RESUMO
We present the results of an investigation into the special traits of conversion of azo dies golden yellow, acid orange, methyl orange, and methyl red under anaerobic conditions in comparison to aerobic conditions. In the presence of oxygen, only methyl red underwent decomposition, while under oxygen-free conditions, all remaining substances were fully discolored under the action of a methanogenous consortium of organisms. The products of reduction of the azo bond are determined in the case of each die. Introduction of additional acceptors of electrons (sulfate and nitrate) had a negative influence on the discoloration of azo dies. Addition of ethanol as an available organic cosubstrate accelerated decomposition of azo dies both under methanogenous and sulfate- and nitrate-reducing conditions. There is no direct correlation between the rates of conversion of azo dies under anaerobic conditions or their toxicity to acetoclastic methanogens. Changes in the morphological composition of the community discoloring an azo die depended on the duration of its impact on microorganisms. The mechanism of the reduction of the azo bond under the action of substances acting as mediators is explained. These substances are products of the metabolism of the microbial community in anaerobic conditions. It is shown that the supposed mediators NADH and sulfide efficiently discolor azo dies in a cell-free system, while riboflavin significantly increased the rate of conversion of substrates in recurrent cycles of discoloration only in the presence of an anaerobic microbial consortium.
