Algae of economic importance that accumulate cadmium and lead: a review

Algae of economic importance that accumulate cadmium and lead: a review

Priscila O. Souza^I; Lizângela R. Ferreira^II; Natanael R. X. Pires^II; Pedro J. S. Filho^III; Fabio A. Duarte^IV; Claudio M. P. Pereira^I; Márcia F. Mesko^II,*

^ILaboratório de Heterociclos Bioativos e Bioprospecção, Centro de Ciências Químicas, Farmacêuticas e de Alimentos, Universidade Federal de Pelotas, Brazil

^IILaboratório de Controle de Contaminantes em Biomateriais, Centro de Ciências Químicas, Farmacêuticas e de Alimentos, Universidade Federal de Pelotas, Brazil

^IIILaboratório de Análise de Contaminantes Ambientais, Central Analítica, Instituto Federal Sul-Riograndense, Brazil

^IVEscola de Química e Alimentos, Universidade Federal do Rio Grande, Brazil

ABSTRACT

Currently, algae and algae products are extensively applied in the pharmaceutical, cosmetic and food industries. Algae are the main organisms that take up and store heavy metals. Therefore, the use of compounds derived from algae by the pharmaceutical industry should be closely monitored for possible contamination. The pollution generated by heavy metals released by industrial and domestic sources causes serious changes in the aquatic ecosystem, resulting in a loss of biological diversity and a magnification and bioaccumulation of toxic agents in the food chain. Since algae are at the bottom of the aquatic food chain, they are the most important vector for transfer of pollution to upper levels of the trophic chain in aquatic environments. Moreover, microalgae are also used for the bioremediation of wastewater, a process that does not produce secondary pollution, that enables efficient recycling of nutrients and that generates biomass useful for the production of bioactive compounds and biofuel.

Keywords: bioactive compounds, macroalgae, metals, microalgae, secondary metabolites

Introduction

Environmental contamination by heavy metals is a growing global problem, which is directly related to anthropogenic actions. For this motive, many techniques for environmental remediation of heavy metals are being studied (Ofer et al., 2003; Bayramoğlu et al., 2006; Rai, 2008, 2010; Rawat et al., 2011). Among these techniques, the application of microorganisms has been widely discussed, mainly in view of their capability to remove pollutants from aquatic environments with good efficiency and relatively low cost. In this context, macroalgae and microalgae have special properties that can be used as a powerful technology to reduce environmental contamination.

In particular, intense human activities can result in high metal concentrations in the environment, leading to numerous problems (Phillips, 1995; MacFarlane & Burchett, 2001). Thus, although low concentrations of some heavy metals are metabolically important to many living organisms, at higher levels they can potentially be toxic (Phillips, 1995; Sunda & Huntsman, 1998; Pinto et al., 2003a). The pollution generated by heavy metals released from industrial and domestic sources causes serious changes in the aquatic ecosystem, resulting in a loss of biological diversity and the magnification and bioaccumulation of toxic agents in the food chain (He et al., 1998).

Aquatic ecosystems such as rivers, ponds and lakes are mainly affected by pollutants and heavy metals discharged in industrial effluents and represent a potential risk to the health of humans and ecosystems (Rai, 2010). According to Rai (2008), several new technologies have been developed for the removal of heavy metals from wastewaters in a feasible way. Nonetheless, these techniques are often only partially effective and of relatively high cost, which can be an obstacle to large-scale investment. Although trace metals can be toxic to aquatic organisms and can be accumulated by several marine species (Bargagli et al., 1996), recent research has shown that some bacteria, fungi, mussels, fishes and algae have the capability to absorb trace metals and thus have the potential to serve as economically viable biological materials for the reduction of environmental pollution (Lourie et al., 2010).

