Role of Padina gymnospora (Dictyotales, Phaeophyceae) cell walls in cadmium accumulation

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Abstract
Padina gymnospora plants were exposed to Cd or Cd combined with Zn for I week and thereafter prepared for atomic absorption spectrophotometry and transmission electron microscopy (TEM). Conventional TEM, energy dispersive X-ray analysis (EDXA) and electron spectroscopic imaging (ESI) were used. Cytochemical tests were applied to determine the presence and abundance of the principal polysaccharide groups in different regions of the thallus. An increase in Cd accumulation of three orders of magnitude occurred during the experimental period. Electron-dense deposits were seen in cell walls from the subapical and median regions of the thallus. Analytical microscopy (EDXA and ESI) and electron diffraction from the electron-dense deposits showed that S colocalized with Cd and Zn in both treatments, and that the Cd/Zn-rich deposits in P. gymnospora cell walls were crystalline. These results indicate that sulphated polysaccharide groups are involved in metal immobilization and in nucleation of the crystalline deposits. Ultrastructural damage was observed only in the Cd + Zn treatment. The higher affinity of the tested stains and probes for cell walls in the median region, compared with those in the subapical region, suggests differences in the amount and/or composition of polyanionic cell-wall polysaccharides along the thallus. Our results indicate that cell walls provide the requirements for the induction of mineralization of different cations, being the main site for metal resistance and accumulation in P. gymnospora.
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