Detoxification Mechanisms of Heavy Metals by Algal-Bacteria Consortia

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28Detoxification Mechanisms of HeavyMetals by Algal-Bacteria Consortia

Enrique J. Pena-Salamanca∗, Ana Lucia Rengifo-Gallego and

Neyla Benitez-CampoApplied Plant Biology Research Group Department of Biology, Universidad del Valle, Cali, Colombia

28.1 IntroductionHeavy elements are defined as chemical elements whosedensity is at least five times heavier than that of water.Among 35 widely occurring metals, 23 are heavy elementsor metals including Ag, As, Au, Bi, Cd, Ce, Cr, Co, Cu,Fe, Ga, Hg, Mn, Ni, Pb, Pt, Te, Th, Sb, Sn, U, V, and Zn(Kvesitadze et al., 2006). In small amounts, most of theseelements are indispensable for many organisms, but theirenhanced doses induce acute or chronic poisoning. Ionsmost essential for life are the representative metal ions:Na+, K+, Mg2+and Ca2+, and the transition metals: Mn,Fe, Co, Ni, Cu, Zn, Mo, and V. The essential metal ions havea variety of functions in biological systems. Their functionsrange from regulators of biological processes to importantstructural components in proteins (Nordberg et al., 2005).

The toxicity of heavy metals is apparent in reducinggrowth and development in microorganisms and plants,which seriously harm the health of animals and humans.The deleterious effects of metal ions can be manifested inmany ways, but their toxicity can be divided into five generalgroups (Kvesitadze et al., 2006):

1 Displacing essential metal ions from biomolecules andother biologically functional units.

2 Blocking essential functional groups of biomolecules, in-cluding enzymes and polynucleotides.

3 Modifying the active conformation of biomolecules, es-pecially enzymes and polynucleotides.

4 Disrupting the integrity of biomolecules.

5 Modifying some other biologically active agents.

The basis for the biological disruption by metal activ-ities is basically based on their ability to bind strongly tooxygen, nitrogen and sulfur atoms, due to their abundancein biological systems, and their role to act as ligands toall essential metal ions (Nordberg et al. 2005). In addi-tion, toxic metal ions can coordinate to essential functionalgroups of proteins which can render the protein inactive.This is especially true of Hg, which has a tremendously highaffinity for sulfur, a common ion used to form amino-acidresidues. In fact, the mechanisms of metal ion toxicity aredirectly related to the modes of metal ion binding in bio-logical systems. A biomolecule that can bind a metal ionmust possess a number of chemical characteristics, such as,a region that has a high concentration of oxygen, nitrogenor sulfur atoms, the number of donor atoms to stabilize themetal ion, and sufficient space in the metal ion space thatallows an appropriate three-dimensional geometry aboutthe metal ion (Gaur and Rai, 2001).

There are three major sources of heavy metals in mostterrestrial ecosystems: the underlying parent material, theatmosphere, and the biosphere. Biotic sources of metals

Handbook of Marine Macroalgae: Biotechnology and Applied Phycology, First Edition. Se-Kwon Kim.© 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.

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442 DETOXIFICATION MECHANISMS OF HEAVY METALS BY ALGAL-BACTERIA CONSORTIA

are originally obtained from one of the other two sources.Particularly, inputs to a system from existing vegetation oc-cur in different ways: inputs from above-ground biomass,from roots, and from below-ground biomass (Wang andLewis, 1997; Perales-Vela et al. 2006). These inputs are alsoconsidered fluxes within the food chain of an ecosystem,since plants are the base of the uptake, transport, and accu-mulation of metal in biological systems (Kvesitadze et al.,2006).

