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Use of XAS to Determine the Chemical Speciation of Bioaccumulated Manganese in Potamogeton pectinatus

Dep. of Plant Biology, 111 Koshland Hall, Univ. of California, Berkeley, CA 94720;

F. W. Lytle

The EXAFS Company, Eagle Valley, HC 74 Box 236, Pioche, NV 89043;

B. N. Smith

Dep. of Botany and Range Science, Brigham Young Univ., Provo, UT 84602.

^* Corresponding author (mlytle{at}nature.berkeley.edu).

ABSTRACT

Manganese (Mn) accumulation was measured in Potamogeton pectinatus L. (sago pondweed) over a growing season. Leaf tissue Mn content increased from 120 mg kg^–1 in June to 4990 mg kg^–1 by October. Our objective was to chemically speciate bioaccumulated Mn in P. pectinatus with x-ray absorption spectroscopy (XAS). Differences were found in Mn valence in leaf tissue between June, October, and January when compared to model inorganic Mn compounds. Manganese in June leaf tissue was most like [Mn²⁺ (H₂O)₆]. By October, average valence of Mn in leaf and root tissues was similar to Mn₃O₄. However, there was little agreement in the chemical structure of Mn in these tissues and Mn₃O₄. Senescent leaf tissue in January differed from October because the valence had shifted back toward Mn²⁺. The Mn³⁺ fraction had leached from leaf tissue between October and January. This coincided with a decrease in total Mn concentration during this same period. Manganese bioaccumulation in this aquatic species occurs in two phases: (i) Mn²⁺ is bound in finite sites and coordinated by water, (ii) once filled, Mn²⁺ is oxidized and stored as a Mn³⁺ oxide within the plant that can be leached upon senescence.

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