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Bioremediation for Sustainable Environmental Cleanup

12.2 Microalgal Remediation of Metals

12.2.1 Metal Tolerance and Resistance in Microalgae

Many algae are found to grow in metal-polluted environments. These algae tolerate high

concentrations of heavy metals. Different groups of algae have varying levels of metal tolerance and

resistance (Table 12.2), and they may be genetically and physiologically fixed (Gaur et al. 2001).

The concentration and nature of metals define the toxicity of heavy metals.

Table 12.2. Metal Tolerance by different Microalgae.

Heavy metal

Microalgae

References

Zinc

Chlorophyceae, Anacystis nidulans, Navicula, Caloneis,

Pinnularia

Reed and Gadd 1989, Gaur et al. 2001

Copper

Cyanophyceae, Caloneis, Eunotia, Cyanidium calarium,

Chlorella vugaris

Hall 2002, Priyadarshini et al. 2019

Cadmium

Chlorophyceae, Cyanophyceae, Nostoc linckia

Brinza et al. 2009, Mona et al. 2011b

Mercury

Chlorella, Pseudochloroccum typicum, Phormidium

ambiguum

Gaur et al. 2001, Priyadarshini et al.

2019

Nickel

Cyanidium calarium, Euglena gracilis

García-García et al. 2018

Lead

Pseudochlorococcum typicum, Scenedesmus quadricauda,

Cladophora aglomerata

Priyadarshini et al. 2019

Chromium

Scenedesmus dimorphus, Lyngbya putealis

Nostoc calcicola, Chroococcum

Toranzo et al. 2020, Kiran et al. 2007a,

Kamra et al. 2007, Mona et al. 2011a

Some metals like Cu, Ni, Mn, Fe are essential for the growth and conditioning of microorganisms

as they are required for the cells (Toranzo et al. 2020). While metals like Pb, Cd, Hg and As are

harmful at very low concentrations, essential metals are also toxic for microbial activity beyond

a threshold level (Hall 2002). Essential metals are displaced by non-essential metals because of

their high ionic force from their area of bindings and make complex bindings with working sites of

microbial cell walls.

12.2.2 Biosorption and Bioaccumulation

Metal removal by microalgae may take place through bioaccumulation or biosorption. In biosorption,

the biomass matrix functions as a sorbent. These activities present a low-cost, sustainable and

reversible solution for the remediation of various contaminants by rapid binding on the functional

groups of the biomass surface. It is independent of cellular metabolism. The biosorption process has

the edge over other regular treatment methods due to minimal use of chemicals, operation receiving

ambient conditions, low cost, high efficiency and little biological sludge. Further, auxiliary nutrients

are not required, and there is a potential for renewal of the biomass (sorbent) and regaining of metals

(Kratochvil and Volesky 1998). A critical review on biosorption of metals by algae demonstrated

excellent biosorptive properties of various groups of algae, particularly microalgae. Biosorption

can take place in pH range of 3–9 and temperature (4–45ºC). Biosorbent particle size is found to be

more suitable between 1–2 mm, and due to the small size, the equilibrium states of adsorption and

desorption are attained fast (Michalak et al. 2013).

Biosorption is a more reliable method for remediate of heavy metals (Sweetly et al. 2014). The

conventional adsorption method mediated through activated carbon is one of the efficient processes

for the extraction of contaminants like heavy metals, but there are disadvantages of this method like

non-recyclable and less cost-efficient (Naimabadi et al. 2020). The algae biosorbents have been

used for regaining of heavy metals. Thus, their use in metal bioremediation enhanced the attention