The need for a cost-effective process and safe methodsfor removing heavy metals from discharging effluents hasresulted in search for other unconventional materials suchas organic or inorganic sorbents (Loy et al., 2004). The useof microbial biomass such as, fungi (Bang et al., 2000), algae(Perales-Vela et al., 2006), and bacteria (Loy et al., 2004) forremoval of heavy metals from aqueous solutions is gain-ing increasing attention. Recently, microbial systems havebeen successfully used as adsorbing agents for the removal ofheavy metals. Microbial populations in metal polluted envi-ronments adapt to toxic concentrations and become metalresistant. Recently, there has been interested in the studyof the metabolic capacity of plant-associated bacteria usedfor phytoremediation strategies. In the rhizosphere, manypesticides as well as trichloroethylene, polycyclic aromaticcompounds, and petroleum hydrocarbons are degraded ataccelerated rates. Although plant-associated bacteria havedynamic and possess varied metabolic capacities, currentstrategies on algal–bacteria consortia are little known andduring the last years there have given a special attention tothose interactions.

In this chapter, the strategies of algal–bacteria consortiato detoxify heavy metals and their potential of biotechnolog-ical applications on heavy metal treatments are discussed.Special attention to transformation processes of metal ionsby algal associated bacteria is given.

28.2 Mechanisms used by algae inheavy metals tolerance andremoval

The ability of eukaryotic algae to survive and reproduce inmetal-polluted habitats may depend on genetic adaptationover extended time periods by mutation, genetic exchange,selection, and changes in physiology, resulting from metalexposure (Shaw, 1989; Pena et al., 2005). The effects of heavymetal toxicity in algae may include:

1 An irreversible increase in plasmalemma permeability,leading to the lost of cell solutes (e.g., K+) and changesin cell volume.

2 A reduction in photosynthetic electron transport andphotosynthetic carbon fixation.

3 The inhibition of respiratory oxygen consumption.

4 The disruption of nutrient uptake processes.

5 Enzyme inhibition, due to displacement of essential metalions

6 The inhibition of protein synthesis

7 Abnormal morphological development and ultraestruc-tural changes.

8 The impairment of motility and loss of flagella in certainmicroalgae

On the other hand, eukaryotic algae have developedsome tolerance and detoxification mechanisms to allowthem to resist metal ions inside their cells, which are shownin Figure 28.1.

28.2.1 Production of extracellularbinding-polypetidesOne of the primary mechanisms observed in micro- and mi-croalgae was related to the production of peptides capable tobind heavy metals (Nies, 1999). These molecules are furtherpartitioned inside vacuoles to facilitate appropriate con-trol of the cytoplasmic concentration of heavy metal ions(Cobbett and Goldsbrough, 2002). Those polypeptides arecommonly named specific ion-chelators or siderophores.The complexing capacity of those ligands has been demon-strated primarily in cyanobacteria and freshwater microal-gae (Butler et al., 1980). They form large extracellular aggre-gates and posses anionic properties that are capable of bind-ing metal cations. The peptides discussed can be groupedinto two categories:

1 Short-chain polypeptides named phytochelatins (PCS)(class III metallothioneins, MT), found in higher plants,algae, and certain fungi (Nicholas et al., 2003).

2 Specific proteins; class II MT (identified in cyanobacteria,algae and higher plants), and class I MT found in mostvertebrates, observed in Neurospora and Agaricus bisporus(not reported in algae) (Robinson, 1989; Rauser, 1990).

The metal-binding polypeptides produced in algae areabundant in both sulfhydryl and carboxyl groups and couldhave affinity for a wide range of metal ions. In the caseof MT they are low molecular weight, cysteine-rich metal

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MECHANISMS USED BY ALGAE IN HEAVY METALS TOLERANCE AND REMOVAL 443

Figure 28.1 Suggested mechanisms involved in plant detoxification for metal ions. Specific details are given in the case ofalgal–bacterial consortia, especially for the algal processes.

binding proteins. Class I and II MT are proteins which areencoded by structural genes (Rauser, 1990; Cobbett andGoldsbrough, 2002). Recently, the genes encoding for PCSactivity were isolated (Perales-Vela et al., 2006). Cysteineis part of the MT II chelating core and is an activator ofthe gene PCS. MT I and II biosynthesis can be induced byheavy metals such as Cd2+, Ag+, Bi3+, Pb2+, Zn2+, Cu2+,Hg2+ and Au2+. MT III are synthesized in the cytosol andare subsequently transported into the chloroplast and mi-tochondria. This was first observed in Euglena organelleswhere almost 60% of the accumulated Cd2+ present in-side the chloroplast was due to the Cd–MT III complexes(Schmitt et al., 2001, Soldo et al., 2005).