Some metals and their compounds have been linked to mechanisms of carcinogenicity and metals such as cadmium and lead have been widely studied in view of their potential carcinogenicity to humans (Beyersmann & Hartwig, 2008). In addition, oxidative stress in living organisms can be related to the toxicity of metals, involving an increase in the concentration of reactive oxygen species and/or a reduction in the cellular antioxidant capacity (Pinto et al., 2003a). Oxidative stress can be associated with the inhibition of photosynthesis, of chlorophyll production or of growth in primary producers. These toxic effects can result from exposure to high concentrations of metals or to exposure of lower concentrations for longer periods, reflecting the fact that the toxicity of heavy metals is largely dose-dependent (Baumann et al., 2009). It has been shown that the photosynthesis of some species of macroalgae can be affected by the accumulation of heavy metals (Gledhill et al., 1997; Baumann et al., 2009). Collen et al. (2003) observed that copper (Cu) and cadmium (Cd) induced oxidative stress in Gracilaria tenuistipitata Zhang & Xia, a red macroalgae in the Gracilariaceae (Rhodophyta) family. Moreover, due to the release of heavy metals and other contaminants into the environment, the difficulty of cultivating Gracilaria has increased (Tonon et al., 2011). Some authors (Pinto et al., 2003a; Torres et al., 2008) have pointed out that exposure to these elements can be a barrier to the growth of many marine organisms, including phytoplankton and macroalgae, which could eventually result in a decrease in biodiversity. Of particular importance is the finding that the lipid composition of algae can be altered by the influence of heavy metals (Vavilin et al., 1998; Rocchetta, et al., 2006). At the same time, it is well known that the oxidation of lipids can occur as a result of oxidative stress, reflecting the production of reactive oxygen/nitrogen species (Pinto et al., 2003b; Leitão et al., 2003). In other experiments (Okamoto et al., 2001; Collen et al., 2003; Rocchetta et al., 2006), it was demonstrated that the levels of polyunsaturated fatty acids (PUFA) are more affected, suffering a greater decrease in the presence of heavy metals. According to Pinto et al. (2011), Cd²⁺ was more toxic than Cu²⁺ and greatly reduced the PUFA concentration in G. tenuistipitata. Here it is important to note that Gracilaria is an increasingly important source of secondary metabolites with antimicrobial, antioxidant and antitumoral activities, principally terpenes, several fatty acids and nitrogenous compounds (Cardozo et al., 2007; Boobathy et al., 2010; Zandi et al., 2010; Falcão et al., 2010; Tonon et al., 2011).

The biosorption capacity of the green algae species Spirogyra spp. and Cladophora spp. To accumulate lead (Pb²⁺) and copper (Cu²⁺) from aqueous solutions was evaluated by Lee & Chang (2011). On the basis of continuous adsorption-desorption experiments, these authors reported that both algal species were excellent biosorbents, with potential for further development. The microalgae Spirogya spp. adsorbed between 10-40 mg g^-1 of Pb²⁺ and between 45-90 mg g^-1 of Cu²⁺ from aqueous solutions containing different concentrations of Pb²⁺ and Cu²⁺. By comparison, the algae Cladophora spp. adsorbed between 5-10 mg g^-1 of Pb²⁺ and between 30-45 mg g^-1 of Cu²⁺.

Several studies have explored the metal binding properties of different biosorbents such as fungi, yeasts, bacteria and algae (Volesky & Holan, 1995; Kapoor & Viraraghavan, 1995). Numerous studies have employed macroalgae and microalgae for the biosorption of metals and the ability of certain species of macroalgae to accumulate and tolerate high levels of metals has been demonstrated. Hence, algae represent an effective, economically viable and environmentally friendly (Yu et al., 1999) alternative for the bioremediation of heavy metals, especially cadmium and lead, the two metals that are subject of the present review.

Bioremediation

Heavy metals discharged into the environment tend to persist indefinitely, sometimes accumulating in living organism via food chain, and are thus considered to represent a potentially serious environmental threat (Kuppusamy et al., 2004). The most effective and least expensive methods for the remediation of waters contaminated by heavy metals have been the focus of much research in recent decades, with the objective of reducing the risk to public health caused by the presence of these wastewater contaminants (Kumar et al., 2009). Compared to conventional treatment methods, biosorption stands out because of the following advantages: high efficiency of removal of metals from dilute solutions; low cost; and minimization of chemical and/or biological sewage. Moreover, it does not require addition of nutrients or regeneration of the biosorbent and makes it possible to recover the metals (Kratochvil & Volesky, 1998). According to Goyal et al. (2003), the biosorption of metals can be performed by many different microorganisms, including bacteria, yeast, fungi and algae. Schiewer & Patil (1997) reported that the efficiency of different biosorbents for the removal of heavy metals can depend on the pH of the solution.

