Case Study Arsenic Transformation by Phytoplankton

Transformation of arsenic by phytoplankton constitutes an interesting case study of coupled ecological and bio-geochemical modeling. Arsenic can exist in a number of species, including arsenate (As(v)), arsenite (As(iii)), methylarsonate (MMA) and dimethylarsinate (DMA). Under oxygenated conditions As(v) is the only thermodynamically stable form, and the other species spontaneously demethylate and oxidize to As(v). However, early field observations in the Pacific Ocean revealed that, although As(v) is the predominant form, other species are present at relatively high concentrations, meaning that there has to be a process continuously producing them. Algae were identified as being responsible for the transformation. In addition, field and laboratory data revealed that the end product of the transformation reaction varies and is a function of the growth rate and/or nutrient status of the algae. A model was proposed for the transformation of arsenic by phyto-plankton, which is presented in Box 2 and Figure 10.

Box 2 Descriptive model of arsenic transformation by phytoplankton

The transformation of arsenic by phytoplankton is linked to the uptake of phosphate. Algae actively take up As(v) (AsO(OH)3) because they cannot differentiate it from phosphate (PO(OH)3). However, because As(v) is toxic, the algae has to detoxify it, which is done by reduction to As(iii), methylation to MMA and DMA, and excretion. The end product of the overall transformation reaction is a function of the phosphorus nutrient status of the algae. Under P-limited conditions the algae take up As(v), reduce it to As(iii), methylate it to MMA and DMA, and then excrete it as DMA. Under P-replete conditions the algae up-regulate their phosphate transport system (luxury uptake), and since As(v) is taken up by the phosphate transport system, it is also taken up at higher rates. The reduction to As(i i i) is fast, but the methylation is slower, causing As(i i i) to build up in the cell and be excreted into the medium.

A quantitative biogeochemical model has been developed for transformation of arsenic by phytoplankton using the concepts presented above. The model simulates uptake using the Michaelis-Menten equation modified for competitive inhibition and upregulation (luxury uptake). The model was calibrated to laboratory data. The results, presented in Figure 11a, illustrate that the model captures the major temporal patterns in the data, including the production of As(iii) early and DMA later in the experiment. Then, the model was integrated with an ecological model and used to simulate arsenic speciation in a lake. The results, presented in Figure 11b, illustrate that the model captures much of the major temporal patterns in the field data, including spring and fall increases of As(iii) and higher DMA in the summer.

Figure 10 Conceptual model of arsenic transformation by phytoplankton.

As(iii)

Figure 10 Conceptual model of arsenic transformation by phytoplankton.

JFMAMJJASOND Date in 1993

Figure 11 Simulated and observed transformation of arsenic by phytoplankton in (a) a laboratory batch experiment and (b) Lake Biwa, Japan.

JFMAMJJASOND Date in 1993

Figure 11 Simulated and observed transformation of arsenic by phytoplankton in (a) a laboratory batch experiment and (b) Lake Biwa, Japan.

See also: Biodegradation; Conceptual Diagrams and Flow Diagrams; Empirical Models; Individual-Based Models; Microbial Models; Scale.

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