A study of zinc metal toxicity on the cellulolytic bacteria in anaerobic passive treatment systems

Abstract

The effective use of anaerobic passive treatment systems (APTS), such as sulfate-reducing bioreactors, to treat acid mine drainage will help to mitigate water contamination from mines located in remote areas as well as cut current treatment costs. One draw back to these systems has been the inhibition of sulfate reduction with high concentrations of metals. APTS contain a complex microbial ecosystem, and metal toxicity could be indirectly affecting sulfate-reduction by inhibiting other important microbes. If microbes such as the cellulolytic -fermenting bacteria are inhibited from producing viable substrate for the sulfate-reducing bacteria, then the rate of sulfate reduction over time in APTS will ultimately decline. We examined the toxic effect of zinc, a common metal found in acid mine drainage, on a pure culture of Cellulomonas flavigena, a cellulolytic - fermenting bacteria. Serum bottles containing C. flavigena, at two protein concentrations of 250 and 500 mg/L, were exposed to initial zinc concentrations of 0, 20, and 40 mg/L and monitored over a 9 hour period. The extent of inhibition on C. flavigena activity correlated best (r2=0.93) with the mass ratio of zinc uptake to cell protein. Final zinc concentrations ranged from 0.9 to 2.2 mg/L. Zinc uptake was operationally defined as the total zinc removed from solution and includes sorption and internalization. Initial and final dissolved zinc concentration did not correlate well with extent of inhibition. In the presence of higher biomass the relative rate of glucose utilization was 20 to 50% higher in the presence of zinc than at lower biomass concentration. The concurrent internalization of metals with sorption and precipitation processes can produce inhibition in the presence of low metal concentration. Thus low effluent metal concentration may not be indicative of the extent of inhibition experienced by the microbes. The inhibitory effect of metals on cellulolytic - fermenting bacteria is an important aspect to consider when establishing the limitations of sulfate reducing biozones.