Due to stricter government regulations, there has been a growing interest in cost-effective remediation technologies (Davis et al., 2003). In this context, bioremediation of polluted areas and wastewater can be an economically viable alternative, especially when the sorbent can be recycled and the heavy metals recovered for resale. Remediation of heavy metals encourages environmental awareness and ameliorates the effects of pollution (Salt et al., 1995). Bioremediation uses naturally occurring biomass as the substrate for chelation of the metal ions, either passively or through non-metabolically mediated processes (Baumann et al., 2009).

Given their abundance in various environmental systems, their adaptability to different environmental conditions (Rajfur et al., 2010) and their ability to accumulate large amounts of heavy metals such as cadmium, lead, zinc, copper, chromium, and manganese (Anastasakis et al., 2011), algae appear to be the most appropriate microorganism for monitoring pollution of water resources by heavy metals (Wallenstein et al., 2009; Rajamani et al., 2007). Indeed, algae have been used for over 40 years for the treatment of wastewater, the first application being described by Oswald & Gotaas (1957). More recently, John (2000; Rawat et al., 2011) introduced the term phycoremediation to refer to remediation by algae. In this context, it is important to emphasize that phycoremediation has several applications in addition to the removal of metals. These include the: (i) removal of nutrients from municipal wastewater and from effluents rich in organic matter; (ii) removal of nutrients and xenobiotic compounds with the help of biosorbents based on algae; (iii) treatment of acidic wastewater and metals; (iv) sequestration of CO₂; (v) transformation and degradation of xenobiotics; and (vi) detection of toxic compounds with algae-based biosensors (Rawat et al., 2011).

Many intrinsic and extrinsic factors can influence the accumulation of metals by algae, such as cellular activity, exposure time, chelating species, and environmental factors such as pH, salinity, organic matter, and temperature (Runcie & Riddle, 2004). Furthermore, structural differences between species influence their absorption capacity (Favero & Frigo, 2002). For the macroalgae Durvillaea antarctica (Chamisso) Hariot, Runcie & Riddle (2004) observed a low metal content that could be ascribed to the low availability of the metals in the surrounding waters.

Recently, macroalgae have been increasingly used as a tool for monitoring marine environments contaminated by heavy metals (Daka et al., 2003; Stengel et al., 2004; Daby, 2006, Baumann et al., 2009; Kumar et al., 2009; Tonon et al., 2011). Many macroalgae are able to accumulate high levels of trace metals, which are sometimes larger than those found in water samples from the same site (Cardwell et al., 2002; Salgado et al., 2006).

In order to determine the heavy metals present in environmental samples, analytical techniques such as atomic absorption spectrometry (AAS) (Carrilho et al., 2003; Zhang & O'Connor, 2005) have been widely used due to the relatively low cost. However, inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP OES) have been increasingly used for metal determination in view of their much lower limits of detection and the capability of multielement detection when coupled with suitable sample preparation procedures (Mesko et al., 2011; Soares et al., 2012).

In the remainder of this review, we shall concentrate on two especially toxic heavy metals, cadmium and lead, and their biosorption by micro- and macroalgae.

Cadmium

Cadmium stands out among the heavy metals because it is relatively easily removed from waste streams, primarily due to its ability to form stable complexes with several different ligands (Ofer et al., 2003). The presence of cadmium in natural waters is extremely undesirable since it is both toxic and a non-essential element for most living organisms (Leborans & Novillo, 1996; Farias et al., 2002).

In a recent research, Tonon et al. (2011) evaluated the absorption of cadmium (Cd) and copper (Cu) by three species of Gracilaria: G. tenuistipata Zhang & Xia cultivated in the laboratory and exposed to the metal and G. birdiae Plastino & Oliveira and G. domingensis (Kützing) collected in their natural environments. G. tenuistipitata bioaccumulated higher concentrations of Cu than Cd, showing that this macroalgae is a metal bio-accumulating organism; the biological function of the accumulated Cd, if any, is currently unknown.

