Effects Of Cd²⁺ Exposure On The Chlorophyll Fluorescence And Growth Of Microalgae

1 Effects of Cd²⁺ exposure on the growth and chlorophyll fluorescence of 7 microalgal strainsThe effects of different concentrations of heavy metal ions(Cd²⁺) and different time(24h, 48h, 72h and 96h) on the chlorophyll fluorescence parameters, relative chlorophyll content and cell density of 7 microalgal strains, Pavlova viridis, Isochrysis galbana (Tahitian), Chlorella sp., Nannochloris oculata, Haematococus pluvialis, Pyramidomonas sp. and Dunaliella salina were investigated. The algae were obtained from the Microalgae Culture Center (MACC), Ocean university of China. The f/2 in natural sea water was controlled under 25±1℃, the salinity was adjusted to 28 by using distilled water, and the light intensity was 100μmol·m^-2·s^-1. The main parameters determined in the present study including the maximal photochemical efficiency of PSII (F_v/F_m), the potential activity of PSII (F_v/F_o), the actual photochemical efficiency of PSII in the light (Yield), the relative electron transport rate (rETR), the photochemical quenching (qP) and the non-photochemical quenching (NPQ). One-way analysis of variance showed that different Cd²⁺ concentrations had significant effects on the fluorescence parameters and growth of the 7 strains of microalgae (P<0.05). The parameters F_v/F_m, F_v/F_o, Yield and rETR decreased in 7 microalgal strains under heavy metal (Cd²⁺) stress. The changed degree of the above parameters increased with the increasing of heavy metal concentration and the increasing exposure time of the 7 microalgal strains. qP decreased except in H. pluvialis under Cd²⁺ stress, NPQ first increased then decreased in P. viridis and D. salina, NPQ increased in I. galbana (Tahitian) with the increasing exposure time, NPQ decreased in the other 4 microalgal strains. Cd²⁺ exposure also caused the decrease of the relative chlorophyll content and cell density of 7 microalgal strains. The corrlation analysis results showed that there were significant negative correlations between the fluorescence parameters (F_v/F_m, F_v/F_o, Yield and rETR), the relative chlorophyll content and cell density of 7 microalgal strains with Cd²⁺ concentrations, the changed degree of the above parameters increased with the increasing Cd²⁺ concentration. The EC50 (the effective concentration causing a 50% inhibition) values was related to the microalgal species. It was calculated as follows: P. viridis (131.278-335.566μmol/L), I. galbana (Tahitian) (9.126-78.831μmol/L), Chlorella sp. (19.048-78.629μmol/L), N. oculata (7.726-16.849μmol/L), H. pluvialis (14.424-29.508μmol/L), Pyramidomonas sp. (12.697-692.628μmol/L), D. salina (24.595-918.333μmol/L), respectively. Therefore, comparison of the EC50 values based on the cell density variation among the 7 microalgal species indicated the order of their tolerance to Cd²⁺ was D. salina > Pyramidomonas sp. > P. viridis > Chlorella sp. > I. galbana (Tahitian) > H. pluvialis > N. oculata.2 Effects of Cd²⁺ and temperature interaction on the growth and chlorophyll fluorescence of 3 microalgal strainsThe effects of different Cd²⁺ concentrations (Chlorella sp.: 0, 20, 40μmol/L; N. oculata: 0, 10, 30μmol/L; D. salina: 0, 70, 140μmol/L) and different temperature (Chlorella sp. and N. oculata: 20, 25, 30℃; D. salina: 15, 25, 35℃) interaction on the chlorophyll fluorescence parameters(F_v/F_m, F_v/F_o, Yield, rETR, qP and NPQ), relative chlorophyll content and cell density of 3 microalgal strains were reported. The 3 strains of microalgae (Chlorella sp., N. oculata and D. salina) were obtained from the Microalgae Culture Center (MACC), Ocean university of China. The f/2 in natural sea water was controlled the salinity at 28 by using distilled water, and the light intensity was 100μmol·m^-2·s^-1. The results showed that the chlorophyll fluorescence parameters, relative chlorophyll content and cell density decreased obviously in 3 microalgal strains with the increasing of heavy metal (Cd²⁺) concentration. F_v/F_m and F_v/F_o in N. oculata were almost the same current between the control and Cd²⁺ concentration (l0μmol/L), it was possible that low Cd²⁺ concentration could stimulate algae's growth and synthetical mechanism in a certain extent. The corrlation analysis results showed that there were very significant negative correlations between the fluorescence parameters (F_v/F_m, F_v/F_o, Yield and rETR), the relative chlorophyll content and cell density of Chlorella sp. and N. oculata with Cd²⁺ concentrations. The relative chlorophyll content and cell density of D. salina were significant negative correlations with Cd²⁺ concentrations after 48h, the changed degree of the above parameters increased with