Metal(loid)s Toxicity and Bacteria Mediated Bioremediation 177

elevated doses of dangerous chemicals and pollutants better than planktonic cells (Davey and

O’toole 2000, Matz and Kjelleberg 2005).

Biofilm-producing bacteria are efficient to bioremediate as they are imprisoned in an EPS

that also immobilizes pollutants during breakdown (Mah and O’Toole 2001). Due to the low

concentration of oxygen towards the center, all the microbes, including aerobes and anaerobes,

heterotrophs (nitrifying bacteria) and sulfate reducers are present in close proximity in the three-

dimensional network of EPS, facilitating quicker degradation of various contaminants in natural and

artificial systems (Sutherland 2001). Toxic metals are removed from the aqueous environment using

EPS from cyanobacteria as a biosorbent (de Philippis et al. 2011). Several enzymes are present in

biofilms EPS, which detoxify heavy metals and organic compounds. Due to the presence of many

negatively charged functional groups in EPS, it acts as a trap for metals and metalloids, allowing

the formation of chelates with toxic metals and organic contaminants and thus facilitating their

elimination (Li and Yu 2014). Different metals like zinc, lead, nickel, magnesium, cadmium, iron,

manganese and copper are known to bind to EPS (Pal and Paul 2008). Nutrient restriction may

lead to enhancement in EPS and copper production and allow to absorb more pollutants from the

polluted site. The EPS of phosphorus-accumulating bacteria in biofilms serves as a reservoir and

aids in the bioremediation and recovery of phosphorus from wastewater (Zhang et al. 2013).

Biofilms may be observed in natural environments such as soil, plants, sediments, streams,

ponds, rivers and man-made environments. Fungus, bacteria, protozoa and algae make up biofilms

that form on the water’s surface.

During bloom periods, fragile structures termed flocs are formed. The floc-activity of activated

sludge plants is used to treat municipal wastewater. Slow-sand filters are used to separate organic

chemicals and metals from the natural environment by using biofilms (Burmølle et al. 2006).

Planctomycetes found in seaweed biofilms in coastal habitats can be used to remove nitrogen from

wastewater by anammox reactions by converting ammonia to dinitrogen anaerobically (Kartal et al.

2010).

Biofilms are extensively used for risk assessment or as an indication for monitoring and assessing

toxic metal pollution. There may be a change in biofilm structural and physiological functions in the

presence of toxic compounds. Microbial biofilms could accumulate contaminants, and contaminants

offer an easy attachment site for biofilms. They are applied as an indicator system (Bengtsson and

Øvreås 2010). Microbes are the first organisms in aquatic habitats to interact with minerals and

pollutants, and as a result, biofilms may be used as a risk assessment tool in aqueous bodies (Fuchs

et al. 1997). Several biofilm marker properties, including biomass production, microbial species,

photosynthetic machinery, pigment formation and enzyme activity, may be used in monitoring

environmental pollution. The microbial species present in river biofilms vary depending on the

season and as well as the level of pollution (Peacock et al. 2004). Contaminated sites with various

toxic metals such as zinc and cadmium may also affect the microbial species; hence, it could be

related to the level of contamination (Brümmer et al. 2000). Biofilm sampling has been shown

to provide a more accurate assessment of heavy metal pollution in aquatic microbial populations

(Bricheux et al. 2013). The algal biofilm could be used as an indicator of pollution due to changes

in biomass in the presence of heavy metals (Ancion et al. 2013). The pigment-formation property

of the biofilm may be altered after exposure to hazardous chemicals, which serves as a biomarker

(Navarro et al. 2002, Dorigo et al. 2004, Dewez et al. 2008).

10.6 Bioremediation Using Extremophiles

During the past several decades, the methods for sustainable bioremediation of toxic metals have

been explored using extremophilic bacteria. Acidophilic bacteria capable of sliving in extremely

acidic environments are used as host cultures for the bioremediation of toxic chemicals (Gumulya

et al. 2018, Saavedra et al. 2020). Acidothiobacillus species, a most ubiquitous acidophilic

and chemolithotrophic bacterium, have been used to develop bioremediation techniques.