XANTHOPHYLL CYCLING IN PHAEOCYSTIS-ANTARCTICA - CHANGES IN CELLULAR FLUORESCENCE

The xanthophyll cycle has been implicated as a possible photoprotectiv e mechanism in higher plants and algae by dissipating excess excitatio n energy via non-photochemical quenching. To examine whether colonial Phaeocystis antarctica Karsten displays xanthophyll cycling, nutrient- replete cultures were initially grown under limiting (40 mu mol quanta m(-2) s(-1)) and saturating (280 mu mol quanta m(-2) s(-1)) irradianc es for photosynthesis and their responses to irradiance transitions we re examined for 1 h under 4 treatments. The in vivo chl-specific absor ption coefficient [a(ph)(lambda), m(2) (mg chl a)(-1)] for the Light- limited cultures was initially lower than the light-saturated cultures while chlorophyll (chl) a-normalized fluorescence yields were similar for both treatments. Increases in irradiance induced increases in the diatoxanthin to diadinoxanthin ratio (DT:DD, w:w) up to 9-fold wherea s parallel decreases in irradiance similarly decreased the DT:DD ratio . Light-induced increases in DT concentration were reduced in cultures exposed to dithiothreitol (DTT), an inhibitor of DD to DT conversion. Short-term changes in DD and DT concentrations were attributed solely to xanthophyll cycling; no de novo synthesis of DD or DT was evident based on a constant sum of DD and DT in the 1 h experimental perturbat ions. It was found that DD and DT de novo synthesis required long-term acclimation; the mass ratio at steady state of (DD+DT)/chl a was 0.1 and 0.4 for the low and high light treatments, respectively. Pooled re sults from treatment and control cultures showed a linear relationship between light-induced changes in DT/chl a concentration and F/chl a ( fluorescence to chi a ratio) and the slopes depended on the initial ph otoacclimated state of the culture. Cellular fluorescence changes appe ared to be physiologically based; a(ph)(lambda) did not change in res ponse to abrupt irradiance changes. Xanthophyll cycling may enable P. antarctica to tolerate both high light environments and sudden changes in irradiance, which occur during austral spring due to shallow mixed layers and intermittent shading by ice or clouds.