Selenium bioaccumulation of the San Francisco Bay

Trace Element Accumulation and Speciation in the Invertebrates of the San Francisco Bay

The bioaccumulation of selenium and several other trace elements in different bird and invertebrate species in the San Francisco Bay and the San Joaquin valley has been well documented. However, the uptake pathways for different trace elements and their chemical forms mobilized from water or sediment into the higher organisms are not well understood. The objective of our research is to identify the key geochemical parameters influencing the bioaccumulation pathways of some common inorganic pollutants in the San Francisco Bay estuary, using non-destructive, synchrotron based in-situ x-ray absorption spectroscopic (XAS) methods. This method has been proven to be versatile in distinguishing different trace element forms in a variety of samples. Using XAS, it is possible to obtain element specific information, such as the coordination envoronment and oxidation state of a given element, within small regions of a sample. As a part of this investigation we have conducted a preliminary investigation on the trace element spatial distributions (Cu, Zn, Fe, Cr, As) and their chemical forms (Se only) in a clam Potomocorbula amurensis collected from the estuary. In addition, we examined the chemical forms of Se in other clam species common in the San Francisco Bay.

Various species of clams were collected from several locations in the San Francisco Bay (Sherman Island, China Camp, Antioc Beach, Rodeo Beach, Berkeley Marina, Grizzly Bay, San Pablo Bay, Suisun Bay and Martinez Bay), and were subsequently cleaned as described by Luoma et al. (Environ. Sci. Technol.26, 485-491, 1992). Samples were then frozen for transport to the synchrotron. X-ray absorption spectroscopic (XAS) studies were performed at the Stanford Synchrotron Radiation Laboratory and Brookhaven National Laboratory. Model compound spectra were also collected for Se in different chemical forms. The relative-concentration maps were prepared for different trace elements using the x-ray microprobe at the Brookhaven National Laboratory.


Figure 1
Figure 1: Top: Optical microscope image of Potamocorbula amurensis (soft tissue without the shell, width ~15mm). Bottom: Se X-ray fluorescence map for the same sample. (Fluorescence intensity is directly proportional to the Se concentration.) The bright red region indicates a relatively high concentration of Se. (Collected by T.K. Tokunaga.)


The relative concentration maps for different trace elements (based on X-ray fluorescence intensity) indicate that Zn, Ca, Cu, Se, and As are concentrated both in the gut and also in the intestinal tract regions of the clam, while Fe, Ti, Mn, and Cr are concentrated only in the intestinal tract region. Overall, the mantle has a much lower accumulation of trace elements. These studies clearly indicate that trace element accumulation is dominant in certain organs of the clam and that such a distribution may depend on trace element biogeochemical factors such as speciation in the environment, residence time, and the ingestion processes and their rates.


Figure 2
Figure 2: Top: Se X-ray absorption spectrum of clam tissue and model compound spectra of Se in various oxidation states. A) selenide, B) clam tissue, C) Se(0), D) selenomethionine, E) selenate. Bottom: Micro X-ray absorption spectrum collected from the guttural regions of the clam. (Collected by T.K. Tokunaga.)


The x-ray absorption spectra of selenium suggest that the absorption edge shifts to higher energy with an increase in oxidation state. Among the examined compounds, Se(-II) has the lowest absorption energy, while Se(VI) has the highest. Seleno-cystine and methionine have absorption energies in between Se(0) and Se(IV). Compared to the model spectra, the Se X-ray absorption spectrum of the clam sample is very noisy due to low Se concentrations. However, the edge position in the clam spectrum is distinctly different from the examined models. The formal oxidation state of Se in the clam may be less than 0 but greater than -I, a species which is expected to be in the R-SeH form (where R is an alkyl group). Although this study suggests that the dominant form of Se is in the form of cysteine, the presence of other forms may not be ruled out in the clam tissue. In addition to these micro-X-ray spectra, we collected X-ray absorption spectra of clam samples collected from different parts of the San Francisco Bay. Our studies suggest that the form of Se varies with location between cysteine (in the Martinez Bay) and cystine (all other sampled locations).