216
A. Naz et al.
9.4 Microbial Growth Inhibition
Wong and Trevors (1988) reviewed the effects of chromium compounds on the
microorganisms including bacteria and algae. Scenedesmus acutus algae stop its
growth at >15 mgL
−1 of Cr
6+ (Travieso et al. 1999), while a mixture of Scenedesmus,
Selenastrum, and Chlorella algae is grown at 100 mgL
−1 of Cr
3+ but could not
tolerate Cr
6+ of same concentration (Brady et al. 1994). Two strains of bacterial
cell Arthrobacter sp. and Bacillus sp. were isolated from contaminated soil with
tannery effluents, and both the bacteria can tolerate 100 mgL
−1 of Cr
6+ in agar
medium, but only Arthrobacter grows in liquid medium. Arthrobacter sp. reduces
up to 50 mgL
−1 of Cr
6+ , while Cr
6+ reduction efficiency of Bacillus sp. is only
20 mgL
−1 . Arthrobacter sp. proven superior to the Bacillus sp. in the ability to
reduce and tolerate toxic Cr
6+ (Megharaj et al. 2003). Staphylococcus epidermidis L02 isolated from chromate contaminated anthropogenic wetland reduced Cr
6+ under
anaerobic condition. Researcher also observed that, the cell number and specific Cr
6+
reduction rate decreases proportionally with the increase of Cr
6+ dose. Due to Cr
6+
toxicity, the microbial cell wall decreases from 50–100 nm as compared to control,
i.e., 100–300 nm (Zakaria et al. 2007), which may have been a reason behind the
reduced hexavalent Cr removal efficiency of certain bacterial species as it limits the
adsorption surface area of the membrane.
Studies have been done on the effect of Cr
6+ on the microorganisms present in the
aerobic mixed cultures present in activated sludge process (ASP) (Gokcay and Yetis
1991). Few researches indicate that even small concentrations (1–5 mgL
−1 ) of Cr
6+
can have a toxic influence on beneficial microbes’ growth (Gokcay and Yetis 1991).
Some studies also refer that after an optimum concentration of Cr
6+ concentration,
there is a gradual decrease in bio-reduction rate as microbes’ lose their tolerance to
Cr- toxicity (Chirwa and Wang 2000). This Cr
6+ dose that affects the Cr-tolerance
of microbes differs from species to species and is an important point of scientific
debate.
Very few works have been done on the remediation of Cr
6+ through anaerobic
microbes. Aerobic condition is proved to be the most favorable for the reduction
of Cr
6+ through aerobic microbes. Cr
6+ reducing dominant genera are Bacillus and
Pseudomonas. Species of Bacillus (B. fusiform NTR9, B.ES29, B. sp. ev3, B. JDM-21, B. sp. CSB-8, B. QC1) and species of Pseudomonas (P. fluorescens, P. fluorescens
LB300, P. Sp. C-171) has been proven to be effective in reducing up to 100 mgL
−1
of Cr
6+ .
Other microbial genera like Arthrobacter, Ochrobactrum, Cellulosimicrobium,
Staphylococcus, Alcaligenes, Streptomyces also found effective to reduce different
concentrations of Cr
6+ . Although few studies have done on Cr
6+ reduction in the
anaerobic condition. Desulfovibrio vulgaris is able to reduce 100% Cr
6+ at the
initial concentration of 20.8 mgL
−1 (Lovley and Phillips 1994), while Achromobacter
sp. Ch-1 and Pannonibacter phragmitetus can able to tolerate high range of Cr
6+
concentration and can reduce Cr
6+ concentration in the range from 250 to 500 mgL
−1
in the both aerobic and anaerobic environmental conditions (Ma et al. 2007; Chai
A. Naz et al.
9.4 Microbial Growth Inhibition
Wong and Trevors (1988) reviewed the effects of chromium compounds on the
microorganisms including bacteria and algae. Scenedesmus acutus algae stop its
growth at >15 mgL
−1 of Cr
6+ (Travieso et al. 1999), while a mixture of Scenedesmus,
Selenastrum, and Chlorella algae is grown at 100 mgL
−1 of Cr
3+ but could not
tolerate Cr
6+ of same concentration (Brady et al. 1994). Two strains of bacterial
cell Arthrobacter sp. and Bacillus sp. were isolated from contaminated soil with
tannery effluents, and both the bacteria can tolerate 100 mgL
−1 of Cr
6+ in agar
medium, but only Arthrobacter grows in liquid medium. Arthrobacter sp. reduces
up to 50 mgL
−1 of Cr
6+ , while Cr
6+ reduction efficiency of Bacillus sp. is only
20 mgL
−1 . Arthrobacter sp. proven superior to the Bacillus sp. in the ability to
reduce and tolerate toxic Cr
6+ (Megharaj et al. 2003). Staphylococcus epidermidis L02 isolated from chromate contaminated anthropogenic wetland reduced Cr
6+ under
anaerobic condition. Researcher also observed that, the cell number and specific Cr
6+
reduction rate decreases proportionally with the increase of Cr
6+ dose. Due to Cr
6+
toxicity, the microbial cell wall decreases from 50–100 nm as compared to control,
i.e., 100–300 nm (Zakaria et al. 2007), which may have been a reason behind the
reduced hexavalent Cr removal efficiency of certain bacterial species as it limits the
adsorption surface area of the membrane.
Studies have been done on the effect of Cr
6+ on the microorganisms present in the
aerobic mixed cultures present in activated sludge process (ASP) (Gokcay and Yetis
1991). Few researches indicate that even small concentrations (1–5 mgL
−1 ) of Cr
6+
can have a toxic influence on beneficial microbes’ growth (Gokcay and Yetis 1991).
Some studies also refer that after an optimum concentration of Cr
6+ concentration,
there is a gradual decrease in bio-reduction rate as microbes’ lose their tolerance to
Cr- toxicity (Chirwa and Wang 2000). This Cr
6+ dose that affects the Cr-tolerance
of microbes differs from species to species and is an important point of scientific
debate.
Very few works have been done on the remediation of Cr
6+ through anaerobic
microbes. Aerobic condition is proved to be the most favorable for the reduction
of Cr
6+ through aerobic microbes. Cr
6+ reducing dominant genera are Bacillus and
Pseudomonas. Species of Bacillus (B. fusiform NTR9, B.ES29, B. sp. ev3, B. JDM-21, B. sp. CSB-8, B. QC1) and species of Pseudomonas (P. fluorescens, P. fluorescens
LB300, P. Sp. C-171) has been proven to be effective in reducing up to 100 mgL
−1
of Cr
6+ .
Other microbial genera like Arthrobacter, Ochrobactrum, Cellulosimicrobium,
Staphylococcus, Alcaligenes, Streptomyces also found effective to reduce different
concentrations of Cr
6+ . Although few studies have done on Cr
6+ reduction in the
anaerobic condition. Desulfovibrio vulgaris is able to reduce 100% Cr
6+ at the
initial concentration of 20.8 mgL
−1 (Lovley and Phillips 1994), while Achromobacter
sp. Ch-1 and Pannonibacter phragmitetus can able to tolerate high range of Cr
6+
concentration and can reduce Cr
6+ concentration in the range from 250 to 500 mgL
−1
in the both aerobic and anaerobic environmental conditions (Ma et al. 2007; Chai