Evaluation of the gene expression changes in Nile tilapia (Oreochromis niloticus) as affected by the bio-removal of toxic textile dyes from aqueous solution in small-scale bioreactor

A laboratory-scale bioremediation unit was designed, built and tested for the bio-removal of several Direct textile dyes. Four experiments were carried out to assess the efficiency of the bioremediation unit using Aspergillus niger fungal strain. Three commonly used Direct dyes and textile dyes mixture (simulated effluent: Direct brown, Direct violet, Direct green) were tested in this study. The strain of A. niger was efficient in the removal of the three Direct dyes. The decolorization percentages of the dyes after 24 h of incubation were 56.2, 51.7, and 95.4% for Direct brown, Direct green, Direct violet dyes, respectively. The percentages increased up to 79.4, 86.4, and 96.7% after 72 h of incubation for the same dyes, respectively. The results also showed that the fungal strain reduced the chemical oxygen demand values of simulated dye effluents from 165 to 564 mg/l with most of the dyes. The assessment of bioremediation products on biomodel was conducted using a fresh water fish. The liver and brain of Nile tilapia were tested to evaluate the expression of genes coding for several proteins related to stress such as metallothioneins (MTs), cytochrome P450 (CYP450), and heat shock proteins (HSPs). To assess the alterations in the gene expression, ten animals from each group were killed after 4 weeks of treatment. The results revealed significant increases in the brain and hepatic mRNA levels of all stress protein genes MT, CYP450, Hsp70a, b, and Hsp47 in the fish groups treated with industrial Direct violet, green, and brown dye water. Exposure of tilapia to bioremediation products after treatment with A. niger fungi reduced the over-expression of the stress protein genes in the brain and liver tissues.