Adaptation of a methanogenic consortium to arsenite inhibition

Abstract

Arsenic (As) is a ubiquitous metalloid known for its adverse effects to human health. Microorganisms are also impacted by As toxicity, including methanogenic archaea, which can affect the performance of process in which biological activity is required (i.e. stabilization of activated sludge in wastewater treatment plants). The novel ability of a mixed methanogenic granular sludge consortium to adapt to the inhibitory effect of arsenic (As) was investigated by exposing the culture to approximately 0.92 mM of As^III for 160 d in an arsenate (As^V) reducing bioreactor using ethanol as the electron donor. The results of shaken batch bioassays indicated that the original, unexposed sludge was severely inhibited by arsenite (As^III) as evidenced by the low 50% inhibition concentrations (IC₅₀) determined, i.e., 19 and 90 μM for acetoclastic- and hydrogenotrophic methanogenesis, respectively. The tolerance of the acetoclastic and hydrogenotrophic methanogens in the sludge to As^III increased 47-fold (IC₅₀ = 910 μM) and 12-fold (IC₅₀= 1100 μM), respectively, upon long-term exposure to As. In conclusion, the methanogenic community in the granular sludge demonstrated a considerable ability to adapt to the severe inhibitory effects of As after a prolonged exposure period.

Keywords: Acclimation; adaptation; arsenic; arsenite; inhibition; methanogenic; toxicity.

Fig. 1

Methane (CH₄) production in the experiments assessing the toxicity of varying concentrations of arsenite (As^III) (expressed as μM) on: (A) acetoclastic methanogens from the unexposed granular sludge: (□) 0; (x) 14; (◇) 20;; (■) 39; (△) 82; (+) 111; (−) 152; and (○) 213; (B) hydrogenotrophic methanogens from the unexposed granular sludge: (□) 0, (x) 39, (◇) 207, (■) 777, (△) 1343, and (+) 1861. The horizontal dotted black lines represent the maximum theoretical concentration of CH₄ that can be produced. The error bars represents the standard deviation of duplicates assays, or quadruplicates in the case of the controls.

Fig. 2

Arsenite (As^III) toxicity to methanogens. Normalized maximum specific acetoclastic (A) and hydrogenotrophic (B) methanogenic activity of the unexposed, original granular sludge, as a function of the initial concentration of soluble As^III. The 50% inhibition concentration (IC₅₀) values are shown in the corresponding graph. The error bars represents the standard deviation of duplicates assays, or quadruplicates in the case of the controls.

Fig. 3

Adaptation of methanogens to arsenic toxicity. Normalized maximum specific acetoclastic- (A) and hydrogenotrophic (B) methanogenic activity of the granular sludge exposed to 0.92 mM arsenite (As^III) (due to arsenate (As^V)-reduction) for 160 d in a continuous-flow bioreactor fed with 4.4 mM ethanol and 1.02 mM As^V as a function of the initial concentration of soluble As^III. The 50% inhibition concentration (IC₅₀) values are shown in the corresponding graph. The error bars represents the standard deviation of duplicates assays, or quadruplicates in the case of the controls.