Cu2+对普通小球藻的光合毒性：初始藻密度的影响

为了研究Cu2+对普通小球藻的光合毒性以及初始藻密度对Cu2+光合毒性的影响,将初始密度为107mL-1的普通小球藻暴露于Cu2+的6个浓度(0、5、10、20、30和40μmol.L-1)中进行96 h的毒性暴露实验,在2、48和96 h分别利用叶绿素荧光仪(MAXI-Imaging-PAM)测定各项叶绿素荧光参数,同时,针对3个不同初始密度的普通小球藻(2×106、5×106和2×107mL-1),测定并比较了其暴露于0、20和40μmol.L-1的Cu2+12 h后,叶绿素荧光参数的变化。不同初始藻密度的毒性实验结果显示,初始藻密度为2×106mL-1时,20和40μmol.L-1Cu2+可完全抑制普通小球藻的光合作用;当初始藻密度增加到5×106和2×107mL-1时,40μmol.L-1Cu2+对普通小球藻的实际光合作用效率仅有约75%和25%的抑制。这表明初始藻密度越大,Cu2+的光合毒性越弱。但随着初始藻密度的增加,初始藻密度的变化对Cu2+光合毒性的影响减弱。初始藻密度为107mL-1时的毒性实验结果显示,暴露于20~40μmol.L-1Cu2+2 h后,普通小球藻的光合作用即受到抑制,且该抑制程度随Cu2+浓度的增加而增强,并随着暴露时间的延长有所缓解。随着Cu2+浓度的增加,最大量子产量(Fv/Fm)、实际量子产量(Yield)、相对电子传递速率(ETR)和光化学淬灭系数(qP)逐渐降低,非光化学淬灭系数(NPQ/4)逐渐上升。研究结果表明,Cu2+对普通小球藻的光合作用有一定的抑制作用,其机理可能为通过引起PSII系统反应中心的部分失活,导致PSII系统反应中心的开放比例减少,引起电子传递速率降低以及ATP和NADPH的合成减慢,从而使光合作用速率下降;初始藻密度对Cu2+的光合毒性大小有较大的影响,故在进行藻类的毒性实验时,也应关注初始藻密度的影响。

Toxicological Effect of Cu2+ on Photosynthesis of Chlorella vulgaris: Influence of Initial Cellular Density

To investigate the toxicity of Cu2+ to the initial photosynthesis of Chlorella vulgaris,Chlorella vulgaris(at the initial cellular density of 107 mL-1) were exposed to Cu2+ at the concentrations of 0,5,10,20,30 and 40 μmol·L-1,and chlorophyll fluorescence parameters were measured by MAXI-Imaging-PAM after the exposure of 2,48 and 96 h,respectively.In addition,Chlorella vulgaris with three different initial cellular densities(2×106,5×106 and 2×107 mL-1) were exposed to 0,20 and 40 μmol·L-1 Cu2+ for 12 h to study the impact of algae cellular density on the toxicity of Cu2+.The results showed that the toxicity of Cu2+ decreased with increasing initial cellular density of Chlorella vulgaris,but the effect weakened as initial cellular density became higher.The quantum yield of PSII was completely suppressed by 20 and 40 μmol·L-1 Cu2+ when the initial cellular density was 2×106 mL-1,whereas the quantum yield only decreased by 75%(25%) under 40 μmol·L-1 Cu2+ when the initial cellular density increased to 5×106 mL-1(2×107 mL-1).The algal toxicity test at initial cellular density of 107 mL-1 showed that the photosynthesis rate was inhibited by 20-40 μmol·L-1 Cu2+ after only 2 h exposure.Such reductions became stronger under higher Cu2+ concentration,but were relieved with longer exposure time.The maximal quantum yield of PSII(Fv/Fm),the actual quantum yield of PSII(Yield),electron transport rate(ETR) and photochemical quenching(qP) declined with increasing Cu2+ concentration,while non-photochemical quenching(NPQ/4) increased at the same time.In conclusion,the toxicity mechanism of Cu2+ to photosynthesis of Chlorella vulgaris was basically due to the deactivation of some PSII reaction centers,which could result in the partial closure of PSII reaction centers,then in turn leading to the deceleration of electron transfer and the synthesis of ATP and NADPH.Moreover,it is found that the toxicity of Cu2+ was dependent on the initial cellular density of Chlorella vulgaris,so the effect of initial cellular density should be taken into account when conducting the ecotoxicological studies of algae.