The cell walls of plants, including those of algae, have the capacity to bind metal ions in negatively charged sites. The authors had already shown that the wild type (walled) strain of the unicellular green alga Chlamydomonas reinhardtii Dangeard was more tolerant to Cd, Co, Cu, and Ni than a wall-less mutant of the same species. The objective of the present study was to determine if the tolerance to metals was associated with an increased adsorption of the same metals to the cell wall. Adsorbed metal was defined as that fraction that could be removed with a solution containing Na(2)EDTA and CaCl(2). The fraction that remained after the EDTA/CaCl(2) wash was considered to be strongly bound in the cell. When exposed to metals, singly, in solution for 24 h, cells of both strains accumulated the metals. The original hypothesis was supported by the results for Cd, Co, and Ni insofar as significantly higher concentrations of these metals were in the loosely bound fraction of the walled strain in comparison with the wall-less strain. However, there are three reasons why the potentially protective effect of the cell wall did not explain differential tolerance of the two strains. After 24 h of exposure (1) less Cd was accumulated internally by the wall-less strain than by the walled strain, (2) very little of the accumulated Cu was in the loosely bound fraction of the walled strain, and (3) the two strains accumulated comparable and relatively high amounts of internal Cu. Unexpectedly, significant amounts of Cd and Cu were also removable from the surface of the wall-less cells. One possible explanation for these apparently externally bound metals in the wall-less strain is that the cells exuded metal-chelating molecules that decreased the ability of metal ions to penetrate the plasma membrane. It was concluded that metal tolerance in this alga must involve a complex of mechanisms involving both internal and external detoxification of metal ions.

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In vivo oxygen evolution above single stomata in Brassica juncea has been used to investigate, for the first time, the effect of Cd-induced stress as imaged by scanning electrochemical microscopy (SECM). SECM images showed a clear stomatal structure-a pore, whose aperture is modulated by two guard cells, serving as the conduit for the oxygen produced. Lower stomatal density and larger stoma size were found in plants treated with 0.2 mM CdCl2 compared with control plants. Either the introduction of Cd caused a slower cell replication in the plane of the epidermis, hence fewer stomata, and/or the number of open stomata was reduced when plants were under Cd-stress. Oxygen evolution above individual stomatal complexes in Cd-treated plants was lower than that from control plants, as determined from the electrochemical current above the middle of each stoma. All guard cells under illumination were swollen, indicating that the stomata were open in both control and treated plants. Thus, decreased oxygen evolution in response to Cd cannot be attributed to simple closing of the stomata, but to a lower photosynthetic yield. SECM provides an excellent tool for monitoring the effects of Cd on photosynthetic activity at the scale of individual stomata.

In three wetlands in southeastern Ontario, ferric hydroxide was deposited on the roots of Typha latifolia (maximum, 67 × 103 ppm); in four other wetlands, deposition was negligible (< 103 ppm). Iron deposition was seasonal, with the peak period in July–August corresponding to peak biomass production of shoots. In one wetland where Carex rostrata and Phragmites australis occurred, seasonal plaque formation was similar. Plaque formation was not obviously related to Eh and pH regimes.

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