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 c u l t u 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.

Southern Ocean phytoplankton physiology in a changing climate

“…Some species acclimate by changing the size of the light harvesting antenna of the individual reaction centre, while others increase the total number of reaction centres, keeping antenna size constant (Falkowski and LaRoche, 1991;Moore et al, 2006). As with temperate diatoms (Ruban et al, 2004;Dimier et al, 2007;Lavaud et al, 2007), SO-and sea ice diatoms are highly plastic to fluctuations in light with a strong dependence on rapid and reversible xanthophyll cycling (Moisan et al, 1998;Moisan and Mitchell, 1999;Kropuenske et al, 2009;Petrou et al, 2011a,c), however, not all species utilise the same strategies, nor have the same level of physiological plasticity (Kropuenske et al, 2009;Mills et al, 2010;Petrou et al, 2011a). This physiological variability results in a wide range of light utilisation efficiency and photoprotective capacities from one phytoplankton species to another, making it important to investigate species-specific photosynthetic activity to understand what drives community composition and ultimately primary productivity.…”

“…This plasticity is not common to all SO diatoms, instead other species exposed to the same stressors, have shown to be less plastic to changes in light, exhibiting limited NPQ capacity and no dynamic control over excitation pressure (Petrou et al, 2011a, highlighting how photoacclimation strategy may be an important determinant of the success of F. cylindrus in polar marine environments (Kang and Fryxell, 1992;Lizotte, 2001;Kopczynska et al, 2007). As with F. cylindrus, the photoacclimation strategy of the prymnesiophyte Phaeocystis antarctica provides us with some insight into the success of this species, including its fast photoacclimation capabilities (Moisan et al, 1998;Kropuenske et al, 2009Kropuenske et al, , 2010 and high investment into carotenoid synthesis under Fe limitation (van Leeuwe and Stefels, 2007). While it appears that these two species utilise similar strategies, studies directly comparing P. antarctica with F. cylindrus found that the former was able to acclimate to changes in irradiance much faster than the diatom, but was much slower to induce NPQ, resulting in more photoinhibitory quenching (Kropuenske et al, 2009;Mills et al, 2010) and a high dependence on photosynthetic protein repair processes (Kropuenske et al, 2009.…”

Fluorescence patches in high-frequency internal waves

“…Krause & Weis 1991), giving an apparent decrease in biomass. In laboratory low-to-high light studies, nonphotochemical quenching resulted in an initial average rate of change in fluorescence of 4% min -1 (SE = 0.8, n = 13) (from graphs and tables in: Kiefer 1973, Sakshaug et al 1987, Cullen & Lewis 1988, Demers et al 1991, Olaizola et al 1994, Moisan et al 1998. A sudden change from high light to low light is predicted to give the opposite effect: an apparent increase in biomass with a time scale of minutes.…”

“…The average rate during summer, although larger, was not significantly different from the laboratory studies (7% min -1 , SE = 1.5; p = 0.15). It may be significant that laboratory studies of non-photochemical quenching generally show the increasing fluorescence response time to be somewhat longer than the decreasing fluorescence response time (Kiefer 1973, Sakshaug et al 1987, Demers et al 1991, Moisan et al 1998, whereas the changes in fluorescence measured in the field were typically symmetric (same rate of increase and decrease during a wave; Figs. 3 & 8).…”

Photosynthetic pigments in 37 species (65 strains) of Haptophyta: implications for oceanography and chemotaxonomy

“…The function of the 4-keto derivatives is unknown. The universally distributed carotenoid pair diadinoxanthin and diatoxanthin, present in all haptophytes examined, have a well-established photoprotective function via the light-regulated epoxide cycle (Stransky & Hager 1970, Siefermann-Harms 1985, Demmig-Adams & Adams 1993, Moisan et al 1998, Lohr & Wilhelm 1999. Further study is needed to understand the function and biosynthetic regulation of all these important marine pigments, and their consequent reliability as chemotaxonomic indicators in field oceanography.…”