Quarterdeck Volume 4, Number 2, Summer 1996
The role of colloids in trace metal
The role of colloids in trace metal speciation
Mary C. Stordal
The bioavailability of trace elements in aquatic systems is not only determined by their concentrations but also the speciation, or chemical form of the element, that is present. A metal ion can interact with inorganic ions such as chloride or sulfate. Organic compounds in the water can also form complex molecules that include the metal ion. Particles in the water can adsorb, or bind with, the trace element. Traditionally, a dissolved element was defined as that which passed through a 0.4 µm filter. Particles, however, exhibit a continuum of sizes and some can pass through such a filter with the metals attached. These particles are called colloids. The attachment or association of a trace element with these finer particles can affect the availability of the element to organisms as well as the distribution, transport, and removal of the element in the aquatic environment.
Observations of trace element movement between the dissolved phase and the particulate phase in natural aquatic environments does not always follow what is predicted from laboratory observations. Generally, some elements are not transferred from the dissolved to the particulate phase as quickly as expected. Some process occurs in the environment which slows things down. One possible mechanism is colloidal pumping. Through this process an element attaches quickly to fine particles which slowly aggregate and coagulate into larger particles that are removed from the water column and buried in the sediments.
My dissertation research focused on the presence of colloidal associations of four trace elements in estuarine environments: arsenic, antimony, mercury and selenium. I chose these elements because they are involved in biological processes (methylation) in aquatic environments as well as physical and chemical processes. Estuaries constitute complex chemical environments stretching from freshwater to saltwater and are ideal for studying processes that control trace-element distributions under a variety of salinities.
I collected surface-water samples from Sabine Lake, Galveston Bay, and Corpus Christi Bay using ultraclean techniques to avoid contamination. I isolated colloids from the water using a cross-flow ultrafiltration system. This process essentially squeezed water and compounds weighing less than 1000 molecular weight out of the water sample by passing it through a filter under high pressure. The colloids were concentrated in a reservoir while the water that passed through the filter, called the filtrate, was collected in another reservoir. I also collected an unfiltered sample to enable comparison and evaluation of the performance of the system. Problems with ultrafiltration were evaluated by comparing the sum of the colloid reservoir and filtrate concentrations to the concentration of all dissolved material in the sample prior to ultrafiltration.
The results showed that arsenic and antimony remained primarily in the filtrate with less than 10% associated with colloids. A different story was told for selenium, of which roughly 35% was associated with colloids. This may reflect the cycling of selenium by phytoplankton in the water. Mercury was most interesting because at least 60% was associated with colloids and a comparison with colloidal organic carbon showed that the mercury was bound to organic molecules. These results have implications for understanding the processes that control the distribution and bioavailability of these elements.
Chemical and phase speciation of metal ion in ocean water.
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The ecology of Pseudo-nitzschia in Monterey Bay, California
M. Celia Villac
In the fall of 1991, the die-off of pelicans in Monterey Bay, California, made news headlines. High levels of a neurotoxin, domoic acid, were found in the stomachs of the birds and in their main food source, anchovies (plankton feeders). Domoic acid in shellfish extended to California, Oregon, and Washington, resulting in the closure of fisheries. The only precedent was domoic acid contamination of blue mussels from a mariculture project in Prince Edward Island, Canada, which was responsible for human deaths and disabilities in 1987-the first report of Amnesic Shellfish Poisoning (ASP). Surprisingly, phytoplankton in both regions was dominated by the diatom genus Pseudo-nitzschia. When domoic acid production was confirmed in cultures of P. multiseries and P. australis, the species that had bloomed in Canada and in Monterey Bay, diatoms were added to the increasing list of organisms responsible for toxic algal blooms.
Pseudo-nitzschia is a genus that is well represented worldwide. Awareness of ASP pressed for a better understanding of distributional patterns and identification at the species level. I visited the west coast (from Monterey Bay northward) during the ASP event. Meanwhile, Greta Fryxell, my advisor, went to southern California. We detected P. australis at every sampling site from Los Angeles to the mouth of the Columbia River. Five other Pseudo-nitzschia species were found in Monterey Bay, providing a good case study of the ecology of this important diatom genus. This became my dissertation research topic.
Through cooperative work with the Monterey Bay Aquarium Research Institute, I assessed the space-time distribution of the Pseudo-nitzschia flora in the bay from September 1991 to October 1993. The study relied to a great extent on concentrated net haul samples, because a large number of cells had to be cleaned of organic matter before identification was made in light and scanning electron microscopes. Differentiation between species was based on the examination of the outline and fine structure of the diatom's siliceous cell wall. Field data were augmented by the investigation of the morphology, domoic acid production, and competition of Pseudo-nitzschia species raised in laboratory cultures.
Of the 21 Pseudo-nitzschia species described, fourteen were found in the bay and four were potentially toxic. Pseudo-nitzschia populations (one to several species at a time) were detected throughout the study period. Increasing cell numbers of the target diatoms showed trends that ranged from the rapid response to local upwelling pulses to the interannual species shifts of species found in nature. Coexistence of dominant species was explained, in part, by the mixing of different phyto-plankton patches in a shorter period of time than is required for competitive displacement to occur, as observed in the laboratory.
Six species were represented among 82 successful cultures. Domo-ic acid production was confirmed for P. australis and P. multiseries. A six-month experi-ment with cultures grown at three temperatures demonstrated the conservative nature of the morphological characteristics used to differentiate species in the field. In a joint effort with other investigators, several of these cultures provided study material for Pseudo-nitzschia characteristics at the molecular level, corroborating species boundaries based on morphology.
The genus Pseudo-nitzschia can be readily recognized by its spindle-shaped cells that form stepped chains by overlap of cell ends. The mere presence of this genus should not cause alarm. The threat of an ASP event must take into account the absolute abundance of a toxic species and its proportion to total phytoplankton. Moreover, the habit of the toxin vector, either sessile or swimming, can play a key role in the transfer of domoic acid to higher trophic levels.
Pseudo-nitzschia viewed through a scanning electron microscope.
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Last updated February 5, 1997