Stohs & Bagchi (1995) suggested that Cd ions might displace zinc and iron from proteins. This could potentially have deleterious consequences for seaweed growth because the liberation of iron ions might induce the Fenton reaction, producing reactive oxygen species (ROS) and total oxidative stress. In land plants, Cd competes for divalent ion carriers and can be transported with protons and type P ATPases. According to Guerinot (2000), the ability to compete for essential metal carriers is particularly important for cadmium (Cd), mercury (Hg) and lead (Pb). Baumann et al. (2009) evaluated the Cd concentration in seven algal species and noted that 10 mmol L^-1 cadmium ion led to the greatest increase in Cd accumulation. The macroalgae Palmaria palmata (Linnaeus) Kuntze had the highest concentrations of Cd and Ascophyllum nodosum (Linnaeus) the lowest. Despite the fact that P. palmata accumulated the highest amounts of Cd and showed a significant reduction in fluorescence, no correlation was found between Cd accumulation and its toxicity. Küpper et al. (1996, 1998) demonstrated that photosynthesis can be affected by exposure to Cd and Zn, which can replace the Mg²⁺ in the chlorophyll molecule, affecting its light-harvesting ability.

Lead

Lead is a more pernicious contaminant in aquatic environments and is rapidly accumulated by organisms (Ribeiro et al., 2010). Moreover, it is able to bind strongly to amino acids, enzymes, DNA and RNA and can induce the production of reactive oxygen species (ROS) like the superoxide radical and hydrogen peroxide that can cause severe oxidative damage to plant cells (e.g., by increasing membrane lipid peroxidation and permeability) (Apel & Hirt, 2004). Lead can inhibit the synthesis of chlorophyll because it changes the absorption of essential elements such as Mg and Fe (Sunda & Huntsman, 1998).

The biological functions of lead in algae are unknown (Pawlik-Skowronska, 2000), but lead is known to have adverse effects on microalgal morphology, growth and photosynthesis when present at high concentrations (Pawlik-Skowronska, 2002).

Baumann et al. (2009) showed that for brown algae there was significant variation in Pb concentrations for all seven species of algae examined. Lead proved to be less toxic than the other five metals evaluated, but was accumulated to a greater extent by all seven algae tested than the other metals. Moreover, none of the treatments with lead affected the fluorescence yield of either species. According to Miles et al. (1972), lead affects light absorption by PSI and PSII and the chloroplast coupling factor. However, Baumann et al. (2009) demonstrated that lead was the only metal out of five tested that did not reduce chlorophyll fluorescence in the species evaluated. The results indicaated that macroalgae, especially Ulva intestinalis Linnaeus, are promising organisms for the bioremediation of waters contaminated by lead, because of their apparent tolerance to Pb and their ability to accumulate lead at high rates.

Applications of algae in the pharmaceutical industry and in environmental remediation

The pharmaceutical industry has shown great interest in the use of algae as a source of biochemically active substances (Burja et al., 2001, Singh et al., 2005, Blunt et al., 2005, Guaratini et al, 2005; Cardozo et al., 2008; Cardozo et al., 2009; Guaratini et al., 2009). The fact that algae may produce chemical prototypes of new therapeutic agents has stimulated bioprospecting for new algal secondary metabolites and the synthetic modification of compounds with potential pharmaceutical applications (Cardozo et al., 2007). In addition to novel biologically active substances, algae also provide compounds essential to human nutrition (Burja et al., 2001; Gressler et al., 2010).

Cardozo et al. (2006; 2007) described the main substances biosynthesized by algae with a potential economic impact on nutrition, public health and the pharmaceutical industry. The diversity of compounds synthesized by marine algae via a variety of metabolic pathways is the result of the defense strategies that they have developed in order to survive in a highly competitive environment. Hence, many of these secondary metabolites are chemically distinct from those found in terrestrial organisms (Burja et al., 2001; Singh et al., 2005; Blunt et al., 2005; Carignan et al., 2009; Wijesinghe & Jeon, 2011). According to Kamatou et al. (2008), the presence of these compounds may help explain some of the traditional uses of medicinal plants.

Algae are ecologically important because they occupy the base of the food chain in aquatic ecosystems and produce half of the O₂ and the majority of the dimethylsulfide released into the atmosphere. In addition, algae are the main source of food for bivalve mollusks in all stages of growth, for zooplankton (rotifers, copepods and brine shrimp) and for the larval stages of crustaceans and some species of fish (Cardozo et al., 2007) .

