Effects of the Headspace Gas Composition on Anaerobic Biotransformation of o-, m-, and p-Toluic Acid in Sediment Slurries

Abstract

Composition of the headspace gas affected the biotransformation pattern of toluic acid isomers in anoxic sediment slurries. Under an N2 atmosphere, o- and m-, and p-toluic acid (20–25 mg L−1) were biotransformed in 100 days, 77 days, and 148 days, respectively, with a lag period of 50 days, 49 days, and 50 days, respectively. Under a CO2 atmosphere, the same toluic acid isomers were biotransformed by the sediment microorganisms in 16–25 days without a lag period. CO2 thus increased the biotransformation rates. The presence of H2, on the other hand, decreased the biotransformation rates: in most cases, adding H2 gas (5% and 20% to the N2 and CO2 atmospheres, respectively) not only increased the lag period but also decreased the maximum biotransformation rates. These effects were especially noticeable for the N2 atmosphere. Under N2, the maximum biotransformation rates of the toluic acid isomers were in the order o-toluic acid>m-toluic acid>p-toluic acid. However, under CO2, the maximum biotransformation rates were reversed, i.e., p-toluic acid>m-toluic acid>o-toluic acid. The presence of the methanogen inhibitor bromoethanesulfonic acid (BESA) slowed the biotransformation rates of p-toluic acid, and this together with the population dynamics of the acetogenic bacteria in the sediment slurries, suggested that acetogenic bacteria were involved in the degradation pathway. However, their exact role remains unclear.