Xanthophyll cycling in Phaeocystis antarctica:changes in cellular fluorescence

Abstract

The xanthophyll cycle has been implicated as a possible photoprotective mechanism in higher plants and algae by dissipating excess excitation energy via non-photochemical q.uenching. To examine whether colonial Phaeocystis antarctica Karsten displays xanthophyll cycling, nutrient-replete cu l tu r~s were initially grown under limiting (40 pm01 quanta m 2 s ~ ' ) and saturating (280 pm01 quanta m-2 S' ) irradiances for photosynthesis and their responses to irradiance transitions were examined for 1 h under 4 treatments. The ~ I I v ~ v o chl-specific absorption coefficient [aml+,(h), m2 (mg chl a)-'] for the light-limiled cultures was initially lower than the light-saturated cultures while chlorophyll (chl) anormallzed fluorrscence 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 whereas parallel decreases in Irradiance similarly decreased the DT:DD ratio. Light-induced Increases in DT concentration were reduced in cultures exposed to dithiothreltol (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 expenmental perturbations. 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 results from treatment and control cultures showed a linear relationship between light-induced changes in DT/chl a concentration and F/chl a (fluorescence to chl a ratio) and the slopes depended on the initial photoacclimated state of the culture. Cellular fluorescence changes appeared to be physiologically based; aaph()L) did not change in response to abrupt irradiance changes. Xanthophyll cycling may enable P antarctica to tolerate both high light environments and sudden changes in irradidnce, which occur during austral spring due to shallow mixed layers and intermittent shading by ice or clouds.