Syntrophic H₂ production enhances the performance of primarily acetate-supplemented reactors treating sulphate contaminated solutions

Abstract

Biological sulfate reduction (BSR) represents a promising bioremediation strategy, yet the impact of metabolic interactions on performance has been largely unexplored. Here, genome-resolved metagenomics was used to characterise 17 microbial communities associated with reactors operated with defined sulfate-contaminated solutions. Pairs of reactors were supplemented with lactate or with acetate plus a small amount of fermentable substrate. At least thirty draft quality genomes, representing all the abundant bacteria, were recovered from each metagenome. All of the 22 SRB genomes encode genes for H2 consumption. And of the total 163 genomes recovered, 130 encode 321 NiFe and FeFe hydrogenases. The lactate-supplemented packed-bed bioreactor was particularly interesting as it resulted in stratified microbial communities that were distinct in their predominant metabolisms. Pathways for fermentation of lactate and hydrogen production were enriched towards the inlet whereas increased autotrophy and acetate-oxidizing SRB were evident towards the end of the flow path. We hypothesized that high sulfate removal towards the end of the flow path, despite acetate being an electron donor that typically sustains low SRB growth rates, was stimulated by H2 consumption. This hypothesis was supported by sustained performance of the predominantly acetate-supplemented stirred-tank reactor, which was dominated by diverse fermentative, hydrogen-evolving bacteria and low-abundance SRB capable of acetate and hydrogen consumption. We conclude that the performance of BSR reactors supplemented with inexpensive acetate can be improved by the addition of a low concentration of fermentable material due to stimulation of syntrophic relationships among hydrogen-producing non-SRB and dual hydrogen- and acetate-utilising SRB.

Integrated Kinetic Modelling and Microbial Profiling Provide Insights Into Biological Sulfate-Reducing Reactor Design and Operation

“…However, the strong agreement between our electron donor predictions based on the sulfate and VFA concentrations suggests that if sulfate reduction is linked to oxidation of these electron donors is likely to be minimal. This is also substantiated by the limited short- and long-term effect on performance of the reduction in yeast extract from 1.0 to 0.4 g/L before the start of the HRT study ( Hessler et al, 2021 ). It is also possible that Desulfovibrio (6) may effectively compete for both lactate and acetate in mixed communities.…”

“…The observed and predicted acetate concentrations leaving the (D) inlet (E) middle and (F) effluent zones of the lactate-supplemented UAPBR are shown. The predicted acetate concentrations were calculated based on the assumptions described in text and that the oxidation of 0.4 g/L yeast extract led to the generation of 268 mg/L acetate, as described in Hessler et al (2021) . Error bars represent one standard deviation from the mean ( n > 4).…”

“…The oxidation of the 0.4 g/L yeast extract, supplied in the medium as a source of micronutrients, was previously found to consistently produce 268 mg/L acetate ( Hessler et al, 2021 ). This estimation could be made after the yeast extract concentration was reduced from 1.0 to 0.4 g/L.…”

“…These OTUs, therefore, are strong candidates for microorganisms which compete for yeast extract components and citrate, the oxidation of which appear to be restricted to the inlet zone of this reactor. Our previous genome-resolved metagenomics study also implicated Spirochaetota in amino acid and citrate fermentation based on encoded metabolism ( Hessler et al, 2021 ). This is consistent with the observed representation of the Spirochaetota in both acetate and lactate UAPBRs inlet zones where the concentrations of these substrates would have been greatest.…”

“…Lastly, in a previous work we performed genome-resolved metagenomics on samples from these six bioreactors at a four-day HRT ( Hessler et al, 2021 ). In this metagenomic study we recovered and assembled 163 recovered microbial genome bins and presented analysis of their encoded metabolisms.…”