Physiological And Molecular Responses Of Microalgae To Environment Stresses

It is well known that the increased marine pollution and the dramatic changes of marine environment, so marine planktonic microalgae may be affected either in terms of population structure or biomass, followed by a further change in the food web and ecosystem. The objective of this study was to examine physiological, biochemical and molecular responses of marine microalgae to three environmental stresses (heavy metals, antibiotics and salinity).Two red tide microalgae (Skeletonema costatum and Nitzschia closterium) were exposed to Cu2+ ( 0, 0.3,0.6,1.2,2.4mg/L) , and they were cultured respectively and co-cultured to test their competition capability. When Cu2+ concentration was 0.3 mg/L, the growth of both algae was stimulated in single culture. The growth of Skeletonema costatum and Nitzschia closterium was inhibited at the concentration of 0.6,1.2,2.4mg/L, and the inhibition rates increased with Cu2+ concentration.There were no significant changes of rETRmax,αand Ik for Nitzschia closterium when exposed to Cu2+. However, there were a significant increase at 0.3mg/L and 0.6mg/L, a derease at 1.2mg/L and 2.4mg/L. In addition, the intracellular ROS of both algaes were enhanced with the increase of Cu2+ concentrations. The growth of Nitzschia closterium was better than that of Skeletonema costatum in single culture when exposed to Cu2+, but ROS levels in Nitzschia closterium lower than those in Skeletonema costatum. Data here may imply that Cu2+ was toxic to both algal species when the concentration was high (≥0.6mg/L), but Skeletonema costatum was more sensitive to Cu2+ than Nitzschia closterium. However,in the co-culture, the opposite results were achieved, and Skeletonema costatum outcompeted Nitzschia closterium. At same time, it was found that 2.4mg/L Cu2+ concentration induced growth arrest of both algaes in co-culture, but two algaes kept a high growth rate in single culture. The competition mechanism needs to further clarified.Different concentrations of antibiotics were adopted (the concentrations of Florfenicol were as follow: 0,0.5,1,2,4,8,16 mg/L, and those of Sulfamonomethoxine sodium were 0,1,4,8,12,16,20 mg/L) to expose microalgae Skeletonema costatum. The results showed that the volumes of Skeletonema costatum increased with Florfenicol exposure concentration, but no significant cell size changes were recorded in the Sulfamonomethoxine sodium exposure group. The growth of Skeletonema costatum was stimulated in the low concentration of Florfenicol (0.5 mg/L,1 mg/L,2 mg/L ), but inhibited significantly in high doses (4 mg/L,8 mg/L,16 mg/L), the inhibition rate of growth was up to 86 % at 16 mg/L. However, when exposed to Sulfamonomethoxine sodium, the growth declined at the lowest concentration (1mg/L), the inhibition became more and more seriously with the increasing doses, and the inhibition rate reached 78%. The photosynthesis of algae cells was inhibited, the content of chlorophyll decreased, integrity of cell membrane lost and intracellular ROS increased for both antibiotics exposure. Data here showed that unicellular algae antibiotics can enhance the ROS production in microalge, which may reulst in dirupting the integrity of plasm membrane, inhibiting photosynthesis, and decreasing cell viability. The 96h EC50 was 5.04mg/L for Florfenicol, but 7.03mg/L for Sulfamonomethoxine sodium, so Skeletonema costatum was more sensitive to Florfenicol than Sulfamonomethoxine sodium.With the climate changes, the salinity of estuary changed rapidly, hypotonic environment appeared frequently. Dunaliella salina cells were exposed to different salinities: 5‰, 10‰, 20‰, 30‰and 35‰for 24h. D.salina cells swelled with the decline of salinities. Compared to the control (30‰), there were 10%,10% and 20% decreases in cell volumes when cells exposed to salinity of 20‰,10‰and 5‰respectively. Significant inhibition of effective quantum efficiency of PSII and photosynthetic oxygen evolution rate were observed with the decline of salinities. At the salinity of 5‰, effective quantum efficiency of PSII and photosynthetic oxygen evolution rate descented by 16% and 52% respectively. The activity of extracellular carbonic anhydrase decreased by29%,29% and 62% respectively when algae cells exposed to 20‰, 10‰and 5‰salinities. Compared to the control (30‰), there were 47%～65% inhibition in CA gene expression when cells exposed to salinity of 5‰～20‰. CA was the significant enzyme of photosynthesis, the impact of CA by salinity changes maybe the reason of photosynthesis influence. There were a great number of ROS accumulated in cells, the relative ROS content was 2 and 1.7 folds to the control when cells exposed to salinity of 5‰and 10‰. Data showed ROS created in PSI. D.salina cells were exposed to 0, 0.25, 0.5, 1, 2.5, 5 mM H2O2 for 24h, CA activity inhibited with the H2O2 concentration increased. CA gene (P60) performed serious inhibition when cells exposed to 2.5 and 5 mM H2O2. The whole experiment showed that ROS could impact photosynthesis by CA influence. ROS not only caused oxidative damages, but were also key regulator for the responses process of D.salina to environment stresses.This thesis investigated the response of three typical microalgaes to marine environment stresses and the response pathway in common. Microalgae kept a proper survival rate in different environment stress by changing cell size, accumulating intracellular ROS, regulating enzyme synthesis and photosynthesis. The experiments could not only provide scientific references for marine pollution assessment, but also confirmed that climate and environment changes may change marine species construction and impact marine ecosystem further.