Extremely high levels of paramagnetic manganese (MnZ') which quench phosphorescent reactions have been found to inhibit the formation of thymine-containing dimers in M . radiodurans. Lowering the concentration of Mn2+ in the culture medium resulted in a lower intracellular concentration of Mn2+, an increase in the UV-sensitivity of this bacterium, and a larger photochemical yield of thymine-containing dimers. High levels of paramagnetic Mn2+ were not found in other test organisms which are more sensitive to UV-irradiation. One interpretation of our data is that in Micrococcus radiodurans Mn2+ may bind to the chromosome and thereby reduce the photochemical yield of thymine-containing dimers.

Low pollutant substrate bioavailability limits hydrocarbon biodegradation in soils. Bacterially produced surface-active compounds, such as rhamnolipid biosurfactant and the PA bioemulsifying protein produced by Pseudomonas aeruginosa, can improve bioavailability and biodegradation in liquid culture, but their production and roles in soils are unknown. In this study, we asked if the genes for surface-active compounds are expressed in unsaturated porous media contaminated with hexadecane. Furthermore, if expression does occur, is biodegradation enhanced? To detect expression of genes for surface-active compounds, we fused the gfp reporter gene either to the promoter region of pra, which encodes for the emulsifying PA protein, or to the promoter of the transcriptional activator rhlR. We assessed green fluorescent protein (GFP) production conferred by these gene fusions in P. aeruginosa PG201. GFP was produced in sand culture, indicating that the rhlR and pra genes are both transcribed in unsaturated porous media. Confocal laser scanning microscopy of liquid drops revealed that gfp expression was localized at the hexadecane-water interface. Wild-type PG201 and its mutants that are deficient in either PA protein, rhamnolipid synthesis, or both were studied to determine if the genetic potential to make surface-active compounds confers an advantage to P. aeruginosa biodegrading hexadecane in sand. Hexadecane depletion rates and carbon utilization efficiency in sand culture were the same for wild-type and mutant strains, i.e., whether PG201 was proficient or deficient in surfactant or emulsifier production. Environmental scanning electron microscopy revealed that colonization of sand grains was sparse, with cells in small monolayer clusters instead of multilayered biofilms. Our findings suggest that P. aeruginosa likely produces surface-active compounds in sand culture. However, the ability to produce surface-active compounds did not enhance biodegradation in sand culture because well-distributed cells and well-distributed hexadecane favored direct contact to hexadecane for most cells. In contrast, surface-active compounds enable bacteria in liquid culture to adhere to the hexadecane-water interface when they otherwise would not, and thus production of surface-active compounds is an advantage for hexadecane biodegradation in well-dispersed liquid systems.Natural biological attenuation is the present strategy for remediating greater than 25% of soil sites contaminated with petroleum hydrocarbons (33). Because of insufficient knowledge about in situ controls on bioavailability and biodegradation of low-solubility hydrocarbon pollutants in soil, it is difficult to predict cleanup end points (33) and to select appropriate remediation strategies. Reported mechanisms for bacterial metabolism of sparingly soluble hydrocarbons are (i) dissolution and diffusion of dissolved hydrocarbons to cells with uptake via active or passive transmembrane transport, (ii) invagination of hydrocarbon non-aqueous-phase liquid (NAPL) into cells ...

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