Objective  A microorganism capable of decomposing malachite green (MG) was identified and its degradation ability studied.

  Method  Indigenous microbes in water samples from aquaculture farms were enriched and domesticated in laboratory to screen for the species that could degrade the organic germicide, MG, a pollutant in aquaculture ponds. Strains showing such capability by physiological and biochemical tests were examined under scanning electron microscopy and identified by a 16S rDNA analysis. A single factor experiment was conducted to determine the effects of temperature, pH, and initial MG concentration on the degradation kinetics of the isolated bacterium. The inoculated culture medium was monitored continuously for the luminescent bacteria toxicity determination and sampled for the high-resolution mass spectrometry analysis to reveal the decomposition pathway.

  Result  An MG-degrading bacterium isolated from the specimens collected at a fish pond in Shanghai was identified as D3 belonging to Citrobacter sp. It exhibited a high rate of growth and MG-degradation on the medium at the pH ranging from 7 to 8 and temperature between 30℃ and 35℃. Its degradation rate under MG 2 mg·L⁻¹ was 96.32% and a half-life of 0.563 0 d with no significant accumulation of leucomalachite green (LMG). Once the MG concentration exceeded 30 mg·L⁻¹, the bacterial growth and degradation effect were significantly hindered. The first-order kinetic degradation function of D3 had a correlation coefficient of 0.916 9–0.963 5. After inoculation for 72 h, the overall toxicity shown on the medium was significantly decreased with more than 50% reduction on the inhibition of the luminescent bacterial growth. Three compounds, 4-dimethylaminodiphenyl ketone (m/z=226.12), N, N-dimethylaniline (m/z=122.10), and 4-dimethylamino-phenol (m/z=138.09), in the metabolites were the degradation intermediates. It appeared that D3 removed the benzene rings in MG step by step to result in those degraded secondary metabolites.

  Conclusion  D3 of Citrobacter sp. might be applied to decontaminate MG in aquaculture ponds.