the increasing Cd²⁺ concentration. Multiple comparison tests showed that the fastness of Chlorella sp. was better under 25℃than 20℃and 30℃, moreover, relative chlorophyll content and cell density were the highest in the three temperature. Compared 20℃and 25℃with 30℃, the sensitivity of N. oculata was greatest of all, the chlorophyll fluorescence parameters, relative chlorophyll content and cell density declined markedly. D. salina adapted well to 25℃, and the parameters decreased under over high or low temperature, it indicated that unconformable temperature could enhance heavy metal (Cd²⁺) stress. The result indicated that electron transport was restrained under over high or low temperature, and prohibitive degree in low temperature was lighter than high temperature.3 Effects of Cd²⁺ and different N concentrations interaction on the growth and chlorophyll fluorescence of 3 microalgal strainsThe effects of different Cd²⁺ concentrations (Chlorella sp.: 0, 20, 40μmol/L; N. oculata: 0, 10, 30μmol/L; D. salina: 0, 70, 140μmol/L) and different N concentrations (55, 880, 7040μmol/L) interaction on the chlorophyll fluorescence parameters (F_v/F_m, F_v/F_o, Yield, rETR, qP and NPQ), relative chlorophyll content and cell density of 3 microalgal strains were investigated. The 3 strains of microalgae (Chlorella sp., N. oculata and D. salina) were obtained from the Microalgae Culture Center (MACC), Ocean university of China. The culture was controlled under 25±1℃, the salinity at 28 by using distilled water, and the light intensity was 100μmol·m^-2·s^-1. The result indicated that Cd²⁺ concentrations and N concentrations had significant effects on the photosynthesis and growth of 3 microalgal strains (P<0.05). The fluorescence parameters (F_v/F_m, F_v/F_o, Yield and rETR) of 3 microalgal strains were significant negative correlations with Cd²⁺ concentrations. Cell density of 3 microalgal strains was significant negative correlations with Cd²⁺ concentrations. And the parameters F_v/F_m, F_v/F_o, Yield and rETR decreased in 3 microalgal strains under low N concentration (55μmol/L), and it also caused the decrease of the cell density and relative chlorophyll content of 3 microalgal strains. The chlorophyll fluorescence parameters, relative chlorophyll content and cell density of the control in Chlorella sp. reached the highest values at 7040μmol/L, and the effect of high Cd²⁺ concentration was strengthened at low and high N concentrations (55, 7040μmol/L). The fluorescence parameters of D. salina decreased at 7040μmol/L. It indicated that high N concentration could reduce toxicity under Cd²⁺ stress. The fastness of N. oculata was better at 880μmol/L, and the relative chlorophyll content and cell density got to the greatest values in N. oculata.4 Effects of Cd²⁺ and different P concentrations interaction on the growth and chlorophyll fluorescence of 3 microalgal strainsThe effects of different Cd²⁺ concentrations (Chlorella sp.: 0, 20, 40μmol/L; N. oculata: 0, 10, 30μmol/L; D. salina: 0, 70, 140μmol/L) and different P concentrations (2.27, 36.3, 290.4μmol/L) interaction on the chlorophyll fluorescence parameters (F_v/F_m, F_v/F_o, Yield, rETR, qP and NPQ), relative chlorophyll content and cell density of 3 microalgal strains were reported. The three strains of microalgae (Chlorella sp., N. oculata and D. salina) were obtained from the Microalgae Culture Center (MACC), Ocean university of China. The culture was controlled under 25±1℃, the salinity at 28 by using distilled water, and the light intensity was 100μmol·m^-2·s^-1. The corrlation analysis results showed that Cd²⁺ concentrations and N concentrations had significant effects on the photosynthesis and growth of 3 microalgal strains (P<0.05). The fluorescence parameters (F_v/F_m, F_v/F_o, Yield, rETR and NPQ) of Chlorella sp. and N. oculata were significant negative correlations with Cd²⁺ concentrations. F_v/F_m, F_v/F_o and NPQ of D. salina were significant negative correlations with Cd²⁺ concentrations, Yield and rETR were significant negative correlations with Cd²⁺ concentrations after 24h. The result indicated that F_v/F_m, F_v/F_o, Yield and rETR of 3 microalgal strains (Chlorella sp., N. oculata and D. salina) all decreased obviously in low P concentration (2.27μmol/L), and the chlorophyll fluorescence parameters, relative chlorophyll content and cell density were lower than the values in P concentrations (36.3, 290.4μmol/L), it reflected distinct limiting phenomenon. Multiple comparison tests showed that the optimal phosphorus concentration was 290.4μmol/L, the chlorophyll fluorescence parameters, relative chlorophyll content and cell density of 3 microalgal strains also reached the highest values at high P concentration (290.4μmol/L).