| Summary: | <p>Abstract</p> <p>Background</p> <p>Heavy-metals exert considerable stress on the environment worldwide. This study assessed the resistance to and bioremediation of heavy-metals by selected protozoan and bacterial species in highly polluted industrial-wastewater. Specific variables (i.e. chemical oxygen demand, pH, dissolved oxygen) and the growth/die-off-rates of test organisms were measured using standard methods. Heavy-metal removals were determined in biomass and supernatant by the Inductively Couple Plasma Optical Emission Spectrometer. A parallel experiment was performed with dead microbial cells to assess the biosorption ability of test isolates.</p> <p>Results</p> <p>The results revealed that the industrial-wastewater samples were highly polluted with heavy-metal concentrations exceeding by far the maximum limits (in mg/l) of 0.05-Co, 0.2-Ni, 0.1-Mn, 0.1-V, 0.01-Pb, 0.01-Cu, 0.1-Zn and 0.005-Cd, prescribed by the UN-FAO. Industrial-wastewater had no major effects on <it>Pseudomonas putida</it>, <it>Bacillus licheniformis</it> and <it>Peranema</it> sp. (growth rates up to 1.81, 1.45 and 1.43 d<sup>-1</sup>, respectively) compared to other test isolates. This was also revealed with significant COD increases (p < 0.05) in culture media inoculated with living bacterial isolates (over 100%) compared to protozoan isolates (up to 24% increase). Living <it>Pseudomonas putida</it> demonstrated the highest removal rates of heavy metals (Co-71%, Ni-51%, Mn-45%, V-83%, Pb-96%, Ti-100% and Cu-49%) followed by <it>Bacillus licheniformis</it> (Al-23% and Zn-53%) and <it>Peranema</it> sp. (Cd-42%). None of the dead cells were able to remove more than 25% of the heavy metals. Bacterial isolates contained the genes <it>copC</it>, <it>chrB</it>, <it>cnrA3</it> and <it>nccA</it> encoding the resistance to Cu, Cr, Co-Ni and Cd-Ni-Co, respectively. Protozoan isolates contained only the genes encoding Cu and Cr resistance (<it>copC</it> and <it>chrB</it> genes). <it>Peranema</it> sp. was the only protozoan isolate which had an additional resistant gene <it>cnrA3</it> encoding Co-Ni resistance.</p> <p>Conclusion</p> <p>Significant differences (p <���0.05) observed between dead and living microbial cells for metal-removal and the presence of certain metal-resistant genes indicated that the selected microbial isolates used both passive (biosorptive) and active (bioaccumulation) mechanisms to remove heavy metals from industrial wastewater. This study advocates the use of <it>Peranema</it> sp. as a potential candidate for the bioremediation of heavy-metals in wastewater treatment, in addition to <it>Pseudomonas putida</it> and <it>Bacillus licheniformis</it>.</p>
|
|---|
