Screening and identification of a photosynthetic bacterium reducing selenite to red elemental selenium

Abstract:

Selenium is essential element for humans and animals but is very toxic at higher concentrations. In four inorganic states of selenate ［SeO^2-_４(Ⅵ)］, selenite ［SeO^2-_３(Ⅳ)］, elemental selenium ［Se^°(0)］and selenide ［Se^2-(-Ⅱ)］, selenite is well known to be more soluble and higher toxic than other three forms. Many microorganisms have the capacity to reduce selenite to red elemental selenium, which provide the potential to cope with the detoxification of pollution and to use the biological availability of red elemental selenium. Strain S3 that was more resistant to sodium selenite was selected from 20 photosynthetic bacteria preserved in laboratory. The red granule produced by S3 was identified as elemental selenium (Se^°) by transmission electron microscopy and Electron-Dispersive X-ray (EDX) analysis. The granule diameter of the red elemental selenium was 5nm～200nm, similar as the Nano-Se that has bioavailability. Morphology, physiology and photosynthetic pigments analysis results showed that strain S3 was essentially consistent with Rhodobacter azotoformans. The 16S rDNA sequence analysis (GenBank accession number DQ402051) 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.The effects of selenite on growth kinetics and the ability to resistant selenite of strain S3 were investigated. In contrast to Rhodospirillum rubrum which was reported not to reduce selenite until the end of exponential growth, strain S3 transformed selenite (1.25mmol/L) at the beginning of the growth, suggesting that strain S3 and Rs. rubrum may employ different strategies to reduce selenite. Strain S3 can grow in the presence of up to 125mmol/L sodium selenite, which is much higher than those which could be resisted to by other bacteria such as Escherichia coli (<20mmol/L) and Ralstonia metallidurans CH34 (<6mmol/L). It is firstly reported that R. azotoformans has the capacity to reduce selenite to red elemental selenium.