Interactions among buffelgrass, phenanthrene and phenanthrene-degrading bacteria in gnotobiotic microcosms.

An experiment was undertaken in gnotobiotic microcosms to determine the role of buffelgrass (Cenchrus ciliaris) and a phenanthrene-degrading bacterium (strain PM600) in the degradation of phenanthrene. The Gram-negative bacterium was identified as a Sphingomonas sp. by 16S rRNA gene sequence analysis and as S. paucimobilis by biochemical tests (API 20 NE strips). Its yellow pigment corresponded to nostoxanthin and its cellular fatty acids were typical of the genus Sphingomonas. Moreover, it was devoid of lipopolysaccharides. Strain PM600 was tested for growth on mineral medium supplemented with No. 2 diesel, hexadecane, mineral oil, pristane, phenanthrene, and pyrene as single carbon sources. It was capable of utilizing phenanthrene only. In the gnotobiotic microcosms silica sand was either or not supplemented with 150 mg of phenanthrene kg(-1) sand, inoculated with strain PM600, and planted to sterile young seedlings of buffelgrass. After 28 days, 67% of the reduction of the phenanthrene concentration was assigned to degradation by the bacterium and ca. 20% to abiotic factors. No statistically significant effect of the young buffelgrass was found. In the absence of phenanthrene, the bacterial population significantly increased in the rhizosphere of buffelgrass. However, in the presence of buffelgrass and phenanthrene, the bacterial population preferentially responded to phenanthrene. The growth of buffelgrass was severely curtailed by phenanthrene in the absence of the bacterium. However, strain PM600 effectively protected buffelgrass against the phytotoxicity of phenanthrene.

Evaluation of agriculture-based phytoremediation in Pacific island ecosystems using trisector planters.

It is difficult to directly evaluate the efficacy of phytoremediation of petroleum hydrocarbon contaminants embedded in deep soil layers, especially if the contaminants are of relatively low concentration and are unevenly distributed. This report describes the greenhouse and laboratory experiments carried out to evaluate a field demonstration project. A trisector planter was designed to simulate field conditions, including soil profiles and field management of the trees selected. The third or bottom section of the planter was spiked with known quantities of 6 diesel-fuel components and the reduction of their concentrations was monitored after 200 days under the influence of the plant root systems. Results are statistically compared; among the three tree species used, milo (Thespesia populnea) and kou (Cordia subcordata) are more effective than false sandalwood (Myoporum sandwicense) in reducing the concentration of the spiked contaminant. Enumerations of populations of hydrocarbon-degrading microoorganisms in the bottom section suggest that biodegradation may be affected by the response of microorganisms to both the "close rhizosphere" (soil within 1 mm of the root) and the "expanded rhizosphere" (soil in the bottom section after root removal). Root exudates leached from the upper sections could be responsible for the expanded rhizosphere effect in the bottom section.