Screening And Characterization Of A Heterotrophic Denitrifying Sulfide Removal Bacterium Strain Pseudomonas Sp. X2

The known theory demonstrated that only autotrophic bacteria, e.g.Thiobacillus denitrificans could accumulate elemental sulfur （S⁰） in the course ofdenitrifying sulfide removal （DSR）. In this study, a heterotrophic strain named X2capable of simultaneous removal of acetate, nitrate and sulfide and excreteextracellular elemental sulfur （S⁰） was isolated from a well-performed expandedgranular sludge bed reactor, which had high efficiency of denitrifying sulfideremoval and overcame the slow growth of autotrophic bacteria. At the same time,the strain’s performance, metabolic pathways and key factors of simultaneousremoval of sulfide, nitrate and acetate were also studied. This work implied thatmore efficient denitrifying sulfide removal and elemental sulfur reclamation couldpossible achieved via enhancing the performance of heterotrophic and autotrophicbacteria in the system.Based on the16S rRNA sequence analysis, the isolate was closely related tothe Pseudomonas sp. at similarity of99%. The extracellular morphology of strainX2was observed by transmission electron microscopy （TEM）.Tests showed that strain X2could remove sulfide, nitrate and acetate efficiently.When the initial sulfide, nitrate and acetate concentrations were set at93.7mgS^2-/L,35mgNO₃^--N/L and54.8mgAc^--C/L, the removal rate reached96%,97%and99%respectively in13h, and insoluble elemental sulfur （S⁰） as the end product. At theend of the reaction, the generation rate of elemental sulfur was99%.The electronic transfer process of strain X2was studied during simultaneousremoval of sulfide, nitrate and acetate. The total provided number of electrons was12.86mmol/L, which was very close to the total accepted number of electrons11.7mmol/L. It was suspected that there was a synchronized metabolic pathwaybetween sulfide, nitrate and acetate. The suggested metabolic pathway for strain X2was provided based on the electron balance, in which sulfide and nitrateconverted toelemental sulfur and nitrite through autotrophic denitrification, then the generatednitrite reacted with acetate and transfered to nitrogen and carbon dioxide byheterotrophic denitrification.The strain presented to tolerate up to300mg/L of sulfide although it couldeffectively reclaim elemental sulfur under100mg/L of sulfide. Besides, sulfide tonitrate ratio (S^2-/NO₃^--N, molar ratios) affects the performance of denitrifyingsulfide removal greatly. In order to obtain an optimal S^2-/NO₃^--N molar ratios,batch tests were carried out and demonstrated that strain X2could efficientlyremove sulfide, nitrate and acetate （with removal efficiency of97%,100%and 100%respectively）, and accumulate elemental sulfur （99%） under the S^2-/NO₃^--Nratio of5/6. As for acetate to nitrate ratio （Ac^--C/NO₃^--N, molar ratios）, the removalrate of sulfide, nitrate and acetate was91%,100%and98%respectively, and thegeneration rate of elemental sulfur was81%under the Ac^--C/NO₃^--N ratio of1.26/1.