Expression of mercuric reductase from Bacillus megaterium MB1 in eukaryotic microalga Chlorella sp. DT: an approach for mercury phytoremediation

A eukaryotic microalga, Chlorella sp. DT, was transformed with the Bacillus megaterium strain MB1 merA gene, encoding mercuric reductase (MerA), which mediates the reduction of Hg²⁺ to volatile elemental Hg⁰. The transformed Chlorella cells were selected first by hygromycin B and then by HgCl₂. The existence of merA gene in the genomic DNA of transgenic strains was shown by polymerase chain reaction amplification, while the stable integration of merA into genomic DNA of transgenic strains was confirmed by Southern blot analysis. The ability to remove Hg²⁺ in merA transgenic strains was higher than that in the wild type. The merA transgenic strains showed higher growth rate and photosynthetic activity than the wild type did in the presence of a toxic concentration of Hg²⁺. Cultured with Hg²⁺, the expression level of superoxide dismutase in transgenic strains was lower than that in the wild type, suggesting that the transgenic strains faced a lower level of oxidative stress. All the results indicated that merA gene was successfully integrated into the genome of transgenic strains and functionally expressed to promote the removal of Hg²⁺.