residue particulate carbon, and spilled organic liquids. Certain manipulations of sorbates or sorbent media may help reveal sorption mechanisms, but mixed sorption phenomena complicate the interpretation of macroscopic data regarding diffusion of HOCs into and out of different matrices and the hysteretic sorption and aging effects commonly observed for geosorbents. Analytical characterizations at the microscale, and mechanistic models derived therefrom, are needed to advance scientific knowledge of HOC sequestration, release, and environmental risk.

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Pro has been shown to play an important role in ameliorating environmental stress in plants and microorganisms, including heavy metal stress. Here, we describe the effects of the expression of a mothbean delta(1)-pyrroline-5-carboxylate synthetase (P5CS) gene in the green microalga Chlamydomonas reinhardtii. We show that transgenic algae expressing the mothbean P5CS gene have 80% higher free-Pro levels than wild-type cells, grow more rapidly in toxic Cd concentrations (100 microM), and bind fourfold more Cd than wild-type cells. In addition, Cd-K edge extended x-ray absorption fine structure studies indicated that Cd does not bind to free Pro in transgenic algae with increased Pro levels but is coordinated tetrahedrally by sulfur of phytochelatin. In contrast to P5CS-expressing cells, Cd is coordinated tetrahedrally by two oxygen and two sulfur atoms in wild-type cells. Measurements of reduced/oxidized GSH ratios and analyses of levels of malondialdehyde, a product of the free radical damage of lipids, indicate that free Pro levels are correlated with the GSH redox state and malondialdehyde levels in heavy metal-treated algae. These results suggest that the free Pro likely acts as an antioxidant in Cd-stressed cells. The resulting increased GSH levels facilitate increased phytochelatin synthesis and sequestration of Cd, because GSH-heavy metal adducts are the substrates for phytochelatin synthase.

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The aromatic C content of humic acids strongly influences the complexation of nonpolar organic solutes by humic substances. A rapid method of estimating the aromatic C content of humic acids would be useful in studies of the fate of nonpolar organic solutes in soils and natural waters. The relationship between the UV absorptivity of nine different dissolved humic substances, measured at a wavelength of 272 nm, and their aromatic C content, as determined by 13C CPMAS NMR, was evaluated. The percent aromatic C (determined by NMR) was strongly correlated (r = 0.94) with the UV absorptivity measurements. It is suggested that determinations of UV absorptivity at 272 nm can be used to provide a quantitative estimate of the aromatic C content of dissolved humic acids.