The quality of the food transferred to the higher trophic levels of the food chain is determined by the chemical composition of algae (such as fatty acids, sterols, amino acids, sugars, minerals and vitamins) (Brown & Miller, 1992; Di Mascio et al., 1995; Guaratini et al., 2007; Dhargalkar & Verlecar, 2009). The nutritional value of algal species depends on several characteristics such as size, shape, digestibility and toxicity (Cardozo et al., 2007). The Chinese, Japanese and Korean diet includes the consumption of several species of red and brown algae (Dawczynski et al., 2007). In addition to this traditional use in the East, people in many other parts of the world also consume or come into contact with algae-derived products used as additives in manufactured food products and processed meat and fruit or in everyday materials such as toothpaste, paint, solid air fresheners and cosmetics (Gressler et al., 2009; 2011).

Algal biomass can be effectively applied in bioremediation because the proteins and polysaccharides of their cell walls can contain anionic carboxylate, sulfate or phosphate groups, which are optimal binding sites for metals (Farias et al., 2002). Several studies have shown that it is possible to enhance the accumulation of metals by algal biomass. Thus, Kumar & Gaur (2011) observed that pretreatment with CaCl₂ generated new sites for metal ion binding by inducing cross-linking between the polymer chains of the exopolysaccharides present in the biomass. Mehta et al. (2002) and Kalyani et al. (2004) found that pre-treatment with HCl increased the metal binding capacity of biomass (by 39 or 70%, respectively), presumably be removing cationic species that were bound to the anionic functional groups, making them available for binding of additional metal ions. An improvement in the biosorption capacity for metal ions could also be induced by an alkaline pre-treatment, reflecting increased deprotonation of the acidic functional groups of the biomass (Sampedro et al.. 1995; Mehta & Gaur, 2001; Nagase et al., 2005; Singh et al., 2007, 2008).

The performance of treatment systems using algal biomass can be reduced by the presence of chelating agents, such as fulvic acid, that can compete with the anionic groups of algal biomass for binding of metal ions (Pascucci & Kowalak, 1999). In addition, there is a decrease in the percentage of metal removed at higher metal concentrations (Pujari & Chandra, 2000) due to the saturation of the available metal binding sites (Dönmez & Aksu, 2002). In this case, the use of a greater amount of biomass may not enhance the overall extent of metal ion binding if the metal ions that initially bind to a dense layer of cells create a screening effect (Zulkali et al., 2006).

Macroalgae

Seaweeds represent a significant portion of global biodiversity. They constitute a large and diverse group of organisms that play vital ecological roles in marine communities and can be classified into three categories according to their pigmentation: brown, red and green algae (Wijesinghe & Jeon, 2011).

Seaweeds are a potentially renewable marine resource and are known to be extremely rich in bioactive compounds (Chandini et al., 2008; Kladi et al. 2004) with novel biological activities (Kashman & Rudi, 2004; Plaza et al., 2008). The brown seaweeds or Phaeophyceae are noted for producing a range of active components, including unique secondary metabolites such as phlorotannins (Wijesinghe & Jeon, 2011). In addition, several components of brown algae have been explored for their antioxidant, antiallergic, antiinflammatory, antiwrinkling and whitening properties.

Compared to other types of biomass, brown algae showed the highest metal binding capacity, making them particularly attractive for the bioremediation of toxic heavy metals (Ofer et al., 2003). According to Davis et al. (2003), the linear polysaccharides known as alginate, which are present in gel form in the stem of algae, are responsible for the biosorption of heavy metals by these algae. Moreover, they noted that the orders Laminariales and Fucales are probably the largest seaweeds, and are the most abundant and widespread, enhancing their potential for cost-effective application in bioremediation. Macroalgae are usually sessile and accumulate metals over time, so that differences in the metal content of macroalgae depend on whether they are located near and far from sources of pollution and can be used to infer the source of metal contamination (Runcie & Riddle, 2004). However, as pointed out by Singh et al. (2007), this application does have its limitations because of the confinement of seaweeds to coastal areas and the difficulty of collecting them during the metal sorption process.

