Adaptation of a methanogenic consortium to arsenite inhibition

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.