28.2.2 Exclusion mechanismThe slow phase of heavy metal accumulation by “bindingsites” is often related with intraprotoplast uptake, or cellexclusion in contrast “rapid” physical binding or biosorp-tion (Robinson, 1989, Mehta et al., 2002). Indeed, changesin the affinity of binding sites within the cell wall matrixreflects, in part active (metabolism-dependent) uptake. Ac-tive transport systems have been described for several heavymetals in algae (Schiewer and Wong, 2000). Some metal-tolerant strains of microalgae may operate an exclusionmechanism, when reducing the internal accumulation ca-pacity (Whitton, 1984). This process implies metal ionssuffer antagonism, such as the case of iron and cadmium,in the marine diatom Thalassiosira weissflora (Stauber andFlorence, 1987). The exclusion mechanism may implicate ametal ion transport system in cadmium but an inhibitionof iron. Consequently, a decreased internal accumulation

in iron appears as a tolerance of the metal by the alga (Penaet al., 2004).

28.2.3 Internal detoxificationThe study of internal detoxification of heavy metal in algaehas received little attention than surface binding and trans-port. However, algae are able to activate a definite set of bio-chemical and physiological processes to resist the toxic ac-tion of environmental contaminants (Gaur and Rai, 2001).In microalgae such as the diatoms Amphora and Naviculacopper is localized intracellularly in electron-dense spher-ical bodies corresponding to polyphosphate granules andin a dense irregular body containing sulfur, calcium, andcopper (Ahner and Morel, 1990; Soldo et al. 2005). Themain processes of detoxification are included: conjugationof the heavy metal with intracellular compounds, and fur-ther compartmentation of conjugates, degradation to com-mon cell metabolites, and finally to carbon dioxide. Thoseprocesses can be regulated by environmental factors suchas temperature, salinity, pH, and others, which implies thesignificance of those interactions with their self-resistancemechanisms (Ospina-Alvarez et al., 2006).

28.2.4 Metal transformationAlgae can carry out chemical transformations of heavymetals such as oxidation, reduction, methylation, anddemethylation. Those ones act as mechanisms of resistance,and most of them involved selective processes to eliminatenon-essential metal ions for growth metabolism (Perales-Vela et al., 2006; Lenis et al., 2007). Methylation has beenobserved in brown algae, to detoxify Hg form aqueous

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444 DETOXIFICATION MECHANISMS OF HEAVY METALS BY ALGAL-BACTERIA CONSORTIA

solution (Davis et al. 2003). The process implies degradationto less toxic compounds, as in the case of the transformationof tributyltin by Chlorella. The compound is debutylatedto di- and monobutyltin molecules; although it shouldbe stressed that light-stimulated photolysis also occurs(Toumi et al., 2000). The transformation of those methyltinderivatives are also related with bacterial transformation.It has also been suggested that the decomposition productsof arseno sugars from macroalgae lead to the formation ofarsenobetaine. This compound is a ubiquitous componentof marine animal tissues and the importance of algae as theinitial source of this molecule in the marine environment iswidely discussed (Davis et al., 2003). Additionally, arseniccan be taken up by certain marine algae and converted tovarious organoarsenicals and such conversion may be adetoxification strategy (Schiewer and Wong, 2000).

28.3 Algal–bacterial mechanismsinvolved in heavy metaldetoxification

Recent studies have demonstrated that algal–bacterial con-sortia have an important role in the cycling of toxic metalsand pollutants. Advances have been made in understandingmetal–microbe interactions and new applications of theseprocesses to the detoxification of metal and radionuclidecontamination have been developed (Lloyd, 2003).

