Effects Of A Symbiotic Bacterium On The Accumulation And Transformation Of Arsenate By Chlorella Salina

Microalgae and bacteria are usually symbiotic in the environment.Algae-bacteria consortia systems may be potentially applied in wastewater treatment and environment remediation.Previous research has showed that the algae-bacteria consortia have a strong removal ability of As(V)in the solution.However,there are still different results abouteffects of symbiotic bacteria on uptake and metabolism of As by microalgae,suggestingthat specific mechanisms about effects of symbiotic bacteria on accumulation and transformation of As by microalgae still need further studies.Therefore,Chlorella salina was used as the tested material in the experiment.First of all,the symbiotic bacterium of C.salina was confirmed to be Halomonas sp.after isolation,cultivation and 16S rRNA identification.Then antibiotics were added to C.salina culture to remove the symbionts from C.salina culture and axenic C.salina culture was obtained.We set up different concentrations and exposure times of arsenate[As(V)]treatment.The uptake,adsorption and transformation of As by the axenic and non-axenic C.salina and transformation of As(V)by the bacterium was determined.Effects of a symbiotic bacterium on the accumulation and transformation of As by C.salina was analyzed.The main results are summarized as follows:1.The symbiotic bacterium isolation,identification and transformation of As(Ⅴ)A symbiotic bacterium was isolated,purified from C.salina culture by streak plate method and it was confirmed as Halomonas sp.by 16S rRNA sequence identification.The symbiotic bacterium was exposed to 0～750 μg·L-1 As(Ⅴ)and arsenic speciation and concentrations in the cultures of the symbiotic bacterium Halomonas sp.Csl after 7-d treatment.The results showed that arsenite[As(Ⅲ)]dominated the As speciation in the bacterial culture and the percentage of As(Ⅴ)was 7.59%-26.80%.Besides,no DMA were detected during the 7 days incubation.This indicated that the symbiotic bacterium has a strong As(Ⅴ)reducing ability.2.Effect of As(Ⅴ)treatment concentrations on the accumulation and transformation of As by axenic and non-axenic C.salinaWe used batch cultures to determine the absorption,adsorption and transformation of As by axenic and non-axenic C.salina exposed to different concentrations of As(Ⅴ)(75,50,300,750 μg·L-1)for 7 d.The results indicated that biomass of non-axenic C.salina was significantly higher than that of axenic C.salina exposed to 150-750μg·L-1As(Ⅴ).Compared with axenic C.salina,the accumulation content of As by non-axenic C.salina was significantly higher exposed to the same concentration of As(Ⅴ).ANOVA showed that the adsorption As content of non-axenic C.salina was significantly higher than axenic C.salina and the absorption As content of non-axenic C.salina was significantly lower than axenic C.salina with the same As(Ⅴ)treatment.The symbiosis of C.salina and a bacterium markedly increased the adsorption of As,but it markedly reduced the uptake and the toxic effect of As(Ⅴ).Arsenate was the major arsenic species in the cells of axenic and non-axenic C.salina.The proportion of As(Ⅲ)was 8.99%-11.52%in the axenic microalga whereas a small quantity of monomethylarsonous acid(MMA)and dimethylarsinous acid(DMA)(0.02%-0.04%)were detected in the non-axenic counterpart.The As removal rate(19.81%-41.08%)by non-axenic C.salina was higher than the bacterium alone(5.14%-14.62%)and axenic C.salina(14.98%-21.08%)after 7-d As(Ⅴ)treatment.3.Effect of As(Ⅴ)treatment time on the accumulation and transformation of As by axenic and non-axenic C.salinaBased on the foregoing results,we selected 300 μg·L-1As(Ⅴ)treatment and determined the uptake,adsorption and transformation of As by axenic and non-axenic C.salina at various time points of incubation.The results indicated the biomass of non-axenic C.salina was significantly higher than that of axenic C.salina under the same treatment time.The intracellular As content of non-axenic C.salina was significantly higher than that of axenic C.salina within 7 days treatment.During the 3-d As(Ⅴ)treatment period,the adsorption As content of non-axenic C.salina was significantly higher than axenic C.salina.However,the results were reversed from the 5th day to 7th day.This demonstrated the symbiotic bacterium had an influence on the uptake and adsorption of As by C.salina.As(Ⅴ)was the major arsenic species in the cells of axenic and non-axenic C.salina.Intracellular proportion of As(Ⅲ)was 1.59%～14.21%in the axenic microalga and the proportion(5.09%～27.07%)of As(Ⅲ)in the non-axenic counterpart was higher.A small quantity of DMA were detected in the non-axenic algal cells from the 3rd day and the proportion was 0.02%-0.04%,indicating that the presence of the bacterium had an effect on the transformation of As(Ⅴ)by C.salina.In addition,non-axenic C.salina can remove 96.23%of arsenic in the solution when cultured on the 4th day.The removal rate was much higher than that of axenic C.salina,showing that algae-bacteria consortia may enhance the removal rate of As in the solution.In summary,the symbiotic bacterium had an influence on the uptake,adsorption and transformation of As and symbiosis of C.salina and the bacterium could enhance the bioremediation of As contaminated water.