Seaweeds role on Biosorption and Biomonitoring of Metal Pollution
Babita Kumari1, Prasant Roy1& P.R. Trivedi2
Marine algae are commonly known as "Seaweeds", found on the rocky seashore area. They are considered as primary producers, for maintaining their role in marine ecosystem. Marine organisms may commonly used as bioindicator for heavy metal pollution which is considered as an environmental problem of worldwide concern (Rainbow 1995, Chan et al 2006). Many researchers referred seaweeds as biosorption indicator, which accumulate the free metal ions, from suspended particulate matter, sediments and phytoplankton. The uptake of metal ion depends on the nature of varied species of seaweeds. Out of three types of seaweeds, researcher investigated brown seaweeds, have the highest role in bioaccumulation of heavy metals in comparison to green and red seaweeds (Voltera & Conti et al. 2000). Lead (Pb), Copper (Cu), Cadmium (Cd), Zinc (Zn) and Nickel (Ni) are important metals observed in seawater. Lead poisoning not only damage bones but also damage reproductive and nervous system, Cadmium cause renal dysfunctuion and bone degradation. However, although copper and zinc required as trace metal but higher concentration may cause harmful for health.
In contrast to seaweed, biomass may serve as an economically feasible and efficient alternative to the existing physicochemical methods of metal removal and recovery from waste water. They bioaccumulate only free metals ion from suspended particulate matter, sediments and phytoplankton. The capacity to uptake metals ion depends on the nature of cellular structure of plant species. Alginates, an important polysaccharide present in the cell wall of brown algae, play an important role in adsorption of water, widely used in food additives. Due to presence of these polysaccharides in food, they strongly absorb the toxic metals comes alongwith food material.
However, investigators put their arguments in favour of absorbing capacity of toxic ions by sulphated fucan. Kuyucak et al (1989) reported the role of functional groups such as hydroxyl (OH‑), Phosphoryl (-PO^Sub3^O^Sub 2^), amino group (-NH2), Carboxyl group (-CooH), Sulphydryl (-SH) group, in adsorption of metal ions. These negative ions may binds up peptidoglycan, teichouronic acid, polysacharides & also protein molecule present in the cell wall of algae. Peptidoglycan ( N-acetyl glucosamine & Beta- 1,4, N-acetylmuranic acid) and peptide chain present in the cell wall of cyanobacteria. Carboxylic group (-COOH) play a major role in metal binding capacity while phosphoryl groups associated with the accumulation of Lipopolysacharides, lipids and peptidoglycan. In relation to amino groups, they are strongly associated with membrane protein and peptide component of peptidoglycan (Chojnacka, Choinacki and Gorecka 2005)
The term "biosorption" used in the accumulation of higher concentration of heavy metals in the extracellular and intracellular matrix of cell wall of dead biomass (Volesky et al. 1995). In several reports, the method involved in the washing of cells with 2M EDTA, which remove the surface bound metals has been reported by several workers. The intracellular metal can be quantified after digesting the cells with EDTA. The quantification of absorbed metal on the cell surface was done by subtracting intracellular metals from the total metal accumulated by the cell. In relation to adsorption contribution, approx. 80% metal accumulation was recorded in comparison to the uptake to total metal accumulation by algal cells.
Young et al. (1993) have shown the interaction of ionic charge among the heavy metal and cellular component on the cell surface. The absorption of cadmium (Cd) and lead (Pb) in different species of Sargassum was studied by Fourest et al. 1997. Raw biomass of different species of Sargassum replaced Mg and Ca ion through Cadmium cations. However, uranium (a radioactive element) recovered from seawater by using immobilizing cells of algae (Nakeijima 1982). Similarly, such ionic exchange through calcium alginate binds with copper was also studied by Chen et al (1993). He found the release of calcium alginate beads with H+ and other electrolyte ions. Raize et al (2004) reported replacement of calcium and magnesium by using cadmium cation (Solution) in the cell wall matrix and created stronger cross-linking. He showed after binding of cadmium and other heavy metals, Ca and Mg concentration were smaller than their concentration in the raw Sargassum. Presence of cations and anions including metal ions significantly affects metal sorption by seaweeds. However, the presence of multi-metallic species in the solution has no significant effect on the biosorption that the presence of multi metalic species in the solution, had no significant effect on biosorption of Gold (Au) by Sargassum natans. Besides, alteration of pH may also affect the adsorption capacity of metals ion, depend on the type of species. The adsorption of heavy metal ion increases with increase of pH. Schiewer et al. (1995) have shown the variation of adsorption of metallic ions among different species of Sargassum such as S. wightii, S. fluitans, S. johnstonii, S. vulgare. Laminaria japonica, a species of seaweeds depends on the pretreatment of biosorption of heavy metal ions at pH 5.3. Subsequently, Gluronic and Manuronic block of alginate in brown seaweeds show a major role for physical properties and reactivity for sorption of heavy toxic metal (Like Ca++ & Cd++). Out of these two block, G block of Alginate strongly binds up Calcium ion more strongly than M- Block (Fourest et al. 1997).
Similarly, biomonitoring and bisorption method was also employed in microalga. For e.g In cyanobacteria, lipopolysacharides, lipids, protein and peptide molecule present in the outer sheath of capsule are anionic in nature. They show strong affinity of metal binding sites (De Philips et al. 2001). He observed lipopolysacharides, lipids, proteins and peptides on the cell membrane. Different strains of bacteria easily capture the toxic metal ions on the outer sheath or capsule present on the cell surface. These potentiality of work fulfilled a great achievement in the environment protection or recovery of precious strategic metals (Tsezos 1985,1986,Volesky, 1987, Malik 2004).
