The Biodegradation of Azo Dyes by Actinobacteria - Microbial Degradation of Synthetic Dyes in Wastewaters - page 295

A small amount of dye in the water (<1 mg l − 1 for some dyes) can cause a very
visible color change (Banat et al. 1996 ), which affects not only the aesthetic aspects
of the water, but also raises environmental concerns (Lu et al. 2010 ; Modi et al.
2010 ). Previously, several researchers have reported the toxicity of synthetic dyes
on ecosystems (Fraga et al. 2009 ; Osugi et al. 2009 ; Dafale et al. 2010 ). The
ef uents containing dyes affect the photosynthetic activity of aquatic plants and

algae by changing the light and gas penetration into water bodies. Hence, waste-
water is also considered very toxic to aquatic organisms, resulting in the disruption
of the ecological balance (Modi et al. 2010 ). Also, azo dyes signi

cantly increase
the chemical and biological oxygen demand in water bodies, which also affects
aquatic life indirectly (Sun et al. 2009 ).
Some dyes are reported to cause irritation allergy, dermatitis, eye irritation and
respiratory tract problems in human beings (Keharia and Madamwar 2003 ; Kousha
et al. 2012 ). Induction of bladder cancer in humans, splenic sarcomas, hepatocar-
cinomas and nuclear anomalies in some experimental animals were commonly
observed (Ra s

was of common occurrence in highly industrialized locations and linked to use of
azo dyes (Wolff and Oehme 1974 ; Chung et al. 1978 ). Furthermore, the mutagenic
activity of disperse azo dyes was also observed in Salmonella species (Ferraz et al.
2010 ). These studies suggest that the azo compounds in the water and the soil
environment can affect the activity and composition of the microbial communities
which are sensitive to these toxins. Similarly, Chen ( 2006 ) found that aromatic
amines, which are common metabolic products of reductive cleavage of azo dyes,
were found highly toxic to bacterium Pseudomonas luteola.
Recent studies have indicated that azo dyes also cause phytotoxicity. Ayed et al.
( 2011 ) reported a reduction of 55 and 30 % in the germination of Triticum aestivum
and Sorghum bicolor respectively with a Methyl Red azo dye. However, degra-
dation products of the dye were found to be non-toxic to the germination of the both
plant species. On the contrary, Chaube et al. ( 2010 ) observed that even the deg-
radation products of Direct Violet 51 and Tartrazine were toxic and a signi

i et al. 1997 ; Puvaneswari et al. 2006 ). Intestinal cancer in 1970 '

cant
decrease in the germination and growth of Triticum aestivum and Phaseolus mungo
was observed. Similarly, the application of dye-contaminated water inhibited the
growth of peas and wheat plants in laboratory conditions, although the same water,
after biological treatment, improved plant growth (Khalid et al. 2013 ; Saba et al.
2013 ).

3 Biodegradation of Azo Dyes by Actinobacteria

Actinomycetes were used mainly for the production of bioactive compounds, such
as antibiotics and neglected earlier in relation to their biodegradation capacities.
Now-a-days, they are becoming potential degraders of organic compounds,
including azo dyes (Table 2 ). These bacteria have gained attention all over the
world because of their versatile nature and enzyme system which is required for the