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Can bacteria improve air quality and prevent acid rain? Scientists believe so, and a group in Japan are engineering a new bacterium that could do the job.

Crude oil contains various organic sulfur compounds. These are released into the atmosphere during the combustion process and cause poor air quality and acid rain. It is therefore desirable to remove sulfur compounds from petroleum before it is used as a fuel.

Conventional oil refining technologies do desulfurize crude oil to some extent. Hydrodesulfurization (HDS) uses high temperature and pressure together with hydrogen and an inorganic catalyst to convert organic sulfur to hydrogen sulfide.

HDS, however, does not efficiently remove one class of sulfur compound: the thiophenes and their alkylated derivatives.

Environmental concerns have led to worldwide legislation to reduce fuel sulfur levels. Meeting these standards using conventional technology will be almost prohibitively expensive.

Deep HDS uses higher temperatures and pressures to remove thiophenes, but still cannot remove alkylated thiophenes.

Moreover, the high temperatures and pressures release vast quantities of carbon dioxide, another environmental nasty. Cheap, efficient and environmentally friendly systems are therefore required for petroleum desulfurization.

Enter biodesulfurization. It has been known for over 30 years that some bacteria can metabolize organic sulfur, turning potentially hazardous compounds into harmless inorganic forms. This has sparked considerable interest in the development of biodesulfurization.

Attention has focused on a Rhodococcus strain, IGTS8, which can metabolize dibenzothiophene (DBT) into a desulfurized carbon skeleton and inorganic sulfite.

This organism has great potential for biodesulfurization: It can grow in oil, it can also desulfurize alkylated DBT, petroleum quality is unaffected, the sulfite end product is water soluble and therefore easily removable, and the desulfurization process is cheap and produces no carbon dioxide.

The only problem with this bacterial strain is that crude oil also contains derivatives of benzothiophene (BT), which it cannot desulfurize. In 1998 a group led by Christopher Oldfield at Napier University in Edinburgh reported the first identified BT-metabolizing bacterial strain.

Last year researchers from the Japanese Petroleum Energy Center (PEC) working in Tsukuba isolated a new Rhodococcus strain, T09, which can desulfurize not only BT but also alkylated BT.

At present a mixed culture of DBT- and BT-metabolizing bacteria would be required to desulfurize both DBT and BT, but ongoing research offers improvements to the system. PEC researcher Toru Matsui is using genetic engineering to introduce the genes responsible for DBT desulfurization into strain T09 to make a new bacterium which can desulfurize both DBT and BT. The creation of such a strain would make biodesulfurization an even more practical technology.

Work is now focusing on optimizing the desulfurization rate of the new strain before large-scale trials are made. If all goes well, the biodesulfurization of petroleum will revolutionize the whole oil-refining process.