| With the strengthening of human activities and the quickly development of industry and agriculture,the problem of cadmium(Cd)pollution has become increasingly serious.Heavy metal of Cd input to the aquatic environment is through discharge of agricultural and industrial waste,surface run-off and atmospheric precipitation,and lead to the continuously increase of Cd concentration in natural water bodies.Cd is one of the most toxic heavy metals,once it goes into the aquatic environment,not only can not be degraded by plankton,but also can be enrich in aquatic organisms and produce toxic effects.The ecological negative effects of Cd pollution has been paid widespread attention.So far,the study on the toxic effects of Cd on algae mainly focus on the photosynthetic inhibitory mechanism.However,the combination effects of environmental factors and Cd toxicity in cyanobacteria were igonored,and Cd transporters in cyanobacteria have not been identified.Light is necessary for the growth of photosynthetic organisms,and light intensity in the aquatic environment is affected by the depth of water,the disturbance of water flow,the biomass of plankton and so on.Our experiment investigated the effects of different light intensities on the Cd toxicity to Synechocystis PCC 6803,which simulated the phytoplankton grow under different water depth with different light intensities,and suffered from different light intensities through water flow disturbance or itself buoyancy regulation.On the basis of this idea,three light intensities were set up to explore the Cd sensitivity of Synechocystis PCC 6803 under different light intensities.Meanwhile,toxic effects of Cd to Synechocystis PCC 6803 under different light intensities,which were prior to adapt to grow under different light intensities were investigated.This study can reveal the effect of light intensity on the Cd toxicity in cyanobacteria,and provide a theoretical basis for explaining the ecological toxicological effects of Cd in aquatic systems.In order to compare Cd toxic effects to Synechocystis PCC 6803 under different light intensities,photosynthetic physiologic parameters and the Cd content of cells exposured to Cd for 5h under the different light intensities were investigated.The results showed that PS? activity,photosynthetic electron transport rate of whole chain,and the energy transfer efficiency of PS? center were significantly reduced with Cd treatment.Upon Cd treatment,cross-section of PS? was increased due to the inactivation of PSII reaction center,while the content of lightning antenna pigments were significantly decreased,indicating that PS? reaction center was more sensitive to Cd toxicity than lightning antenna pigments.NPQ as a PS? protective mechanism has been declined with gradient concentrations of Cd treatment,and the rate of decline were increasing with increase of light intensity,that indicated Cd treatment under high light intensity exacerbated the protective strategy of PS? and accelerates PS? damage.Pigment analysis showed that chlorophyll and carotenoid were insensitive to short-term Cd treatment.In contrast to the inhibitory effect of Cd on PS? activity,PSI activity was less inhibited by Cd treatment.Meanwhile,increasing cyclic electron transport around PSI also compensates the inhibition of PSI activity to a certain extent.Intracellular Cd content measurements showed that Cd accumulation in cells were significantly enhanced with increasing of light intensity,suggesting that the light intensity could promote Cd to enter cell and increase photosynthetic system damage.Synechocystis PCC 6803 that were prior to adapt to different growth light intensities and exposed to Cd treatment under different light intensities were also showed that high treated light enhance Cd toxic effects.To sum up,the results showed that the effects of Cd toxicity on cyanobacteria were highly coupled with light intensity,and high light promote Cd accumulation and accelerate the damage of photosynthetic apparatus.This study can provide a theoretical basis for understanding the ecological toxicological effects of Cd in natural water environment.Many studies have shown that Cd can be introduced into cells by bivalent cation transporters.The pathway of Cd transport to algae cells is little known,and Cd specific transporters in cyanobacteria have not been identified.It is most likely that Cd could transport into cells through other divalent metal transporter.On this basis,15 kinds of metal ion transport knocked-out mutants of Synechocystis PCC 6803 were chosen to explore the Cd transporter by comparing Cd toxic effects with chlorophyll fluorescence analysis.It showed that slr1392 knocked-out mutant was some higher resistant to Cd toxicity compared to wild-type Synechocystis,which suggested ferrous ion transporter possibly also involved in Cd transport in Synechocystis.This study will lay a foundation for revealing the Cd transport mechanism in cyanobacteria. |
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