Overall, algae and bacteria share a variety of strate-gies for heavy metal tolerance. In some cases, metal tol-

erance is the outcome of their metabolism or is an in-trinsic property related to their cell wall structure or thepresence of extracellular polymeric substances. In othercases, the resistance mechanisms include active transportefflux pumps, or intra- and extracellular sequestration. Inbacteria, they involve particularly enzymatic transforma-tion, toxic chemical species by redox reactions, methylation,alkylation/dealkylation and reduction in the sensitivity ofcellular targets to metal ions (Nies, 2003). In some plantbacterial association, these organisms underwent a vari-ety of plasmid-mediated adaptation (Brinza et al., 2007).The understanding of how those consortia resist metals canprovide insight into strategies for detoxification or removalof pollutants from the environment. Both microorganismsand algae have adapted to the presence of different metal-toxic environments by developing a variety of mechanisms(Loutseti et al., 2009). A number of mechanisms are pro-posed to explain how this consortium regulates the detox-ification and transformation of essential and non-essentialmetal ions along with their biotechnological applications(Figure 28.2).

28.3.1 BiosorptionRecently microbial systems like fungi, bacteria, and algaehave been successfully used as adsorbing agents for re-moval of heavy metals (Lee et al., 2002; Wang and Cheng,2009). Microbial populations in metal-polluted environ-ments adapt to toxic concentrations of heavy metals andbecome metal resistant. Different species of Aspergillus,Pseudomonas, Sporophyticus, Bacillus, Phanerochaete, etc.,

Figure 28.2 Main biological mechanisms involved in the detoxification of metals by plant-associated bacteria.

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ALGAL–BACTERIA CONSORTIA IN THE RED ALGAE BOSTRYCHIA CALLIPTERA (RHODOMELACEAE) 445

have been reported as efficient chromium and nickel reduc-ers (Bapat et al., 2003). Bacteria were used as biosorbentsbecause of their small size, ubiquity, and ability to growunder controlled conditions; and their resilience to a widerange of environmental situations. Bacteria and algae havethe ability to act as biological materials to accumulate heavymetals through metabolically mediated or physicochemicalpathways of uptake or binding (Vieira and Volesky, 2000).These bioprocesses involve biosorptive (passive) uptake bydead biomass or bioaccumulation by living cells.

The main drawback in the use of algal–bacterial con-sortia as biosorptive materials is their ability to interact asion-exchange synthetic resins and cell surface sequestrationfor metal ions. Nevertheless, biosorption methods seem tobe more effective than their physicochemical counterpartsin removing dissolved metals at low concentrations (be-low 2–10 mg/l) (Pena et al., 2004) and demonstrate higherspecificity, which avoids overloading of binding sites byalkaline-earth metals (Bunke and Buchholz, 1999).

28.3.2 BioaccumulationNickel-resistant bacterial populations isolated from thegreen alga Rhizoclonium riparium (Cladophorales) exhib-ited reduced bioaccumulation when cells were in station-ary phase (Pena et al., 2004). In contrast, during the mid-log phase of cellular growth, metal uptake rate was higher,demonstrating the enhancing of Ni(II) removal by Micro-coccus sp. The initial condition was a Ni(II) concentrationof 50 mg/l, pH 7, temperature: 35 ◦C. Similar studies havebeen reported by Enterobacter cloacae (Leung et al., 2000),Bacillus circulans (Tian-Wei et al., 2004) and Asperigillus sp.(Nasseri et al., 2002). All studies demonstrated that Micro-coccus isolate were more effective for the removal of Cr(VI)and Ni(II) when compared with other microbial biomassreported. The initial metal ion concentration plays a role indetermining the bioaccumulative capacity of bacterial iso-late species. As the heavy metal concentrations increased,the cellular growth of all the isolates can be inhibited, de-pending on the metal ion kinetics (Prasenjit and Sumathi,2005).

