pH 6.0 and 40 minutes of time period are suitable conditions for chromium reduction. Another S. platensis was also studied by Khuntia et al., (2011) [49] for the reduction of hexavalent chromium. In addition, 99.7% of chromium was reduced at an optimum pH of 0.5 and biomass dose of 5 g L−1. According to Katircioğlu et al., (2012) [50], a cyanobacterial strain Oscillatoria sp. H1 obtained from the Mogan Lake in Ankara, Turkey, was also observed for hexavalent chromium (VI) removal from aqueous solutions. Maximum biosorption of Cr(VI) was found at pH 6.0 which remain unaffected to temperature between 20°C and 40°C. Increase in biosorption was observed by increase the dried free and immobilized live and heat inactivated biomass (0.04 g) and 30 beads, respectively.
2.3.2 Green Algae
The smaller freshwater green algae Pseudokirchneriella subcapitata, formerly known as Selenastrum capricornutum Pintz, amplifies metal binding sites, leading to an increase in bioaccumulation and consequential increase the capacity to accumulate chromium [51]. Spirogyra sp. was found to be a cost effective and eco-friendly biosorbent while studied using different concentrations of chromium (1.0, 5.0, 15.0, and 25.0 mg L−1), different dosages of dead algal biomass (0.1, 0.2, and 0.3g) with variation time, pH, and temperature [52]. Chatterjee and Abraham, 2015 [53] observed maximum biosorption in the dried biomass of the Spirogyra sp. (2.5 g L−1) at pH 6.0 when it was treated with 10 mg L−1 chromium concentration for one hour. Sphaeroplea sp. was treated with different chromium concentration with variation in time period, in its natural and acid treated form to study the biosorption capacity. Maximum result was observed at pH 5.0 in the acid treated alga (158.9 mg g−1) than its natural form (29.85 mg g−1) [54].
2.3.3 Diatoms
Sbihi et al., (2012) [56] have observed maximum Cr(VI) biosorption capacity 93.45 mg g−1 of Cr(VI) in Planothidium lanceolatum at a concentration of 0.4-g dried diatoms per liter with a Cr(VI) concentration of 20 mg L−1. Hence, it was proved to be a potent microalga for biosorption of hexavalent chromium.
2.4 Mechanism Involved in Hexavalent Chromium Reduction in Microalgae
Polysaccharide is the basic building block present in the cell walls of prokaryotic and eukaryotic microalgae following proteins and lipids [57]. They contain functional groups like phosphate, amino, sulfhydryl, thiol, and carboxylic groups which are mostly capable of binding to the heavy metals as per their specificity and affinity as seen in Figure 2.2.
Cyanobacteria are able to synthesize metallothioneins (intracellular metal binding proteins) [58]. These are low molecular weight proteins (6,000 to 8,000 amu) rich in cystein residue and bind to metal ion in metal thiolate cluster. It has been reported that cyanobacterial species like Oscillatoria sp., Gleocapsa sp., and Spirulina sp. have the ability of synthesizing siderophores possessing metal chelating properties [50]. It has also been noticed that the heavy metals get deposited in polyphosphate bodies (intracellular storage compartments). Besides this they have also some other advantages like larger surface area, high binding affinity, simple nutrient requirement, and greater mucilage volume which help them to act as biosorbents [55]. Besides this, another mechanism involves the partitioning of metal ion between cell wall and exopolymer sheath [58].
Figure 2.2 Schematic diagram: Mechanism of Cr(VI) reduction through micro-algal biomass.
Except extracellular chromium reduction, intracellular reduction can also be taken as a major mechanism. Algal cells are found to be better source of Cr(VI) reduction, so there must be presence of Cr(VI) reducing enzymes in their cells like bacteria and fungi. The protoplasm of these cells contains some components such as NADH, proteins, low molecular weight carbohydrates, fatty acids, amino acids, and flavoproteins which can completely reduce Cr(VI) to Cr(III). Generally, in chromium-rich region an oxidative stress condition is created inside the cell leading to the generation of several harmful reactive oxygen species (ROS). In order to avoid this situation, the cell starts to produce special kind of protein, enzyme, or any substance which can able to reduce, remove, or transform the Cr(VI). Besides this, microalgae also release electron through photosynthesis and they have a very unique metabolic process compensating the electron for the reduction of Cr(VI) [16, 59].
According to the findings of Nacorda et al., (2010), there is an initial rapid phase of passive extracellular biosorption process [60]. It was carried out following a slower active intracellular bio-absorption. This method is quite similar to the biphasic uptake take place in bacteria, fungi and other microbes. It is also reported that the longer is the incubation time the higher is the amount of Cr(VI) absorbed by Chlorella vulgaris. Another reason behind this bio-absorption may be due to the high storing capacity of the protoplasm.
2.5 Conclusion
Although, chromium is pervasive metal in the environment and Cr(VI) is reported as toxic with several carcinogenic, mutagenic, and a few more hazards, which are affected to behavioral, physiological, biochemical, and immunological aspects. Although bacteria, fungi, and other algal forms are able to convert the hexavalent chromium to trivalent chromium (nontoxic form) but in addition to the common mechanism found in bacteria and all other microbes, the microalgae uses some special mechanism like the residues of flavonoids and the electrons release during photosynthesis for the conversion of hexavalent chromium to trivalent form. Microalgae are potential candidate for the detoxification of Cr(VI), which would be used for the treatment of chromium contaminated water and soil in an eco-friendly manner.
References
1. Hayat, S., Khalique, G., Irfan, M., Wani, A.S., Tripathi, B.N., Ahmad, A., Physiological changes induced by chromium stress in plants: an overview. Protoplasma, 249, 599, 2012.
2. Choudhury, S., Role of Chromite Mineralisation in Orissa. Orissa Rev., 57–60, 2006.
3. Mertz, W., Chromium occurance and its function in biological systems. Physiol. Rev., 49, 165, 1969.
4. Anderson, A.J., Mayer, D.R., Mayer, F.K., Heavy metal toxicities: levels of nickel, cobalt and chromium in the soil and plants associated with visual symptoms and variation in growth of an oat crop. Aust J Agric Pest., 89, 47, 1972.
5. Zhitkovich, A., Voitkun, V., Costa, M., Formation of the amino acid-DNA complexes by hexavalent and trivalent chromium in vitro: Importance of trivalent chromium and the phosphate group. Biochem., 35, 7275, 1996.
6. Costa, M., Toxicity and carcinogenicity of Cr(VI) in animal models and humans. Crit. Rev. Toxicol., 27, 431, 1997.
7. Jordão, C.P., Pereira, J.L., Jham, G.N., Chromium contamination in sediment, vegetation and fish caused by tanneries in the State of Minas Gerais, Brazil. Sci. Total Environ., 207, 1, 1997.
8. Khasim, D.I., Kumar, N.V., Hussain, R.C., Environmental contamination of chromium in agricultural and animal products near a chromate industry. Bull. Environ. Contam. Toxicol., 43, 742, 1989.
9. Dhakate, R. and Singh, V.S., Heavy metal contamination in groundwater due to mining activities in Sukinda valley,
