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Recently, the search for biological nitrogen removal method from wastewaters and exhaust air has come up with several promising methods; however, most of them just took advantage of some special bacteria combined with industrial procedures. On the contrary, our iGEM project aim to reduce nitrogen oxides and oxidize sulfide compounds at the same time.

During the processes of denitrification, sulfide compounds and nitrate act as electron donor and acceptor respectively. This reaction is also known as sulfide-driven denitrification. Researchers have reported that E. coli can perform such reaction when expresses sqr gene from R. capsulatus. Herein, we enable certain type of cyanobacteria to take advantage of sulfide and reduce nitrogen oxide compounds into nitrogen. The BLAST result shows that sqr genes are homolog in R. capsulatus and Synechocystis sp. PCC 6803. For denitrification, we plan to get access to Thiobacillus denitrificans, the well-known chemolithotrophic organisms. Nevertheless, we later found it difficult to obtain the specific strain we need. According to NCBI database, enzymes for denitrification such as nir, nor, nos share great similarity between Thiobacillus denitrificans and Pseudomonas aeruginosa PAO1, so we adopted P. aeruginosa PAO1 instead and expressed the enzymes mentioned above in Synechocystis sp. PCC 6803.

These denitrifying enzymes are functional under aerobic condition, yet like all cyanobacteria, Synechocystis sp. PCC 6803 produces oxygen during photosynthesis. Fortunately, when sulfide presents in the environment and sqr is expressed, it will cease producing oxygen and use sulfide as an electron donor for carbon dioxide photoassimilation. Together with dsrI and dsrII enzymes from Desulfovibrio desulfuricans, our engineered organisms are capable of reducing three major oxides pollution – nitrogen, sulfur and carbon oxides.