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Bioremediation for Sustainable Environmental Cleanup

10.4.1.6 Biotransformation

Microbes interact with toxic metals and alter their form to a reasonably less toxic form via multiple

biological reactions such as condensation, hydrolysis, formation of new carbon bonds, isomerization

and introduction of functional groups, etc. (Table 10.2) (Guo et al. 2019). In Bacillus species

converts Cr (VI) to Cr (III) under physical conditions like pH (7–9), temperature (between 30ºC

and 40ºC), and Cr (VI) levels (50–250 mg L^–1) (Lei et al. 2019). Under optimal conditions of pH 7

and at 37ºC, this species completely reduced the Cr (VI) concentration of 120 mg L^–1 in just 48 hr.

By using acetate as a carbon source, a heterogenous anaerobic colony containing Anaerolineaceae,

Spirochaeta and Spirochaetaceae exhibited the capability to the reduction of Cr (VI) and V (V)

with high efficiency of 97 and 99.1%, sequentially (Wu et al. 2019). In the presence of hematite and

dissolved organic compounds, Geobacter sulfurreducens showed the capacity to eliminate Cr (VI).

Microorganisms use a sequence of methylation reactions for the conversion of As to volatile

forms. Microorganisms transform arsenic trioxide (As2O3) into volatile toxic tri-methyl arsine

(CH3)3As and increase the mobility and release of As into the environment. Acinetobacter sp.

and Micrococcus sp. are likewise found to be converting poisonous As (III) into nontoxic and less

soluble As (III) and lessen its toxicity. In methylation of Hg, bacteria convert mercuric ions into

methyl mercuric, which increases the bioavailability of Hg through food sources in the aquatic

environment. The biochemical processes of methylation of Hg and SO4

2– in contaminated sites are

significantly associated with reducing the pollutant levels in sediment pore water (Hines et al. 2012).

10.5 Biofilm in Toxic Metal Removal

An aggregation of microbes consisting of one or more strains bonded to a substratum and

encapsulated in a self-synthesized composite including water, proteins, carbohydrates and extra

cellular DNA, is known as a biofilm (Jia et al. 2013). It is expected that diverse microbial species

found in biofilm communities, each with its unique metabolic breakdown mechanism, can degrade

numerous pollutants collectively or individually (Gieg et al. 2014). As they fight for nutrients and

oxygen, biofilm-producing bacteria are adapted to thrive in adverse circumstances. It is considered

that biofilm-mediated remediation is an economical and environment-friendly alternative strategy

of removing toxic chemicals. Biofilms are useful for bioremediation because they accumulate,

precipitate, and eliminate many environmental pollutants. Indigenous populations in severely polluted

regions create bacterial biofilms to help them survive, flourish and cope with the harsh environment.

Gene expression in biofilms varies and is unique, analogous to free-floating planktonic cells. Several

genes play an important role in the degradation of pollutants through different metabolic pathways.

Chemotaxis and motility in bacteria are very important in biofilms formation (Horemans et al.

2013). Microorganisms use various motility behaviours such as chemotaxis, swimming and quorum

sensing to coordinate the movement of microbe toward pollutants and improve biodegradation

(Pratt and Kolter 1999).

Under normal environmental conditions, bacteria surviving in the form of biofilm enclosed

in a polymeric substance are known as exopolysaccharides (EPS) and offer a favorable assembly

to biofilm-producing microorganisms in bioremediation (Lacal et al. 2013, More et al. 2014). The

EPS is composed of polysaccharides, amino acids, fatty acids, nucleic acids, proteins and humic

compounds (Flemming and Wingender 2001). The composition of these EPS is variable in different

species and depends upon growth conditions, surface for attachment for biofilms and type of physical

stress (Jung et al. 2013). Biofilms form filamentous and mushroom-like structures that grow rapidly

in flowing and stagnant water, correspondingly (Miqueleto et al. 2010). In the presence of predatory

protozoa, bacteria frequently adapt to produce biofilms in the form of vast inedible microcolonies

that allow survival and persistence under hostile environments (Edwards et al. 2000). The biofilm

matrix protects microorganisms against adverse environmental conditions, mechanical stress, pH

stress, antibiotics, antimicrobial compounds, dehydration, predators, solvents, UV radiations and