Bioaccumulation Of Metals (Pb～(2+) And Cd～(2+)) And Its Effects On Growth, Morphology And Pigment Contents Of Cyanobacteria

Excessive metal concentration in water bodies poses significant hazard to human, animal and plant health and to the environment in general. Due to the potential toxicity and high persistence of metals, water polluted with these elements is a matter of great concern. Since trace amounts of certain metals often referred to as"heavy metals"could be harmful to the plant cell, the mechanisms preserving a stable metal environment within the cell are of outstanding importance. Supply of the cell with the necessary amounts of metals is guaranteed by uptake systems or systems which release metal stores from intracellular compartments. Many of metals have a direct influence on various physiological and biochemical processes of microalgae. However, different organisms proved to have different sensitivities to the same metal, while the toxicity of different metals to same organism may also be varied. Microalgae, on the other hand, are potential alternatives to conventional means of heavy metals removal from the contaminated water bodies. In this context, Cyanobacteria, due to their ubiquitous occurrence in nature and metabolic uptake with continuous growth have gained a paramount attention. The edible filamentous cyanobacterium Spirulina platensis along with the unicellular Synechocystis sp. PCC 6803, a model organism widely used in genetic, physiological and morphological studies were used in this study to assess the bioaccumulation of Pb²⁺ and Cd²⁺ and its subsequent consequences.A slight growth stimulation in Spirulina platensis could be seen at low initial concentrations (1, 2 and 3μg/mL) of both metals. The pigment contents (chlorophyll a andβcarotene) were also shown to be positively influenced by both metals at low concentrations (1 and 2μg/mL). However, at high concentrations (30, 50 and 100μg/mL), the growth and the pigment contents were severely affected. Toxicity measured as LC50 at 96 h was 75.34 and 37.84μg/mL respectively for Pb²⁺ and Cd²⁺. Apart from a few cases of filament breakages at elevated concentrations, morphological abnormalities influenced by metals in S. platensis were not specific.Low exposure of Pb²⁺ could slightly stimulate the growth of Synechocystis sp. PCC 6803 (0.2, 3.8 and 6.5 % stimuations respectively at 0.5, 1 and 2μg/mL). However, at > 4μg/mL Pb²⁺, the species could no longer withstand against Pb²⁺. Whereas Cd²⁺even at 0.5μg/mL, could inhibit the growth of Synechocystis sp. The highest inhibitions recorded at the end of 6 d incubation period were 53.14 and 70.07 % respectively for Pb²⁺ and Cd²⁺ at 8μg/mL. Cells exposed to 6μg/mL Pb²⁺, resulted in 36.56, 37.39 and 29.34 % reductions in chlorophyll a,βcarotene and phycocyanin contents respectively. The corresponding reductions at the same Cd²⁺concentrations were 57.83, 48.94 and 56.90 %. Lethal concentration (LC50, 96 h) values were 12.11 and 3.47μg/mL respectively for Pb²⁺ and Cd²⁺. Alterations in the ultrastructure became evident with the increased (> 4 Pb²⁺μg/mL) metal exposure. Membrane system (plasma and thylakoid) seemed largely deteriorated at 8μg/mL Pb²⁺.Synechocystis sp. PCC 6803 was shown to be highly sensitive to both metals, while Spirulina platensis impressed some resistance. Lethal concentration (LC50, 96 h) values revealed that both species were more vulnerable to Cd²⁺ than Pb²⁺. Both metals decreased pigment contents (chlorophyll a,βcarotene and phycocyanin) in a dose-dependent manner and the reductions caused by Cd²⁺ were higher than those of Pb²⁺. The maximum metal accumulations (155.63 and 199.83 mg/g-dry weight, respectively for Pd²⁺ and Cd²⁺) and the highest bioconcentration factor (BCF) values (20,657 and 24,980 respectively for Pb²⁺ and Cd²⁺) were recorded from Synechocystis sp. PCC 6803. Despite the comparatively low accumulated amounts displayed by Spirulina platensis, both species sound good accumulators of metals with high bioconcentration factor (BCF) values. However, the practical viability of the species as potential bioremovers of metals must be further confirmed by the studies performed under field conditions.