In a comparative study reported by Kumar et al. (2009), the ability to accumulate cadmium and lead was evaluated for five green marine macroalgae by employing initial metal concentrations in the range of 20 to 80 mg L^-1 and different contact periods. The Pb uptake values for Cladophora fasicularis (Mertens ex C. Agardh) Kützing ranged from 5.68 to 33.53 mg g^-1, while Cd uptake values ranged from 4.08 to 18.78 mg g^-1. The Cd uptake values for Ulva lactuca varied from 3.89 to 7.84 mg g^-1 and those for Pb uptake from 6.19 to 25.07 mg g^-1. For Chaetomorpha sp, the Pb uptakes were between 7.52 and 35.08 mg g^-1 and the Cd uptakes between 7.98 and 31.55 mg g^-1. Caulerpa sertularioides (S.G.Gmelin) M.A.Howe showed Cd uptake values in the range of 1.19 to 20.51 mg g^-1 and Pb values in the range of 6.03 to 21.58 mg g^-1. Valoniopsis pachynema (G. Martens) Borgesen had Cd uptakes in the range of 7.69 to 17.31 mg g^-1 and Pb uptakes from 6.42 to 37.71 mg g^-1. The efficiency of cadmiun absorption varied in the order: Chaetomorpha sp. > C. sertularioides > C. fasicularis > V. pachynema > U. lactuca; for lead, the corresponding order was: V. pachynema > Chaetomorpha sp. > C. fasicularis > U. lactuca > C. sertularioides. During the experimental exposure of the seaweeds to these two heavy metals, the concentration of free metal ion decreased significantly, demonstrating that seaweeds can be excellent biosorbents.

Figueira et al. (2000) used several species of the brown seaweeds Durvillaea sp., Laminaria sp., Ecklonia sp. and Homosira sp., pre-saturated with Ca, Mg and K, and Hashim & Chu (2004) examined seven species of brown, green and red seaweeds in order to assess their ability to remove cadmium from aqueous medium.

Gosavi et al. (2004) demonstrated that four genera of macroalgae (Ulva sp., Enteromorpha sp., Chaetomorpha sp. and Cladophora sp.) accumulated significant amounts of Fe, Al, Zn, Cd, Cu, As and Pb, noting that cadmium was absorbed better by Cladophora sp. (1.6±0.3 mg g^-1), while Chaetomorpha sp. and Enteromorpha sp. absorbed lead better. According to Thomas et al. (2003), brown algae are the most effective and promising substrates for Pb accumulation. Farias et al. (2002) evaluated eleven species of macroalgae from the Antarctica; the highest levels of trace metals were found in Monostroma hariotii Gain and Phaeurus antarcticus Skottsberg. However, M. hariotii was not able to accumulate As, Cd and Pb, which are relevant because of their potential toxicity to living organisms. Table 1 provides an annotated compendium of literature reports published in the last decade on the application of macroalgae for the biosorption of metals.

Microalgae

As an important biological resource with multiple applications, microalgae have attracted great interest (Sigaud-Kutner et al., 2002; Pinto et al., 2003b; Rawat et al., 2011). At the same time that they bioremediate wastewater, they provide biomass that can be used to sequester carbon dioxide (Olguin, 2003; Munoz & Guieyesse, 2006; Briens et al., 2008; Singh & Gu, 2010 ) and to produce biofuels (methane, ethanol, hydrogen, butanol etc.). Particularly advantageous features of microalgae as a source of biomass for the production of biodiesel include a high growth rate and short regeneration time, a high lipid content, the minimal requirement of land area, and the use of wastewater as the source of nutrients for growth, without the use of chemicals such as herbicides and pesticides (Rawat et al., 2011). The main disadvantage is the difficulty of separation of the microalgae, which are usually unicellular, from their suspensions (Moreno-Garrido, 2008).

The growth of microalgae can be indicative of water pollution since they typically respond to ions and toxins (Rawat et al., 2011). Thus, the remediation of wastewater by using microalgae is an environmentally friendly process that does not generate secondary pollutants and yields biomass that can be reused, enabling efficient recycling of nutrients (Munoz & Guieyesse, 2006). Besides their use in bioremediation and biofuel production, microalgae can also be used as additives in animal feed and for the extraction of added-value products such as carotenoids and other biomolecules (Rawat et al., 2011; Hobuss et al., 2011; Soares et al., 2012).

