Multiple syntrophic interactions in a terephthalate-degrading methanogenic consortium

Abstract

Terephthalate (TA) is one of the top 50 chemicals produced worldwide. Its production results in a TA-containing wastewater that is treated by anaerobic processes through a poorly understood methanogenic syntrophy. Using metagenomics, we characterized the methanogenic consortium inside a hyper-mesophilic (that is, between mesophilic and thermophilic), TA-degrading bioreactor. We identified genes belonging to dominant Pelotomaculum species presumably involved in TA degradation through decarboxylation, dearomatization, and modified β-oxidation to H(2)/CO(2) and acetate. These intermediates are converted to CH(4)/CO(2) by three novel hyper-mesophilic methanogens. Additional secondary syntrophic interactions were predicted in Thermotogae, Syntrophus and candidate phyla OP5 and WWE1 populations. The OP5 encodes genes capable of anaerobic autotrophic butyrate production and Thermotogae, Syntrophus and WWE1 have the genetic potential to oxidize butyrate to CO(2)/H(2) and acetate. These observations suggest that the TA-degrading consortium consists of additional syntrophic interactions beyond the standard H(2)-producing syntroph-methanogen partnership that may serve to improve community stability.

Figure 1

TA-degrading laboratory-scale anaerobic hybrid bioreactor. (a) Schematic of the laboratory-scale anaerobic TA-degrading hybrid reactor operated with a temperature gradient from ∼46 °C at the bottom to 50 °C in the upper zone. Inserts illustrate freshly grown biofilm biomass on the surface of the media after 2 and 11months of enrichment; and (b) performance of the reactor over 480-day operation. Under the initial operational conditions (that is, TA-loading rate of 0.70–0.78 gTA/d.l, and hydraulic retention time (HRT) of 4 days), TA removal efficiency was gradually improved to 72.7% by day 124. By shortening the HRT (3 d on day 127, 2 d on day 168 and then to 1.5 d on day 182) and increasing the TA loading concentration (to 3.2 g on day 364), the TA loading rate was increased to 2.13 gTA/d.l by day 364. Concurrently, the TA removal efficiency increased over the operation period reaching a 99% removal efficiency by day 308. During the entire operation no sulfate reduction activity was detected. Samples were removed at the indicated time points (arrows) and the genomic DNA was extracted for 16S rRNA clone library construction and metagenomics analysis.

Figure 2

Bacterial population dynamics of the TA-degrading bioreactor as revealed by 16S rRNA clone library. Samples (number of 16S rRNA sequences) from inner to outer of the ring chart were day 221 biofilms (287), day 280 biofilms (254), day 346 biofilms (337), day 346 sludge bed (289), day 430 biofilms (352) and day 430 sludge bed (287).

Figure 3

Metabolic reconstruction of the TA-degrading syntrophic community. The metagenomic data revealed pathways for the degradation of the aromatic (TA) compound, the recycling of the intermediates (H₂/CO₂, acetate and butyrate) and the subsequent syntrophic methanogenesis.