An Alaskan feast for oil-eating microbes.

The Exxon Valdez oil spill, which fouled Alaskan waters on March 24, 1989, provided the first major test of whether hydrocarbon-eating aquatic microbes can help clean oilstained beaches. Ten weeks after the spill, Environmental Protection Agency scientists began seeding small patches of blackened shore with fertilizers containing nitrogen and phosphorus to spur the growth of indigenous, oil-noshing bacteria (SN: 6/17/89, p.383). After initial signs that the fertilizer accelerated microbial breakdown of beached crude oil (SN: 7/15/89, p.38), regulators approved expansion of the test.

Between Aug. 1 and Sept. 15, 1989, cleanup crews applied either liquid fertilizers or granular, slow-release fertilizers at 750 sites along more than 74 miles of beaches in Prince William Sound and the Gulf of Alaska. On heavily oiled beaches, crews first rinsed the shores with pressurized water. Over the next two years, these and other, less heavily oiled shores received some 1,600 additional applications of fertilizer.

Although treated beaches whitened more quickly than untreated ones, suggesting that microbial cleaning occurred faster there, researchers were unable to determine exactly how much oil had disappeared. Complicating the problem was the fact that all beaches experienced changes in oil composition, including natural weathering (loss of volatile constituents) and some microbial breakdown. Differences in the initial amount of oil present and the type of beach also made the standard technique of comparing weights of treated and untreated beach material all but useless here. Finally, researchers could not tell how well the bacteria had degraded the more complex and potentially toxic compounds within crude oil, such as polycyclic aromatic hydrocarbons (PAHs).

James R. Bragg, a chemical engineer with Exxon Production Research Co. in Houston, says he and his co-workers ultimately solved that problem by comparing levels of hopane - a petroleum hydrocarbon ignored by microbes and unaffected by weathering -- to compounds that microbes do eat. The results for one treated beach showed that bacteria degraded about 60 percent of total hydrocarbons and 45 percent of the PAHs within three months, Bragg says. Overall, he reports, in south central Alaska, oil degraded up to five times faster and PAH levels dropped five times faster on beaches stimulated with sufficient fertilizer than on beaches left alone.