The release of municipal and industrial wastewater into bodies of water results in serious environmental changes (Arora & Saxena, 2005; Bashan & Bashan, 2010). Eutrophication, induced by a richness of organic matter and of inorganic chemicals such as phosphates and nitrate, can be particularly problematic (Olguin, 2003; Godos et al., 2009; Bashan & Bashan, 2010). Eutrophication can be avoided with microalgae because they use the wastewater as a food source for their growth (Rawat et al., 2011) and the accumulation of biomass (Munoz & Guieysse, 2006; Pittman et al., 2011). A wide range of microalgae, such as Chlorella sp., Scenedesmus sp., Phormidium sp., Botryococcus sp., Clamydomonas sp. and Spirulina sp. (Olguin, 2003; Chinnasamy et al., 2010; Kong et al., 2010; Wang et al., 2010), can be effectively employed to treat domestic wastewater. Using a consortium of 15 isolated native algae, Chinnasamy et al. (2010) found > 96% removal of nutrients from treated wastewater.

The rapid decline in the levels of metals, nitrates and phosphates in wastewater upon microalgal treatment (Wang et al., 2010), demonstrates the efficiency of microalgae for the removal of metals and nutrients, while meeting the stringent requirements of international standards (Rawat et al., 2011).

Microalgae are a source of peptides with the special ability to bind heavy metals (Perales-Vela, 2006). These proteins form organometallic complexes that partition into the vacuoles to facilitate control of the cytoplasmic concentrations of metal ions, thereby preventing or neutralizing their potential toxic effects (Cobbett & Goldsbrough, 2002). Prokaryotes use a mechanism that is different from that of eukaryotes, which use the consumption of ATP to drive the efflux of heavy metals or enzymatic changes in metal speciation for detoxification (Nies, 1999). These peptides can be classified into two categories: (Robinson, 1989; Rauser, 1990; Steffens, 1990; Thiele, 1992):

1. Short-chain polypeptides, synthesized enzymatically and called phytochelatins or class III metallothioneins, are found in higher plants, algae and certain fungi;

2. Proteins encoded by genes, which include the class II metallothioneins (found in cyanobacteria, algae and higher plants) and class I metallothioneins (found in most vertebrates, in Neurospora and Agaricus bisporus, but with no records so far in algae).

Initially, when the short-chain polysaccharides were discovered they were named phytochelatins (PC) because they were isolated from higher plants, explaining the prefix 'phyto", and had the ability to chelate cadmium ions (Grill et al., 1985; Steffens, 1990). However, the class II metallothioneins proved to be effective in plant responses to stress by heavy metals and the name of the PC was changed to class III metallothioneins (Mt III) (Rauser, 1990). Howe & Merchant (1992) showed that the microalgae Chlamydomonas reinhardtii P.A. Dangeard could sequester about 70% of the cadmium present in the cytosol by the action of Mt III.

In a study by Avilés et al. (2003) with the flagellated protist Euglena gracilis exposed to cadmium, 79% of the metal was accumulated in mitochondria and there was an increase in the concentration of Cys and glutathione in cells treated with cadmium. In addition, 17% of the total Mt III found in the treated cells was concentrated in the mitochondria. According to Mendoza-Cózatl et al. (2004), the presence of Mt III and Cd²⁺ in chloroplasts and mitochondria of Euglena may be the result of the following processes:

(1) the Mt III are synthesized and sequester Cd²⁺ in the cytosol; the Cd-Mt III complexes are then subsequently transported inside the chloroplast and mitochondria;

(2) the Mt III are synthesized in these two organelles and bind Cd²⁺ transported as free ions, forming the HMW complexes;

(3) both processes co-exist and the Mt III are synthesized in the three cellular compartments.

Microalgae are often grown in two commercial systems: open raceway ponds and closed photobioreactors (Hollnagel et a;, 1996; Chisti, 2007; Munoz & Guieyesse, 2006; Chinnasamy et al., 2010). The former system is inexpensive and allows the removal of nutrients from domestic wastewater, while photobioreactors, despite increased productivity, and not feasible on a large scale for phycoremediation due to economic limitations (Chinnasamy et al., 2010). The separation of algal biomass can be accomplished by methods such as centrifugation, flocculation, sedimentation, microfiltration and combinations of these (Grima et al., 2003; Munoz & Guieyesse, 2006; Danquah et al., 2009; Mutanda et al., 2011). Hobuss et al. (2011) reported a preliminary study of biodiesel production by the microalgae Chlorella vulgaris Beijerinck cultivated in a photobioreactor; the biodiesel was obtained in a significantly shorter time and with good lipid productivity.

