Photosynthetic Bacteria Selenium Transformation

Selenium has been shown to be an essential micronutrient for mammals, birds, and several bacteria, but it can be highly toxic depending on its concentration and speciation. Its essential role as a nutrient is due to the association with the active site of the enzyme glutathione peroxidase, and evidences suggest that low selenium status is linked to an increased risk of Chronic Keshan disease. Considering the toxicity and bioavailability of various forms of selenium, it is a reasonable pathway to obtain active Se-containing substance using methods of biotransformation. Phototrophic microorganisms belonging to the group containing the purple bacteria are of great nutritious value, and have been shown to be particularly resistant to a variety of metal and transition metal oxyanions, including selenium. It provides the potential to cope with the absence of selenium and to produce the nutritious food rich in selemum.The main research aspects and results are as following:First, using sodium selenite as inorganic selenium source, stain YJ-7 and S3 that were more resistant to sodium selenite were selected from 20 photosynthetic bacteria preserved in laboratory. Then, cultured in fermentation media with sodium selenite in different concentrations, strain S3 growing well and transforming more selenite was isolated by measuring the cell concentrations and selenium-transforming activity. It was demonstrated that strain S3 can transform 83.4% of 0.2g/L selenite, and its cell concentration was higher than 5×10⁸ cells/ml. Morphology, physiology and photosynthetic pigments analysis results showed that strain S3 was essentially consistent with Rhodobacter azotoformans. The 16S rDNA sequence analysis suggested that strain S3 was clustered together with R. azotoformans in phylogenetic tree, with the sequence identity of 99%. Based on all the results of taxonomy, strain S3 was identified as R. azotoformans S3. Its GenBank accession number is DQ402051.Second, various mechanisms have been proposed to explain the biological dissimilatory of selenite, but few papers have discussed the effect of selenium on the Rhodobacter azotoformans, and little attention has been paid to selenium's role on the growth and yield of Rhodobacter azotoformans. The effect of selenite on growth kinetics, the varieties of cells and the main compositions of the cell were investigated by using R. azotoformans S3. Photosynthetic cultures were able to completely reduce as much as 0.12g/L selenite, and can endure up to 10g/L selenite. The presence of selenite in the culture medium strongly affected cell division. In the presence of a selenite concentration of 1.0g/L cultures reached final cell densities that were only about 18% of the control final cell density. Elemental selenium expelled by was demonstrated through electron microscopy and EDX analysis. Contents of many amino acids such as Glu, Cys, Leu, Lys were reduced in selenite existing, and some such as Tyr was increase. Influenced by selenite, the content of pigment changed obviously.The medium compositions and fermentation conditions were optimized by single factor experiments. Through the measure of cell growth and selenite transformation, the optimal medium included 0.4% sodium lactate, 0.75% yeast extract, and the initial concentration of sodium selenite was 1.0g/L. The transform-content of selenite was above 8 times (0.95g/L) after optimization than before, and the efficiency was up to 95%.The main goal of our study on selenite transformation was to produce photosynthetic bacteria rich in bioavailable forms of selenium. First, we studied the acute toxicity of the selenium existing in photosynthetic bacteria. Its acute toxicity LD₅₀ in mice was 86(59.0～125.2) mg/kg-bw Se, which was much better than selenite (LD₅₀:15.72(13.38～18.47) mg/kg-bw Se) and organic selenium(LD₅₀:30～50mg/kg·bw Se), and a little higher than Nano-Se(LD₅₀:112.98(89.95～141.90)mg/kg·bw Se). Next, we would make further research on the bioavailability of the photosynthetic bacterial selenium.