Effect of extracellular polymeric substances (EPS) on Pb2+ accumulation by Aureobasidium pullulans

Abstract

In Pb 2+ accumulation by Aureobasidium pullulans, the time to reach an equilibrium state was not dependent on the initial cell dry weight. The Pb 2+ accumulation capacity was increased from 56.9 to 215.6 mg Pb 2+ /g cell dry weight as the biomass was stored from 1 to 53 days, and correlated with the amount of excreted extracellular polymeric substances (EPS). It was observed that Pb 2+ accumulated only on the surface of the intact cells of A. pullulans due to the existence of EPS, whereas Pb 2+ penetrated into the inner cellular parts of the EPS-extracted cells. IntroductionThe presence of heavy metal ions in municipal wastewater may have detrimental effects on the water puri®cation systems. In the interest of public health protection studies concerning the sources of heavy metal ions, their removal mechanisms in wastewater treatment plants, and the fate of heavy metal ions discharged into surface streams and/or land are deemed necessary. In recent years, the investigation of bioaccumulation/biosorption for the removal of heavy metal ions by microorganisms and their microbial products has been increased [1±3]. Microorganisms bind metal ions via any one (or more) of several known mechanisms, including adsorption, inorganic precipitation, complexation, ion exchange and active transport [4].In particular, Pb 2+ , well recognized for its detrimental effect on the environment where it accumulates throughout the food web, is generated from mining, metal, dyestuff, electric, and petroleum industries, etc. It is well known that Pb 2+ is easily accumulated in microorganisms.For instance, equilibrium metal uptake values using freeze-dried Rhizopus arrhizus increased in the order: Sr 2+ < Mn 2+ < Zn 2+ < Cd 2+ < Cu 2+ < Pb 2+ , and were positively correlated with the covalent index of the metal ions [5]. The work of Leusch et al. [6] indicated that the order of adsorption for metal ions in the biomass particles of Sargassum¯uitans was Pb 2+ > Cd 2+ > Cu 2+ > Ni 2+ > Zn 2+ . It has also been described that fungal biomass is capable of sequestering and accumulating heavy metal ions [7,8]. Fungal pigments are implicated in metal resistance and/or uptake by some fungal cultures. Biosynthesis of metalbinding pigment melanin, produced as a response to copper-induced culture stress, has been studied in a polymorphic fungus Aureobasidium pullulans [9]. Nevertheless, the relationship between the process of heavy metal ion uptake and extracellular polymeric substances (EPS) in fungi has not been fully understood.In the present study, in an attempt to elucidate the detailed process of Pb 2+ accumulation in A. pullulans, the critical effect of EPS was investigated using transmission electron microscope (TEM). Materials and methods Microorganism and growth conditionAureobasidium pullulans KFCC (Korean Foundation of Culture Collection) 110245 was aerobically cultivated with 100 ml medium containing (as g/l) 200, sucrose; 20, yeast extract; 5, K 2 HPO 4 ; 2, MgSO 4 á 7H 2 O; 15, NaNO 3 in a rotary-shaker incubator (150 rpm) at 30°C for 72...

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