throug hout the entire photodamage treatment. Photodamage involved exposure t o single-turnover flashes fired at a low (non-actinic) frequency. The damage caused by the light stress thus applied was monitored by change s in photosynthetic properties and pigment composition. Cells preadapt ed in the light resisted photodamage better than those kept in darknes s. The low light grown cells were more vulnerable to photodamage than the high light grown cells. Our experimental approach permitted the eq uilibria between the components that participate in the xanthophyll cy cle to be set without addition of inhibitors. Regardless of the total amount of violaxanthin being present, its conversion to anthera- and z eaxanthin is a prerequisite for protection. The protection is most eff ective for photosystem II. It appeared that antheraxanthin accumulates as a result of photodamaging flashes provided that these are fired in the presence of background light, i.e. with zeaxanthin present. From this, it is newly derived that the xanthophyll cycle operates in full in the light, including epoxidation of zeaxanthin. The latter conversi on was also demonstrated in vitro, via nonenzymatic oxygen-dependent t urnover of zeaxanthin into violaxanthin.