Metal resistance of soil algae [Chlorophyta] occurring in post-flotation Zn-Pb and Cu-tailing ponds

Algae as pioneer organisms are important in extreme environments. We isolated several green algae (Chlorophyta) from Zn/Pband Cu-enriched ground samples of post-flotation tailing ponds, containing 17200–18400 mg Zn kg⁻¹, 3017–6566 mg Pb kg⁻¹ or 1420 mg Cu kg⁻¹. The algae with different morphologies belonged to following classes: Chlorophyceae and Trebouxiophyceae. Their Zn-, Pb- and Cu- resistance in comparison with soil green algae (Chlorophyceae) isolated from the unpolluted control soil was evaluated under laboratory conditions on the basis of 96h-EC₅₀ (effective metal concentration which causes 50% inhibition of algal growth after 96 h exposure). Among isolated algae Dictyococcus cf. varians Gerneck em. Starr from the Zn/Pb-tailing pond was highly resistant both to lead (EC₅₀ 48 μM) and zinc (EC₅₀ 126 μM), but sensitive to copper (EC₅₀ 2 μM). Stichococcus minor Nägeli and Chlamydomonas boldii Ettl from the Cu-tailing pond were resistant to copper (EC₅₀ 17.8 μM and 10 μM, respectively). Simultaneously, S. minor revealed co-resistance to Zn (EC₅₀ 251 μM), while C. boldii to Pb (EC₅₀ 38.9 μM). Geminella terricola J.B. Petersen (Chlorophyceae), isolated from the unpolluted control soil, revealed high sensitivity to the three metals (Zn-EC₅₀ 44.6 μM; Pb-EC₅₀ 10.2 μM and Cu-EC₅₀ 6.4 μM). Simultaneously, G. terricola accumulated intracellularly higher amounts of Zn (7.1 amol μm⁻³) and Cu (5.5 amol μm⁻³) than all the algae from the polluted ground samples (Zn: 1.2–6.4 amol μm⁻³ and Cu: 0.4–2.7 amol μm⁻³). It also accumulated high amounts of Pb (6.0 amol μm⁻³), but two-fold lower than D. cf. varians. Using cytochemical staining of metals dark pink Pb-rhodizonate complexes were detected in thick cell walls of the Pb/Zn-resistant D. cf. varians and in thick mucilage layers of the Cu/Pb-resistant C. boldii. However, in the Pb-sensitive S. minor Pb-complexes were detected inside deformed cells. Pink-orange Zn-dithizone complexes were mainly distributed inside the cells of the Zn-resistant D. cf. varians. The results obtained suggest that thick cell walls or envelopes may be partly responsible for the higher Pb-resistance of some studied algae. However, in the case of micro-nutrients like Zn or Cu other resistance mechanisms (biochemical / physiological) may be involved. It seems that algal species or ecotypes living in the grounds of metal post-flotation tailing ponds have been adapted to heavy metals present in their habitats and may be useful for remediation of such degraded environments.