Photosynthetic acclimation to photon irradiance and its relation to chlorophyll fluorescence and carbon assimilation in the halotolerant green alga Dunaliella viridis

This work describes the long-term acclimation of the halotolerant microalga Dunaliella viridis to different photon irradiance, ranging from darkness t o 1500 mu mol m(-2) s(-1.) In order to assess the effects of long-term phot oinhibition, changes in oxygen production rate, pigment composition, xantho phyll cycle and in vivo chlorophyll fluorescence using the saturating pulse method were measured. Growth rate was maximal at intermediate irradiance ( 250 and 700 mu mol m(-2) s(-1)). The increase in growth irradiance from 700 to 1500 mu mol m(-2) s(-1) did not lead to further significant changes in pigment composition or EPS, indicating saturation in the pigment response t o high light. Changes in Photosystem II optimum quantum yield (Fv/Fm) evide nced photoinhibition at 700 and especially at 1500 mu mol m(-2) s(-1). The relation between photosynthetic electron flow rate and photosyntetic O-2 ev olution was linear for cultures in darkness shifting to curvilinear as grow th irradiance increased, suggesting the interference of the energy dissipat ion processes in oxygen evolution. Carbon assimilation efficiencies were st udied in relation to changes in growth rate, internal carbon and nitrogen c omposition, and organic carbon released to the external medium. All illumin ated cultures showed a high capability to maintain a C:N ratio between 6 an d 7. The percentage of organic carbon released to the external medium incre ased to its maximum under high irradiance (1500 mu mol m(-2) s(-1)). These results suggest that the release of organic carbon could act as a secondary dissipation process when the xanthophyll cycle is saturated.