28.3.3 Biotransformation andbiomineralizationIn microbial populations, the most widely studied bio-transformation mechanism involves enzymatic reductionof metal ions to less toxic, volatile elemental (Nies, 1999).In addition to metal reduction, another strategy is the pro-duction of organic acids, and the generation of sulfuricacid through bioxidation of sulfur (e.g., by Thiobacillusspp.) (Gadd, 2000). A recent development has been the se-

quential extraction of copper by bacterial associated to rootplants on macrophytes (e.g., by Eichornia crassipess) (Penaet al., 2005; Kumar-Rai, 2008). The mechanism involvespreacidification by sulfur-oxidizing bacteria and the subse-quent immobilization of the metals through organic acidproduction. In some bacteria, the transformation mech-anism involves the presence of genes that form a specificion-resistance operon (HgII), that not only detoxifies thision but also transports and self-regulates resistance (Bru-ins et al., 2000). This same set of genes also encodes theproduction of a periplasmic binding protein that regulatesthe biomineralization of mercury compounds to less toxicmolecules which can be easily transport to cytoplasm fordetoxification (Loy et al., 2004).

28.4 Algal–bacteria consortia inthe red algae Bostrychiacalliptera (Rhodomelaceae)

During the last 20 years, the potential uses of macroalgaeepiphytic on mangrove aerial roots have been studied asbiomonitors of estuarine contamination (Pena, 1998; Pena,et al. 1999, 2005). Particularly, the metal concentrationsof macroalgae and associated bacterial populations haveextensively studied in the Buenaventura estuary, on the Pa-cific coast of Colombia (Pena et al., 2004; Ospina-Alvarezet al. 2006; Pena, 2008; Rengifo, 2010). More recently, thepercentage of chromium removal in the algae–bacteriumassociation exposed to a set of different metal concentra-tions in vitro conditions were studied (Rengifo, 2010).

The monitoring of the estuarine pollution was motivatedby the increase concerning of heavy metal pollution in thebay. The metals of concern, specifically chromium, cop-per, and lead, among others, enter waterways from a widerange of both natural and anthropogenic sources (Penaet al., 2004). While external inputs of metals into estuariesare important, estuarine sediments themselves may becomethe main sources of contaminants to estuarine waters. Bue-naventura bay is surrounded by extensive mangrove habi-tats that contain macroalgae attached to roots, tree trunksand mud surfaces (Pena, 1998). Despite the continual expo-sure of these algae to the high contaminant concentrationsfound within estuarine ecosystems, few studies have ex-amined the effects of contaminants on the interactions inalgal–bacterial communities in these tropical habitats.

Wild plants of Bostrychia calliptera associated with bac-terial populations collected from Dagua River were moni-tored in the laboratory. The trial was conducted in syntheticseawater with two levels of chromium, 5 and 10 ppm, us-ing bioreactors according to four treatments: unprocessed

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446 DETOXIFICATION MECHANISMS OF HEAVY METALS BY ALGAL-BACTERIA CONSORTIA

plant material (algae–bacteria), plant material with antibi-otic (algae–antibiotic), sediment and/or suspended matterin surface algae (natural bacterial consortium or CBN),and the control without the presence of B. calliptera orbacteria. The experimental design followed a model of twofactors (Concentration of chromium × Types of combina-tion) with repeated measures using one factor. The behav-ior of microbial populations and the chromium decreaseconcentration percentage was monitored by using atomicabsorption spectroscopy (AAS).

Results showed greater bacterial growth at higherchromium concentrations (10 mg/l) compared to thosewith the treatment exposed at 5 mg/l. Additionally, signif-icant differences were obtained for both, bacterial popula-tion to the total concentration of chromium in the algae-bacteria systems and CBN, algae–bacteria being the mostefficient treatment to remove the metal at the highest metalconcentration (Figure 28.3).

