PHOTOINHIBITION OF PSII IN EMILIANIA HUXLEYI (HAPTOPHYTA) UNDER HIGH LIGHT STRESS: THE ROLES OF PHOTOACCLIMATION, PHOTOPROTECTION, AND PHOTOREPAIR¹

Abstract

The response of the coccolithophorid Emiliania huxleyi (Lohmann) W. H. Hay et H. Mohler to acute exposure to high photon flux densities (PFD) was examined in terms of PSII photoinhibition, photoprotection, and photorepair. The time and light dependencies of these processes were characterized as a function of the photoacclimation state of the alga. Low-light (LL) acclimated cells displayed a higher degree of photoinhibition, measured as decline in Fv /Fm , than high-light (HL) acclimated cells. However, HL cultures were more susceptible to photodamage but also more capable of compensating for it by performing a faster repair cycle. The relation between gross photoinhibition (observed in the presence of an inhibitor of repair) and PFD to which the algae were exposed deviated from linearity at high PFD, which calls into question the universality of current concepts of photoinhibition in mechanistic models. The light dependence of the de-epoxidation state (DPS) of the xanthophyll cycle (XC) pigments on the timescale of hours was the same in cells acclimated to LL and HL. However, HL cells were more efficient in realizing nonphotochemical quenching (NPQ) on short timescales, most likely due to a larger XC pool. LL cells displayed an increase in the PSII effective cross-section (σPSII ) as a result of photoinhibition, which was observed also in HL cells when net photoinhibition was induced by blocking the D1 repair cycle. The link between σPSII and photoinhibition suggests that the population of PSII reaction centers (RCIIs) of E. huxleyi shares a common antenna, according to a "lake" organization of the light-harvesting complex.

Interpretation of fast repetition rate (FRR) fluorescence: signatures of phytoplankton community structure versus physiological state

“…Growth rates and cellular physiology are thus not in balance with the nutrient supply. Inverse relations between F v /F m and σ PSII are observed when PSII function becomes compromised by nutrient starvation (Kolber et al 1988, Geider et al 1993b, Steglich et al 2001 or photoinhibition by visible or UV radiation (Vassiliev et al 1995, Ragni et al 2008. Such a decline of F v /F m in response to stress can be interpreted to indicate a decline in the proportion of RCIIs that are functional: when the pool of functional and damaged RCIIs shares a common , Vassiliev et al 1995.…”

“…Significantly, within a taxon, F v /F m decreases and σ PSII (478) increases in response to nutrient starvation and other environmental stresses, e.g. photoinhibition and/or UV radiation (Vassiliev et al 1994, Ragni et al 2008). This pattern of change essentially reflects that observed in the taxonomic shift from diatoms to picoplankton (compare Fig.…”

PSII photoinhibition and photorepair in Symbiodinium (Pyrrhophyta) differs between thermally tolerant and sensitive phylotypes

“…1 in Ragni et al 2008). We accounted for the differences in photosynthetically utilisable radiation (PUR) between the white fluorescent lamps used for growth and the blue LED by using a conversion factor calculated as described in Ragni et al (2008). The PUR of the fluorescent light source was ca.…”

“…The tubes were distributed along a light gradient in 3 parallel rows in an incubator cooled with a thermostatic bath set to the growth temperature and illuminated by a block of blue light-emitting diodes (LEDs; 465 nm peak height; see Fig. 1 in Ragni et al 2008). We accounted for the differences in photosynthetically utilisable radiation (PUR) between the white fluorescent lamps used for growth and the blue LED by using a conversion factor calculated as described in Ragni et al (2008).…”

“…We used fluorescence-based methods to determine the rates of PSII photoinhibition and photorepair, and changes in the PSII antenna size (σ PSII ), electron transport rates (ETR RCII ), and photochemical quenching (q P ) and NPQ. Rates of damage and repair were discriminated by incubating samples in the presence and absence of a chloroplast protein synthesis inhibitor, lincomycin (Lesser et al 1994, Bouchard et al 2005, Ragni et al 2008, since it blocks de novo synthesis of the D1 protein and thus prevents the replacement of the damaged protein during the PSII repair cycle.…”