Effects of hydrogen and acetate on benzene mineralisation under sulphate-reducing conditions

Abstract

Syntrophic mineralisation of benzene, as recently proposed for a sulphatereducing enrichment culture, was tested in product inhibition experiments with acetate and hydrogen, both putative intermediates of anaerobic benzene fermentation. Using [ 13 C 6 ]-benzene enabled tracking the inhibition of benzene mineralisation sensitively by analysis of 13 CO 2 . In noninhibited cultures, hydrogen was detected at partial pressures of 2.4 Â 10 À6 AE 1.5 Â 10 À6 atm. Acetate was detected at concentrations of 17 AE 2 mM. Spiking with 0.1 atm hydrogen produced a transient inhibitory effect on 13 CO 2 formation. In cultures spiked with higher amounts of hydrogen, benzene mineralisation did not restart after hydrogen consumption, possibly due to the toxic effects of the sulphide produced. An inhibitory effect was also observed when acetate was added to the cultures (0.3, 3.5 and 30 mM). Benzene mineralisation resumed after acetate was degraded to concentrations found in noninhibited cultures, indicating that acetate is another key intermediate in anaerobic benzene mineralisation. Although benzene mineralisation by a single sulphate reducer cannot be ruled out, our results strongly point to an involvement of syntrophic interactions in the process. Thermodynamic calculations revealed that, under in situ conditions, benzene fermentation to hydrogen and acetate yielded a free energy change of DG 0 = À 83.1 AE 5.6 kJ mol À1 . Benzene mineralisation ceased when DG 0 values declined below À 61.3 AE 5.3 kJ mol À1 in the presence of acetate, indicating that ATP-consuming reactions are involved in the pathway.

Protein-SIP enables time-resolved analysis of the carbon flux in a sulfate-reducing, benzene-degrading microbial consortium

“…A comparison of the degradation and formation rates revealed that around 90% of the carbon of 13 C-benzene was degraded to 13 CO 2 . This agrees with previous findings, where also 87-95% of carbon derived from benzene was converted to CO 2 (Herrmann et al, 2010;Rakoczy et al, 2011):…”

“…Acetyl-CoA is then either further degraded to CO 2 or released as acetate by the Clostridiales, depending on the energy level. Deltaproteobacteria further metabolized the putatively released acetate, and interspecies hydrogen transfer possibly takes place between Clostridiales and Deltaproteobacteria as suggested previously (Rakoczy et al, 2011). Our study illustrates the power of protein-based SIP, in combination with a comprehensive metagenome data set, to decipher the carbon flow within complex microbial communities.…”

“…The group I organism might be able to switch between the described pathways of metabolization of acetylCoA, depending on the level of reducing equivalents present. Assuming a syntrophic interaction with interspecies hydrogen transfer, the release of acetate would be necessary to prevent the accumulation of hydrogen that would otherwise inhibit a further elimination of reducing equivalents (Rakoczy et al, 2011). Strong indications for acetate and hydrogen as intermediate metabolites of benzene mineralization by the studied consortium have been recently gained in inhibition experiments, in which acetate and hydrogen supplementation transiently stopped benzene mineralization, even at low concentrations (Rakoczy et al, 2011).…”

“…Assuming a syntrophic interaction with interspecies hydrogen transfer, the release of acetate would be necessary to prevent the accumulation of hydrogen that would otherwise inhibit a further elimination of reducing equivalents (Rakoczy et al, 2011). Strong indications for acetate and hydrogen as intermediate metabolites of benzene mineralization by the studied consortium have been recently gained in inhibition experiments, in which acetate and hydrogen supplementation transiently stopped benzene mineralization, even at low concentrations (Rakoczy et al, 2011). The hypothesis of a syntrophic interaction is also in accordance with a recent study of van der Zaan et al (2012), who found that Peptococcaceae were involved in anaerobic degradation of benzene with different electron acceptors.…”

Analytical model for BTEX natural attenuation in the presence of fuel ethanol and its anaerobic metabolite acetate

“…The biodegradation of ethanol led to the formation and accumulation of acetate (Figure 1). Although not toxic, the presence of acetate as low as 0.3 mM (Rakoczy et al, 2011) could hinder the thermodynamic feasibility of BTEX degradation by syntrophic microorganisms typically associated with sulfate-reducing and methanogenic conditions (Ulrich and Edwards, 2003). Benzene and toluene degradation in our column experiments only occurred after the onset of acetate removal (Figure 1).…”