干旱区生态环境知识资源中心

This study presents an assay for the detection and quantification of soil metal superoxides and peroxides in regolith and soil. The O-2 release (OR) assay is based on the enzymatic conversion of the hydrolysis products of metal oxides to O-2 and their quantification by an O-2 electrode based on the stoichiometry of the involved reactions. The intermediate product

[GRAPHICS]

from the hydrolysis of metal superoxides is converted by cytochrome c to O-2 and by superoxide dismutase (SOD) to 1/2 mol O-2 and 1/2 mol H2O2, which is then converted by catalase (CAT) to 1/2 mol O-2. The product H2O2 from the hydrolysis of metal peroxides and hydroperoxides is converted to 1/2 mol O-2 by CAT. The assay method was validated in a sealed sample chamber by using a liquid-phase Clark-type O-2 electrode with known concentrations of

[GRAPHICS]

and H2O2, and commercial metal superoxide and peroxide mixed with Mars analog Mojave and Atacama Desert soils. Carbonates and perchlorates, both present on Mars, do not interfere with the assay. The assay lower limit of detection, when using luminescence quenching/optical sensing O-2-electrodes, is 1 nmol O-2 cm(-3) or better. The activity of the assay enzymes SOD and cytochrome c was unaffected up to 6 Gy exposure by gamma radiation, while CAT retained 100% and 40% of its activity at 3 and 6 Gy, respectively, which demonstrates the suitability of these enzymes for planetary missions, for example, on Mars or Europa. Key Words: Geochemistry-Oxygen-Geological conditions for the development of life-Mars-Europa. Astrobiology 16, 126-142.