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

<jats:p>The response of the coccolithophorid <jats:italic>Emiliania huxleyi</jats:italic> (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 <jats:italic>F</jats:italic><jats:sub>v</jats:sub><jats:italic>/F</jats:italic><jats:sub>m</jats:sub>, 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 (σ<jats:sub>PSII</jats:sub>) 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 σ<jats:sub>PSII</jats:sub> and photoinhibition suggests that the population of PSII reaction centers (RCIIs) of <jats:italic>E. huxleyi</jats:italic> shares a common antenna, according to a “lake” organization of the light‐harvesting complex.</jats:p>