Although many researcher approach the biological method for binding large amount of metal but in comparison to fungal and other biosorbent, a few researcher have been conducted experiments to resolved the problem to increase the sorption of maximum metals by algal biosorbents. In Spirulina, Cacl^ Sub 2^, pretreatment increase Pb sorption capacity nearly 84-92% (Gong et al. 2005). Similar result was also observed in Ecklonia maxima for Pb, Cu & Cd by using Cacl^Sub 2^ (Feng & Aldrich, 2004), besides Cacl^: Sub 2^ mineral acids (Hcl & HNO^Sub3^). Subsequently pretreatment of dil. Hcl, increases the sorption of Cu & Ni in Chlorella vulgaris. However, the pretreatment of Cacl^Sub 2^ followed by thermal treatment which show most uniform cylinder or fibre like shapes for biosorbent (Yu, Kaewsarn & Duong, 2000).For e.g., the functional group present on the cell surface dominantly active in sorption of uranium. Gaur et al. (2007) showed the role of dried biomass of spirogyra in the sorption of pb (II) [116.1 mg/gm] and Cu (II) [115.3 mg/gm] occurred at 0.1 gm/lit biomass and 100 mg/lit metal concentration in the solution. pH was adjusted at 4.5 & 5.0. pH is the major influencing the adsorption surface charge studies showed that the availability of free sites depended on pH. With increasing pH the surface charges site of Ca Alginate become more negative so uptake of metal ion increasing with increase of pH. Crist et al. (1993) reported that with decreasing pH, negative sites become available for the sorption of copper ions so that the removal of copper increases at high pH and pH increases about 0.1-1 unit from the initial pH during the adsorption of pH. Lao et al. (2006) found that lead sorption by L. japonica was strictly pH dependent, and maximum removal of lead on various pretreatment biosorbers was observed at pH 5.3. Similarly, Chen et al. (1997) reported the functional group between metal ions, evaluated an important role in adsorption of toxic metals. A fixed pH the number of functional group is fixed so that sites available for metal ions uptake decreases with increases ionic strength. The use of dead marine macroalgae biomass showed more effective than living cells in the removal of mettalic ions (Sheng et al. 2004, Bispat et al. 2005).
The heavy metal binding capacities of the corresponding seaweeds were directly proportional to their respective total carboxyl group content and related to electronegativity of the element investigated (Ca, Zn, Cd, Cu & Pb). The uronic acid composition or sequence of the alginate component did not affect the metal uptake properties of the biosorbents. Besides these, the alginate leaching owing to its solubilization by Na ions was observe to decrease with increasing intrinsic viscosity of the extracted alginate, related to its molecular weight and with increasing apparent acidic dissociation constant related to the alginate density inside the biomass (Fourest et al. 1996).
By following the method of Langmuir & Freundlich adsorption isotherm, some researcher found among two species of seaweeds, the potentiality of Padina sp. has been found a higher capacity in the adsorption cadmium compared to Acetabularia in aqous industrial waste effluents. Furthermore, the different concentration of copper and zinc in Padina sp. was greater than in Acetabularia, however the concentration of Fe in the Padina was greater than in Acetabularia. Similar study was done by Campanell et al. 2001, which might concluded that phaeophyta family showed a greater tendency to concentrate heavy metals compared to Chlorophyta (Malea et al. 1995). Foresberg et al (1987) demonstrated the role of phenol content in Ascophyllum in the recovery of toxic metal on the cell surface. The fluctuation of metal content observed due to variation of salinity and dynamic factors such as water movements, currents and winds in oceanic level.
The metal ions in water are cationic in nature which further adsorbed on to cell surface (Grif et al. 1981, Xue, Stumm & Sigg, 1988, Grist & Martin & Grist 1991, Romero-Gonzalez, William & Gandoner 2001, Skowroniki et al. 2000). This functional group has a specific dissociation constant (pKa), which dissociate the functional group into cation and anion at specific pH (Eccles, 1999, Niu & Volesky, 2000).
Although different techniques have been employed to determine the concentration of metal ions depending on the nature of cellular structure of different species of algae. "Spectroscopy" used for the determination of oxidation state of metal bound on algal cell. Besides, TEM was employed to determine the location of cadmium adsorption by Ectocarpus siliculoses (Winter, Winter & Pohl 1994). Similarly, the role of cell surface in algae in adsorption of heavy metal ions was reported by Klimnek et al. 2001).
Furthermore the inherent of metal accumulation capability of algae could be used to alleviate the burden of toxic metal load and to recover precious metal (For e.g. Gold & Silver) from waste water. In some cases the absorbing capacity of toxic metals depends on the amount of algae i.e. biomonitoring of metal pollution in water body (De Philips et al. 1994). By following the method of active and passive method, a simple method was employed for distinguishing the adsorption of intracellular metal in algal cells. Out of these two methods, passive method proved to be beneficial. Because it take a short interval of time in the absorption of metals on the cell surface. However, active method is dependent process; the captured metal ions inducted first on the cell membrane, before crosses the cytoplasm of cell wall. Hence overall literature survey shows that seaweeds are good source of absorption of Marine metal Pollution.
Department of Paramedical Science, 1Indira Gandhi Technological Medical Sciences University, Ziro, Arunachal Pradesh (India),
2Indian Institute of Ecology & Environment, New Delhi, (India).