Microbial extracellular polymeric substances: central elements in heavy metal bioremediation

Abstract

Extracellular polymeric substances (EPS) of microbial origin are a complex mixture of biopolymers comprising polysaccharides, proteins, nucleic acids, uronic acids, humic substances, lipids, etc. Bacterial secretions, shedding of cell surface materials, cell lysates and adsorption of organic constituents from the environment result in EPS formation in a wide variety of free-living bacteria as well as microbial aggregates like biofi lms, biofl ocs and biogranules. Irrespective of origin, EPS may be loosely attached to the cell surface or bacteria may be embedded in EPS. Compositional variation exists amongst EPS extracted from pure bacterial cultures and heterogeneous microbial communities which are regulated by the organic and inorganic constituents of the microenvironment. Functionally, EPS aid in cell-to-cell aggregation, adhesion to substratum, formation of fl ocs, protection from dessication and resistance to harmful exogenous materials. In addition, exopolymers serve as biosorbing agents by accumulating nutrients from the surrounding environment and also play a crucial role in biosorption of heavy metals. Being polyanionic in nature, EPS forms complexes with metal cations resulting in metal immobilization within the exopolymeric matrix. These complexes generally result from electrostatic interactions between the metal ligands and negatively charged components of biopolymers. Moreover, enzymatic activities in EPS also assist detoxifi cation of heavy metals by transformation and subsequent precipitation in the polymeric mass. Although the core mechanism for metal binding and / or transformation using microbial exopolymer remains identical, the existence and complexity of EPS from pure bacterial cultures, biofi lms, biogranules and activated sludge systems differ signifi cantly, which in turn affects the EPS -metal interactions. This paper presents the features of EPS from various sources with a view to establish their role as central elements in bioremediation of heavy metals.

Bacterial-assisted cadmium phytoremediation by Ocimum gratissimum L. in polluted agricultural soil: a field trial experiment

“…increased soil cadmium solubility and is related to the production of exopolysaccharide (EPS) (Prapagdee et al 2012). EPS forms complexes with metal cations by electrostatic interaction and promotes metal plant uptake (Chen et al 1995;Pal and Paul 2008). Therefore, the cultivation of aromatic O. gratissimum L. plants coupled with Arthrobacter sp.…”

Detection Techniques for Extracellular Polymeric Substances in Biofilms: A Review

“…Induces hydrolysis of EPS, disrupts cells (Denkhaus et al 2007;Pal and Paul 2008) Steaming High protein yield, insignificant cell lysis (Zhang et al 1999) Releases significant quantities of hexose sugar (Brown and Lester 1980) Centrifugation Effective, does not cause cell lysis…”

“…EPS mostly contain bacterial secretions, shedding of materials from the cell surface, cell lysates and hydrolysates, and the adsorption of organic constituents from the survival environment (Sheng and Yu 2006;Pal and Paul 2008). EPS are a complex mixture of biomolecules (proteins, polysaccharides, nucleic acids, lipids, and other macromolecules) that are secreted by microorganisms and that hold microbial aggregates together Stewart and Franklin 2008).…”

“…In fact, in an immobilized but dynamic microbial environment (Sutherland 2001b), EPS mediate the transition from reversible to irreversible adhesion of single cells, consequently forming a cohesive, 3D polymer network that interconnects and transiently immobilizes biofilm cells. EPS are also validated in the degradation and sorption of organic and inorganic compounds (Pal and Paul 2008) and barrier system of cells resistant to hostile environments, and serve as sources of carbon and energy for biofilm growth (Wu et al 2012).…”

Biosorption of Mn (II), Co (II) and Cr (VI) in a horizontal rotating tubular bioreactor: experiments and evaluation of the integral bioprocess model

“…EPS is polyanionic in nature, with enhanced capability for cation ion bonding. Moreover, enzymatic activities in EPS also assist in the detoxification of heavy metals by transformation and subsequent precipitation in the EPS (Pal and Paul, 2008).…”