Study On Mercury Tolerance And Mercury Reduction Mechanism Of Two Newly Isolated Mercury Resistant Bacteria Strains

As a result of urbanization,poverty and low standard of living,the tendency for humans to improve their way of life has led to an increase in the creation of chemical,metallic and petroleum industries globally.These industries through their operational processes generate a lot of toxic waste and these wastes most of the time are not treated and are discharged directly into water bodies,land and air thereby polluting the environment.These wastes are usually toxic heavy metals that are not easily biodegradable and can persist in the environment for a long period with adverse health effects.One such toxic heavy metal is mercury which is known to have a density of 13.53 g/cm³ and exhibits characteristics of non-biodegradability.Various remediation methods have been used to combat this toxic heavy metal but their disadvantages far outweigh the advantages except for only one method which involves the use of microorganisms such as bacteria to reduce the toxic heavy metal such that they are now harmless in the environment.This method is gaining a lot of attention globally because it's a very cheap,environmentally friendly,non-generation of secondary pollutants and can be used in any place to remediate the environment.In this research,water samples were collected from a sewage outlet in Xigu district in Lanzhou City.High mercury resistant bacterial strains were screened and isolated and experiments based on their tolerance to mercury,conversion rate,and optimum conditions in terms of concentration,pH and temperature,the study of their anti-mercury mechanism and the verification of their mercury tolerance by applying the strains on two indicator organisms,Chlorella vulgaris,and Broad bean plant were carried out.The results of the research are:?1?Two highly mercury tolerant strains were isolated and can grow on a solid medium of 120 mg/L of Hgcl2.The two strains were tagged H6 and H9 and using 16srRNA gene sequence analysis for identification,H6 had 96%similarity with Stenotrophomonas terrae strain R-32768 and H9 had 100%similarity with Stenotrophomonas rhizophila strain ICE234.Therefore both strains were identified as Stenotrophomonas terrae and Stenotrophomonas rhizophila and both strains had a mercury conversion rate of 64%and 46.4%respectively after 24 h.Their conversion rate was lower than what was reported in the literature.?2?Both strains had the same mercury tolerance concentration in the liquid medium as60 mg/L.The pH required for both strains for their optimum growth was 7 and 8 at an optimum temperature of 25?.This implies that a neutral and alkaline environment is best suited for their growth.?3?The results of real-time quantitative PCR?Q-PCR?showed that the reduction process of mercury was completed under the control of mer operon.merR,as the promoter gene of mer operon,initiates the expression of the whole operon.merT and merC are involved in the migration of mercury into bacteria,merA is responsible for reducing Hg²⁺to Hg⁰.In H6 strain,merT and merC genes began to transport Hg²⁺at 6 h and 8h respectively.In H9 strain,merT and merC genes were expressed at 8 h at the same time,and then the expression of merA gene was the highest at 12 h.?4?In the medium containing mercury,Chlorella vulgaris and Vicia faba were used to verify the mercury reduction rate of the two strains.The results showed that the two strains could reverse the growth and decline of Chlorella vulgaris in the medium containing mercury,and both of them could effectively reduce the micronucleus rate of Vicia faba cells,indicating that the two strains did have certain reduction ability to mercury,and could reduce the toxicity of Hg²⁺to the environment.