The natural consortia bacteria associated with the redalgae (CBN–algae) showed higher chromium removal(Figure 28.4) suggesting their active role in the transfor-mation processes of this metal in aqueous marine solutionsat environmental levels.

28.5 Biological treatment of heavymetals

Conventional methods of heavy metal treatment are oftenexpensive, hence alternative cost-effective technologies gen-erally based on biological processes are being developed to

Figure 28.3 Bacterial growth at different metal concentra-tions (5 and 10 mg/l). CBN (natural bacterial consortia); AB(isolated algal–bacteria strains).

Figure 28.4 Percent of heavy metal removal by algal–bacteriaconsortia exposed to different chromium concentrations. (A)Percent removal at 5 mg/l. (B) Percent removal at 10 mg/l.

remediate heavy metal pollution (Vieria and Volesky, 2001).Bioremediation exploits microorganisms to deal with heavymetal pollution in a variety of methods such as bioleach-ing, biosorption, oxidation/reduction reactions, bioprecip-itation and biomethylation. These techniques aim to changethe speciation of the heavy metals, making them either moremobile in order to improve their removal, or decreasingtheir toxicity and mobility. Phytoremediation is a special sit-uation in which plants and their associated microorganismsare used to assimilate and remove contaminants from theenvironment. Phytoremediation of heavy metals comprisesseveral processes (Salt et al., 1995). Although phytoremedia-tion is a promising method, it is restricted to contaminationat shallower depths and requires longer times compared toother methods (Pena et al., 2005). Microorganisms can helpplants to overcome heavy metal toxicity stress, either by de-creasing metal toxicity or by counteracting the plant’s stressresponse. In addition, they can assist the plants by ren-dering heavy metals more bioavailable, so improving theiruptake. Recent advances demonstrated that all bioreme-diation/phytoremediation technologies rely on the geneticand biochemical capacities of the interactions of plant andmicroorganism to protect themselves against the toxic ef-fects of heavy metals (Wang and Cheng, 2009). An under-standing of the ways how algal–bacteria consortia cope withtoxic concentrations of heavy metals is therefore essential in

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BIOTECHNOLOGICAL APPLICATIONS 447

order to exploit them for detoxification and removal ofheavy metals.

28.6 Biotechnological applicationsThe efficiency of any biotechnological applications on heavymetal bioremediation depends on the activity of the mi-croorganisms involved which is, in turn, affected by envi-ronmental conditions, operational parameters and the lo-cal composition of the overall algal–microbial community(Ospina-Alvarez et al., 2006; Perales-Vela, 2006). When opt-ing for a biological remediation strategy, important ques-tions to be answered include:

� Are the organisms with the desired characteristics andactivities present at the contaminated site?

� What is their activity?

� How is the algal–microbial community composition andfunction influenced by environmental parameters andprocess conditions?

Algae-bacterial associations have been traditionally usedfor pollution control, especially for the removal of inor-ganic nutrients (Toumi et al., 2000). The most commonarrangements used are high rate algal ponds (HRAP) andthe patented Algal Turf Scrubber (ATS), which employssuspended biomass of common green algae (Chlorella,Scenedesmus, Cladophora), and bacteria (Cyanobacteriasuch as Spirulina, Oscillatoria, Anabaena) or consortia ofboth. The above mentioned algal systems have been testedfor heavy metal removal (Pena et al., 2005). Toumi et al.(2000) compared the heavy metal removal rates of tradi-tional waste stabilization ponds (WSP) and a HRAP whereboth were receiving urban polluted water with trace concen-trations of Zn2+, Cu2+ and Pb2+. It was found that HRAPhad a higher removal rate per unit volume per day, withvalues up to 10 times more efficient in the case of Cu2+.The values obtained could have resulted from the high pHachieved as a result of algal photosynthesis that enhancesmetal precipitation.

