METAL ACCUMULATION BY AQUACULTURED SEEDLINGS OF INDIAN MUSTARD

Indian mustard (Brassica juncea (L.) Czern) seedlings grown in aerated water (aquacultured) were able to accumulate various metals from arti ficially contaminated water over a range of environmentally relevant m etal concentrations. Seedlings concentrated the divalent cations Pb(ll ), Sr(ll), Cd(ll), and Ni(ll) 500-2000 times and concentrated the mono valent Cs(I) and hexavalent Cr(IV) 100-250 times from artificially con taminated water containing the competing ions Ca, Mg, K, SO4, and NO3. Seedlings also removed Cd from artificially contaminated water over a broad Cd concentration range. At the lowest Cd concentration studied, Cd levels were reduced to below 10 ppb (mu g/L). In the absence of co mpeting ions, Cd accumulation in seedlings increased 47-fold. This sug gests that a better understanding of the biological processes governin g uptake and accumulation of Cd by seedlings should allow the applicat ion of modern genetic engineering techniques to improve their selectiv ity and capacity for Cd removal from waters containing high levels of competing ions. As a first step in this process, we have started to de fine the tissue and cellular localization of Cd, its accumulation rate s and possible uptake mechanisms, and the role of intracellular chelat es in Cd detoxification. Intracellular Cd accumulation in seedlings wa s mediated by saturable transport system(s) and was inhibited competit ively in shoots and noncompetitively in roots by Ca2+, Zn2+, and Mn2+. Phytochelatins, the Cd-binding peptides known to be involved in Cd re sistance in mature plants, also accumulated in B. juncea seedlings exp osed to Cd. Using X-ray absorption spectroscopy, we determined that ov er time the percentage of Cd bound to phytochelatins in vivo increased from 34% after 6 h of Cd exposure to 60% after 72 h. The remaining in tracellular Cd appeared to be octahedrally coordinated by oxygen atoms possibly from organic acids. Our results suggest that the use of aqua cultured seedlings of B. juncea could provide a novel approach to the treatment of various metal-contaminated waste streams such as landfill , mining and various industrial runoffs, and leachates.