Toxicity and uptake of Iron ions by Synechocystis sp. E35 isolated from Kucukcekmece Lagoon, Istanbul

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Abstract

The study demonstrates the potential of using unicellular cyanobacteria species, isolated from Kucukcekmece Lake, Turkey, for biological Iron removal from aqueous solutions. EC50 at 96 h was estimated to be 13.92 mg/L for Synechocystis sp. E35. The optimum pH value and incubation temperature for the resistant isolate were 7.0 and 23 °C, respectively. The Iron biosorption/bioaccumulation by Synechocystis sp. E35 was evaluated by fractionating the Fe content as the remaining metal in supernatant, the adsorbed metal on the cell surface and the intracellular accumulation. Synechocystis sp. E35 adsorbed appreciable quantities of Iron ions on the cell surface within 5 min. Metal ions were adsorbed onto the surface of cells followed by active uptake resulting in the transportation of the metal ions across the cell membrane and into the cytoplasm. Intracellular metal uptake increased with increasing metal concentrations from 10 to 15 mg/L. Extracellular polysaccharides (EPSs) were clearly seen in SEM images of Synechocystis sp. E35 grown at a 10 mg/L metal concentration. EPS region was analyzed with Energy Dispersive X-Ray Analysis (EDXA) and the SEM images further confirmed our experimental observations about the Iron biosorption/bioaccumulation mechanism.

Introduction

Heavy metal pollution of water through the discharge of industrial wastewater has become one of the most serious environmental concerns threatening the ecosystem of water bodies. Many industries including metal plating, mining, battery, metallurgy, ceramic and chemical industries have seriously contributed to the release of toxic heavy metals to water streams (by runoff). These heavy metals pose a serious threat to the environment, animals and humans because of their toxicity. Conventional treatment technologies, such as ion exchange, chemical precipitation, and reverse osmosis are often ineffective and/or expensive particularly for the removal of heavy metal ions at low concentrations [1], [2]. Most of these processes further compound the problem by generating toxic sludge. Therefore economically viable and eco-friendly technologies such as biosorption and/or bioaccumulation are required to reduce heavy metal concentrations to acceptable environmental levels. The bioprocess of metal removal is classified into two categories: biosorption by non-living or non-growing biomass and bioaccumulation by living cells [3].

Some heavy metals are important co-factors of enzymes and critical components of electron transport reactions in biological systems, but they can be toxic if taken in excess quantities [4]. Hence, toxicity gains importance in biosorption/bioaccumulation studies with living cells. In biosorption/bioaccumulation studies, microorganisms (algae, bacteria, yeast and fungi) have been traditionally used for remediation of wastewaters or soils contaminated with heavy metals [5]. Küçükçekmece Lake is one of the natural Lagoon lakes in Turkey and is a habitat of various endemic species [6]. Iron, copper and zinc were found in the water and the sediment structure of Küçükçekmece Lake at toxic levels [7].

Waste waters from automobile manufacture, metal cleaning, plating and metal processing industries contain Fe concentrations ranging over 10–120 mg/L [8], [9], [10]. Iron biosorption has been studied by various investigators using a variety of different biomass types including bacteria and fungi as biosorbents [11], [12], [13]. However, algae and cyanobacteria (blue-green algae) are the dominant group of microorganisms in the lakes due to carbon limitation and high nutrient concentrations. Hence, biosorption/bioaccumulation potential of algae and cyanobacteria grown in the lakes should be known to estimate the fate of metal pollutants in lakes.

Cyanobacteria (blue-green algae) are a diverse group of prokaryotes widespread in different habitats [14]. These prokaryotic organisms quickly respond and adapt to stress conditions in general and heavy metals in particular [15]. Cyanobacteria also produce metal-binding proteins which internally sequester and detoxify high concentrations of metals within the cell [16]. Therefore, this study aims at demonstrating the potential of unicellular cyanobacteria species isolated from Küçükçekmece Lake in removing Iron ions from aqueous solutions.

Section snippets

Kucukcekmece Lagoon

Kucukcekmece Lagoon, located in the European part of Istanbul in Turkey, has typical spoon shaped topography (Fig. 1). The surface area of the lake is approximately 17 km2, and the water volume is 145 million m3 at sea level. Untreated wastewaters, both domestic and industrial (metal, textile, plastic etc.), are routinely being discharged into the creeks of Kucukcekmece Lagoon [6].

Isolation, identification and culture conditions of microorganisms

E1, E4, E8, E35 and E37 were isolated from Kucukcekmece Lake (İstanbul), Turkey. Samples were isolated by the method

Toxicity of Iron on cyanobacterial isolates

The bacterial cells were cultivated in microplates containing growth medium with different concentrations of Fe ions (1, 5, 10, 15, 20 mg/L) to investigate the toxic effect of Iron (Fe) on cyanobacterial isolates (Fig. 2). The effect of Fe toxicity on bacterial isolates was determined after an incubation period of 96 h by evaluating the EC10 and EC50 values by probit analysis (Table 1). These values were used as estimates of Fe concentrations where statistically significant toxic effects started

Conclusions

Iron is an essential nutrient for living organism and is required as a cofactor in enzymatic reactions involving the transfer of electrons. However, Fe could be toxic to living organism when supplied in excess. Synechocystis sp. E35, isolated from Kucukcekmece Lagoon, was evaluated for its efficacy for accumulating toxic levels of Iron in the growth medium. Our results clearly indicated the potentiality of the isolated cyanobacterial isolates in biosorption/bioaccumulation of Fe. The SEM images

Acknowledgement

This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK). Project Code: 105Y116.

References (30)

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