Photoacclimation Characteristics And Mechanisms Of Sargassum Thunbergii

Living in the intertidal zone, macroalgae are exposed to drastically changing environmentsresulting from immersion and emersion alternation. Most of the species are sessile, thus theyexperience a complicated pattern of irradiance controlled by the diurnal changes and thesuperimposed tidal rhythm. In response to widely fluctuating environment, macroalgae haveevolved many adaptive mechanisms, of which light-induced photoinhibition has been proven to bethe most important one. In brown macroalgae, because of the lack of rapid allosteric changes inthe light harvesting complexes, enhanced heat dissipation by de-epoxidized xanthophyllsrepresents the major photoprotective response, thereby preventing inactivation and damage to theirphotosynthetic apparatus. Therefore, the photoinhibition of intertidal brown macroalgae deservesspecial attention for the purpose of full understanding of plant defense mechanism of light damage.Sargassum thunbergii is a representative intertidal brown macrolaga along the northwesternPacific coast. Two experimental techniques including chlorophyll fluorescence measurements(PAM) and high performance liquid chromatograph (HPLC), will be used to systematicallyexplore the process and mechanisms of photoacclimation in S. thunbergii from three viewpoints ofecology, phytophysiology and biochemistry. The main results showed as following.1. Photoacclimation characteristics of Sargassum thunbergii germlings underdifferent light intensitiesThe photosynthesis and growth responses of Sargassum thunbergii germlings to differentlight intensities (10,60, and300μmol photons m2s1) were investigated. Maximumphotochemical efficiency (Fv/Fm), rapid light curves (RLCs), and photochemical andnon-photochemical quenching (qP and NPQ) were estimated by a pulse amplitude-modulatedfluorometer. The photosynthesis of S. thunbergii germlings exhibited different properties tooptimize light capture and utilization. The excitation pressure (1qP) was rapidly increased toapproximately0.27showing that germlings responded to high light by chronic photoinhibitionwith an accumulation of closed reaction centers, which ultimately resulted in a slow growth. Thiswas accompanied by a reduced Fv/Fmwith time and a development of high capacity for NPQ.Although Fv/Fmin moderate-light germlings did not fully recover overnight, germlingsdemonstrated a less severe chronic photoinhibition considering the reduced degree of excitation pressure accumulation of approximately0.15. The relative stability of photosynthetic capacity(rETRmax, Ek, and α) could endow germlings with the highest relative growth rate (RGR) ofapproximately9.3%day1in moderate light. By contrast, low-light germlings demonstrated highFv/Fmand Fo, corresponding high α collectively suggested greater efficiency of light absorptionand energy transformation. Sustained increases in electron transport capacity (rETRmaxand Ek)occurred in low-light germlings, which resulted in a stable RGR of over8.2%day1.Consequently, S. thunbergii germlings are considered to prefer low light regimes and have arelative capacity of moderate and high light tolerance. However, the light acclimation tooversaturating conditions is at the cost of slow growth to maintain survival.2. Highly efficient photoprotective responses to high light stress in Sargassumthunbergii germlings, a representative brown macroalga of intertidal zonePhotosynthetic responses to sudden exposure to high light stress (600μmol photons m2s1) and the potential for subsequent recovery were assessed in Sargassum thunbergii germlingsgrown under three different light intensities of10μmol photons m2s1(low light, LL),60μmolphotons m2s1(moderate light, ML) and300μmol photons m2s1(high light, HL). Alltreatments exhibited high capacity for dynamic photoinhibition, with the fast reaction kinetics ofFv/Fmduring both inhibition and recovery period, and with the rapid induction of maximum NPQ(within minutes). HL-germlings characteristically demonstrated a high NPQ value of approx.5.5,allowing a flexible and reversible response to stress. Besides the significant contribution of NPQto photoprotection, photosynthetic capacity (ETRmax) in LL-germlings was as great as that inHLgermlings, suggesting that energy dissipation through photochemical electron transport systemcould also reduce probability of photodamage. NPQ in S. thunbergii germlings appeared to be notdirectly controlled by a transthylakoid proton gradient (ΔpH) due to the lack of “light activatedstate”. Furthermore, inhibition of xanthophylls cycle with DTT considerably blocked NPQpreinduction of preillumated germlings, and a slow NPQ relaxation occurred upon disruption of ΔpHby NH4Cl, collectively indicating the importance of xanthophyll cycle to NPQ. These resultssuggested that S. thunbergii germlings could tolerate sudden high light by down-regulation ofphotosynthetic capacity, based on highly efficient photoprotective responses, including energydissipation through xanthophyll cycle and photosynthetic electron transport. The photoprotectionwas efficiently independent on the light history of germlings. The high photosynthetic plasticitywith immediate response to rapidly changing lightmay be a central feature explaining the survival of germlings in highly variable light environments of intertidal habitat.3. The importance of sustained retention of de-epoxidated pigments for the rapiddevelopment of NPQ in Sargassum thunbergii, a representative brown macroalgaof intertidal zoneThe characteristics of non-photochemical quenching (NPQ) and xanthophyll cycle inresponse to high light stress (1200μmol photons m-2s-1) were investigated in Sargassumthunbergii. Results showed that the ΔpH-dependent quenching component qE is absent in NPQ ofS. thunbergii, as evidenced by the slow induction and relaxation of NPQ and strong relationshipbetween NPQ and xanthophyll cycle. Moreover, inhibition of xanthophyll cycle with dithiothreitolconsiderably blocked NPQpreinduction of preilluminated S. thunbergii, and during illumination, aslow NPQ relaxation occurred upon disruption of ΔpH by NH4Cl only in the presence ofxanthophyll cycle, further indicating that NPQ in S. thunbergii did not directly controlled by ΔpH.The rate of violaxanthin de-epoxidation was decreased with increasing concentrations of NH4Cladded before illumination showing that the role of ΔpH might be only related to activateviolaxanthin de-epoxidase enzyme. In the absence of qE, the xanthophyll cycle was crucial for arapid and efficient photoprotection response to high light stress for S. thunbergii. The size ofxanthophyll cycle pigment pool (ΣXC) was as high as17mol mol-1Chl a×100, and thede-epoxidation state of this pool (DPS) reached up to0.5, allowing S. thunbergii to accumulateefficiently de-epoxidated pigments to induce an extremely high NPQ capacity approx.10.Furthermore, preillumination significantly accelerate the induction of NPQ in S. thunbergii. Therapid formation and remarkable slow epoxidation of antheraxanthin resulted in a retention of highantheraxanthin concentrations after preillumination for several minutes, which could significantlyaccelerate the accumulation of zeaxanthin. The significant negative linear correlations betweenhalf-time of NPQ induction and the retention of zeaxanthin suggested that zeaxanthin directlyaccelerated NPQ induction. Consequently, the sustained retention of de-epoxidated pigmentsplayed a key role in response to considerable variation in incident irradiance, allowing S.thunbergii to be widely distributed in the intertidal zone.