The use of high rate algal ponds (HRAP) for the treatment of wastewater results in the production of large amounts of algal biomass, which can be converted into biofuels in many ways, including anaerobic digestion to give biogas, transesterification of lipids to obtain biodiesel, fermentation of carbohydrates into bioethanol and high temperature conversion to bio-crude oil (Mesple et al., 1996; Munoz & Guieyesse, 2006; Park et al., 2011). Moreover, HRAP are an effective system for phytoremediation, replacing conventional tertiary treatment nutrient removal, which has a cost four times higher than that of conventional primary treatment (Mesple et al., 1996; Olguin et al., 2004; Moreno-Garrido, 2008; Godos et al., 2009; Garcia et al., 2009). The main advantages of this treatment are that microalgal photosynthesis releases oxygen and there is no need for mechanical aeration because the microbial degradation of organic matter is heterotrophic.

Considering the ability of microalgae to degrade organic pollutants, dangerous species of Chlorella sp., Ankistrodesmus sp. and Scenedesmus sp. have demonstrated success in the treatment of refinery wastewater and wastewater from paper mills (Pinto et al., 2002). Cerniglia et al. (1979, 1980) evaluated the ability of algae to biodegrade the organic pollutants present in municipal waste by stimulating cell growth in the presence of pollutants; they found that cyanobacteria and eukaryotic microalgae biotransformed naphthalene into four main non-toxic metabolites (1-naphthol, 4-hydroxy-4-tetralone, cis-dihydronaphthalene diol and trans-dihydronaphthalene diol).

Inthorn et al. (2002) showed that green microalgae (C. vulgaris, Scenedesmus sp. Chlorococcum sp. and Fischerella sp.) and cyanobacteria (Lyngbya spiralis Geitler, Tolypothrix tenuis Kützing, Stigonema sp. and Phormidium molle (Kützing) Gomont) efficiently removed Pb (II), Cd (II) and Hg (II) ions. Recently, there has been a growing use of the unicellular microalgae C. reinhardtii in bioremediation (Macfie & Welbourn, 2000; Adhiya et al., 2002). Bayramoğlu et al. (2006) isolated wild-type C. reinhardtii from a polluted part of the Kizilirmak river, taking advantage of the fact that species growing in polluted areas have a higher resistance and ability to accumulate heavy metals. They showed that Ca-alginate bead with immobilized biomass of the microalgae were biosorbents capable of removing Hg (II), Cd (II) and Pb (II) ions from aqueous media.

The cyanobacterium Microcystis novacekii (Komarek) Compère, present in many tropical countries, is found in eutrophic and polluted environments (Singh, 1998), indicating that this species may be resistant to exposure to toxic agents, including heavy metals (Pradhan et al., 2007). According to Ribeiro et al. (2010), the biomass of M. novacekii had a maximum sorption capacity of 70 mg g^-1 at 21±2 °C and pH 5.0, higher than that of other biosorbents used to remove lead from water. The use of active biomass was not feasible for the removal of lead due to precipitation of the metal and cell growth was inhibited by concentrations of free metal ions in excess of 0.5 mg L^-1. In contrast, inactive cells showed a high capacity for absorbtion of Pb²⁺ from aqueous solution and equilibrium was reached quickly. Some of the most important applications of microalgae for the biosorption of metals are outlined in Table 2.

Conclusion

Based on data obtained in a number of studies, there is clear potential for the use of macroalgae and microalgae for the bioremediation of metals. In addition, there is somewhat of an advantage of microalgae over macroalgae due to the ease of collection, preparation and testing of the former. However, further studies of metal biosorption by algae are needed in order to obtain specific relationships correlating the affinities of algae for certain metals with ecological, physiological, biochemical and molecular parameters.

Acknowledgements

The authors thank the CNPq, CAPES, FAPERGS and FAPESP for funding and fellowship support.

Received 29 Nov 2011

Accepted 29 Jan 2012

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Publication Dates

• Publication in this collection
  05 June 2012
• Date of issue
  Aug 2012

History

• Received
  29 Nov 2011
• Accepted
  29 Jan 2012