A Model for Arsenic Transformation by Algae

Ferdi L. Hellweger, K. Farley, U. Lall, and Dominic M. Di Toro, Ph.D.

Field and laboratory data in the literature indicate that algae reduce and methylate arsenic. The data further show that the product of this transformation reaction is a function of the growth rate of the algae. At high growth rates arsenite (As(III)) is produced. At lower growth rates di-methyl arsenic (DMA) is produced. In batch experiments this leads to a pronounced As(III) peak during the exponential growth phase followed by a more gradual appearance of DMA during the stationary phase. In lakes this leads to high As(III) concentrations during bloom conditions (e.g. spring and fall) and high DMA concentrations in the summer. A mathematical model was constructed to simulate the transformation of arsenic by algae. The model explicitly accounts for (state variables) algae biomass concentration, intra- and extra-cellular concentrations of NH3, NO3, PO4, As(V), As(III), mono-methyl arsenic (MMA) and DMA. Processes include uptake of As(V) by active transport (subject to PO4 competition); reduction of As(III) inside the cell, excretion of As(III) by active transport; methylation to MMA and DMA inside the cell; excretion of DMA by passive diffusion. To simulate the strong growth rate dependence of the overall transformation reaction it is necessary to link a number of reactions to the nutrient status (e.g. intra-cellular PO4 concentration) or growth rate. The model is applied to two data sets from the literature.

Presented at:

Proctor Academy
Andover, New Hampshire