| In this work, the rate of photosystem II (PSII) damage was measured in photoautotrophic cultures of Dunaliella salina (a green unicellular alga) grown at light intensities in the 50-3000 {dollar}mu{dollar}mol photons {dollar}rm msp{lcub}-2{rcub} ssp{lcub}-1{rcub}{dollar} range. In the presence of the chloroplast translation inhibitor lincomycin, PSII photodamage (monitored as loss of the D1 protein) proceeded with first-order decay kinetics with respect to the initial active PSII concentration. The rate constant of PSII photodamage was a linear function of cell growth irradiance. Loss of D2, another PSII reaction center protein, was also observed, occurring with kinetics similar to those of D1. Increasing rates of photodamage as a function of irradiance were accompanied by an increase in the steady-state level of a higher molecular weight protein complex ({dollar}sim{dollar}160 kD) that cross-reacted with D1 and D2 antibodies. The steady-state level of the 160 kD complex in thylakoids was also a linear function of cell growth intensity. These observations suggest that photodamage to D1 converts the D1/D2 heterodimer into a {dollar}sim{dollar}160 kD complex, which helps to stabilize the reaction center proteins until degradation and replacement of D1 can occur.; The linear dependence of the rate of photodamage on growth irradiance may imply that the probability of photodamage is a direct function of the number of charge separation reactions that occur at PSII. Alternatively, the rate of photodamage could be regulated by the redox state of the primary plastoquinone acceptor of PSII, {dollar}rm Qsb{lcub}A{rcub},{dollar} which depends on the relative rates of light absorption and forward electron transport. To investigate the effect of the redox state of Q{dollar}sb{lcub}rm A{rcub}{dollar} on PSII photodamage, the rate of electron transport and the light-harvesting capacity in Dunaliella salina were modulated by varying the amount of inorganic carbon in the culture medium. Cells were grown under high irradiance either with a limiting supply of inorganic carbon, provided by an initial concentration of 25 mM NaHCO{dollar}sb3,{dollar} or with supplemental CO{dollar}sb2{dollar} bubbled in the form of 3% CO{dollar}sb2{dollar} in air. NaHCO{dollar}sb3{dollar}-grown cells displayed low rates of photosynthesis and had a small PSII light-harvesting chlorophyll antenna size (60 Chl molecules). The half time of PSII photodamage for cells grown at high irradiance and on NaHCO{dollar}sb3{dollar} as the sole carbon source was 40 min. When switched to supplemental CO{dollar}sb2{dollar} conditions, the rate of photodamage was retarded to a t{dollar}sb{lcub}1/2{rcub} = 70{dollar} min. Conversely, CO{dollar}sb2{dollar}-supplemented cells displayed higher rates of photosynthesis and a larger PSII light-harvesting chlorophyll antenna size (500 Chl molecules). They also showed a rate of photodamage with t{dollar}sb{lcub}1/2{rcub} = 40{dollar} min. When depleted of CO{dollar}sb2,{dollar} the rate of photodamage was accelerated (t{dollar}sb{lcub}1/2{rcub} = 20{dollar} min). (Abstract shortened by UMI.)... |