Adey et al. (1996) developed a system using consortia offilamentous cyanobacteria and suspended green algae fortreating polluted underground waters. This research provedtheir advantage for the efficient removal of heavy metals,and also the removal of chlorinated and aromatic organiccompounds was observed. The authors hypothesized thatbacteria could have aided the biodegradation of aromaticcompounds. Algae degradation of these chemicals has beenrecently reported and this is a growing field of research inenvironmental microbiology (Davis et al., 2003).

The use of both living and death algal–microbial biomassfor removal of heavy metals from aqueous solutions us-ing biosorptive mechanisms is gaining increasing attention.Biosorption is regarded as a potential cost-effective biotech-nology for the treatment of high volume low-concentrationcomplex wastewaters containing heavy metals (Wang andChen, 2009). It has been found that development of biore-actors with living cells for improving biosorption activitydepends on properties of adsorbent and molecules in thetransfer from the solution to the solid phase. It has alsobeen reported that biosorption capacities for heavy met-als are strongly pH sensitive and that adsorption increasesas solution pH increases (Ospina-Alvarez et al., 2006). Ithas been found that the plant-associated bacteria possessedmaximum sorption capacity for the cationic metal ions atpH values between 4 and 6. At pH below 3, uptakes of cop-per, nickel and zinc were negligible, probably due to thecation competition effects with oxonium (hydronium) ionH3O+ (Klimmek et al., 2001).

In commercial applications, another factor affectingbiosorption activity, beside pH are the multi-omponentmetal solutions (Volesky and Naja, 2005). The sorption pro-cesses were found to be slower in a mixed-metal solutionthan in the single-component metal solutions, and equi-librium was reached after 5 h of the experiments. Reach-ing the equilibrium point, copper and zinc were bound46 %, nickel 30 % and chromium 20 %. Moreover, duringthe next 5 hours there was no evidence in further uptakeof metal ions (Volesky and Naja, 2005; Wang and Cheng,2009). It can be concluded that the kinetics of biosorptionappears to be faster in the single-component systems inthe comparison with the multicomponent one. It is prob-ably due to the absence of competitive processes betweenmetals and biomass. Generally, for the future of biosorp-tion technology, there are two trends of biosorption de-velopment for metal removal. One trend is to use hybridtechnology (algae/bacteria biomass) for pollutants removal(Tsezos, 2001), especially using living cells. Another trendis to develop good commercial biosorbents, just like a kindof ion exchange resin, and to exploit the market with greatendeavor (Volesky, 2007).

Recently, molecular and non-molecular methods forthe identification and characterization of plant-associatedbacteria and their specific properties have been used toassess the composition and activity of those consortiafound at heavy metal-contaminated sites. These techniquespromise to become complementary tools to classic chem-ical and physiological analysis (heavy metal concentrationsand speciation, redox potential, etc.) for monitoringspatial and temporal changes in microbial communitycomposition and function. Advances in understanding ofthe roles of these interactions in such processes, especially

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for algal–bacterial consortia, together with the abilityto fine-tune their activities using the tools of molecularbiology, has led to the development of novel or improvedmetal bioremediation processes during the last years (Lloydand Lovley, 2001).

28.7 Conclusions and futureremarks

Significant advances have been made in understanding theroles of algae and bacteria in mineral cycling, and in theapplication of these processes to the bioremediation ofmetals. Additional advances are expected in the study ofalgal–bacterial interactions, focused on the use of new tech-niques, such as genomic approaches, which will undoubt-edly make an impact in the area of environmental biotech-nology.

Extensive surveys of heavy metal tolerant algal-associated bacteria are needed in order to obtain new datafor specific strains that can be isolated for biotechnologicalapplications such as biosorptive commercial designs. Stud-ies that revise particular detoxification/resistance mecha-nisms should be verify to increase current knowledge of howthey can involve in commercial applications for remediationof heavy metals in aqueous solutions. Especial attention isneeded to identify candidate enzymes for genetic manipu-lation, responsible for the production and transportationof specific molecules involved in uptake and detoxificationprocesses in algal–bacteria consortia.

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