Study On Interaction Between Selenite And Purple Bacteria

Selenium is one of necessary trace elements in life activity, but various adverseecological effects can be induced if its dose is too high or too low. Nowadays themetabolism of selenium by microbes has been lucubrated, while the usage ofanoxygenic phototrophic bacteria (APB) is limited for the general lower tolerance andreduction ability to sodium selenite (Se(Ⅳ)), meantime, the systematic study of thepigment synthesis rules in APB by Se(Ⅳ) is lack. At present, even though thewholeprocesses of Se(Ⅳ) adsorption and reduction by APB is clarified, the relationshipbetween adsorption process and reduction process is still dimness. What’s more, thedata from electron microscope and energy spectrum can be taken as the evidence ofintercellular accumulated Se(0) and excreted Se(0), whereas the dynamic process ofexcretion is difficult to be observed. Thus, taken Marichromatium gracile YL28as acomparison, a Rhodobacter sphaeroides YL75strain possessing high tolerance toSe(Ⅳ) is selected to study systematically the regulation of bacterial growth andphotosynthetic pigment synthesis by Se(Ⅳ) as well as the adsorption and reductionprocesses of sodium selenite and the excretion process of Se(0).A YL75strain with higher tolerance to sodium selenite and a YL28strain withlower tolerance relatively were screened from the APB strains kept in our laboratory,using selective medium of sodium selenite Se(Ⅳ). The values of EC50for YL75andYL28were5.5mmol/L and1.0mmol/L, respectively, under light anaerobic condition.Both two strains had the ability of removing selenite, and bacterial growth andremoval rate of Se(Ⅳ) could be obviously affected by the initial concentration ofSe(Ⅳ). The total amount of photosynthetic pigments in YL75was decreased firstly,then increased and finally decreased again with the increased concentration of Se(Ⅳ),at the same time, decreased BChl a synthesis amount, increased intermediatesaccumulation and transform to downstream metabolism pathway of Car were alsoobserved. In contrast, the total amount of photosynthetic pigment in YL28wasdecreased gradually, and low concentration of Se(Ⅳ) could promote the synthesis ofRhodopin. The relative contents of various components was little change with theincreased concentration of Se(Ⅳ).The results of the interaction between the resting cell of YL75and Se(Ⅳ) showed that the removal amount of Se(Ⅳ) is positive correlated to the concentration ofbacterial suspension(OD660), and the most removal amount was achieved when thevalue of OD660was more than4.0and pH was7.0. The removal amount of Se(Ⅳ) byYL75was increased with the increased concentration of Se(Ⅳ), while it wasdecreased at0.7mmol/L of Se(Ⅳ). The competition of PO43-and Se(Ⅳ) suggestedthat Se(Ⅳ) might enter the cell through the channel of phosphate. Adsorption ofSe(Ⅳ) was mainly occurred at0℃in YL75, and the adsorption amount was0.10mg/g. Intercellular reduction of Se(Ⅳ) was mainly occurred at25℃, and theadsorption reduction amount was2.61mg/g. Thus the reduction amount of Se(Ⅳ)was about2.51mg/g. Similarly, the reduction amount of Se(Ⅳ) in YL28was0.81mg/g. The results of adsorption dynamics and adsorption thermodynamics showedthat the monomolecular layer adsorption was dominated in the process of Se(Ⅳ)adsorption.The dynamic excretion process of intercellular Se(0) was observed by differentialspectrophotometry. The excretion of intercellular Se(0) was preliminarily judged bythe increased turbidity of the reaction system of resting cells and Se(Ⅳ) in YL28andYL75. The data from microscopy showed that some dense particulate matters wereexisted in cell. The dead resting cells of YL75could not be seen during the reaction,while the partial death could be found in YL28. The results suggested that reductiveSe(0) was excreted out of cell by the resting cells of YL75, whereas intercellularreductive Se(0) was released to extracellular by fractured resting cells of YL28.In conclusion, YL75had high tolerance and reduction ability of Se(Ⅳ). Firstly, thevariation rules of intercellular accumulated photosynthetic pigments in two strainscaused by Se(Ⅳ) were clarified, at the same time, the adsorption and intercellularreduction rules of Se(Ⅳ) in two APB strains were also clarified. Then the dynamicexcretion process of intercellular Se(0) was observed with the simple differentialspectrophotometry. This method applied experimental thinking for monitoring thedynamic excretion process of Se(0), but the general applicability needed a furtherstudy.