MEPS 169:113-121 (1998)  -  doi:10.3354/meps169113

Xanthophyll cycling in Phaeocystis antarctica: changes in cellular fluorescence

Tiffany A. Moisan*, Miguel Olaizola**, B. Greg Mitchell

Scripps Institution of Oceanography, University of California, La Jolla, California 92093-0218, USA
**Present address: Aquasearch Inc., 73-4460 Queen Kaahumanu Hwy. Suite 110, Kailua-Kona, Hawaii 96740, USA

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 quenching. To examine whether colonial Phaeocystis antarctica Karsten displays xanthophyll cycling, nutrient-replete cultures were initially grown under limiting (40 µmol quanta m-2 s-1) and saturating (280 µmol quanta m-2 s-1) irradiances for photosynthesis and their responses to irradiance transitions were examined for 1 h under 4 treatments. The in vivo chl-specific absorption coefficient [a* ph(λ), m2 (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 whereas 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 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; a* ph(λ) 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 irradiance, which occur during austral spring due to shallow mixed layers and intermittent shading by ice or clouds.

KEY WORDS: Antarctic · Phaeocystis · Phytoplankton · Xanthophyll cycling · Fluorescence · Absorption · Diatoxanthin · Diadinoxanthin

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