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 H₂/CO₂ and acetate. These intermediates are converted to CH₄/CO₂ 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 CO₂/H₂ and acetate. These observations suggest that the TA-degrading consortium consists of additional syntrophic interactions beyond the standard H₂-producing syntroph–methanogen partnership that may serve to improve community stability.

Three mesophilic bacteria (strains AMX 26B^T, UR374_02 and 12-3^T) isolated respectively from an anaerobic digester, human urine and urban riverside soil were characterized. Cells were Gram-negative, motile, non-sporulating, straight to curved rods with one polar flagellum and had a strictly respiratory metabolism with O₂as the preferential terminal electron acceptor. Phylogenetic analysis based on 16S rRNA gene sequences revealed that all strains clustered within theXanthomonadaceaebranch of theProteobacteria. Isolates AMX 26B^Tand UR374_02 exhibited 100 % 16S rRNA gene sequence similarity and both were related to strain 12-3^T(99·6 % similarity). The closest relative of all the isolates wasPseudoxanthomonas broegbernensisDSM 12573^T(similarity 97·1–97·5 %), and they were equidistantly related toXanthomonasspecies (95·4–96·6 %),Stenotrophomonasspecies (95·3–96·1 %) andPseudoxanthomonas taiwanensisATCC BAA-4040^T(95·3–95·4 %). Chemotaxonomic and biochemical data (branched-chain cellular fatty acid pattern without C_13 : 0iso 3-OH, ubiquinone with eight isoprenoid units, limited range of substrates used, ability to reduce nitrite but not nitrate with the production of N₂O) supported their affiliation to the genusPseudoxanthomonas. The results of DNA–DNA hybridization and/or phenotypic analysis allowed them to be differentiated from the twoPseudoxanthomonasspecies with validly published names and showed that strain 12-3^Twas genomically and phenotypically distinct from the other two isolates. On the basis of these results, two novel species of the genusPseudoxanthomonasare proposed:Pseudoxanthomonas mexicanasp. nov., consisting of strains AMX 26B^T(=ATCC 700993^T=CIP 106674^T=JCM 11524^T) (type strain) and UR374_02 (=DSM 15133), andPseudoxanthomonas japonensissp. nov., consisting of strain 12-3^T(=CCUG 48231^T=CIP 107388^T=JCM 11525^T). The report of these two novel species leads to the emendation of the description of the genusPseudoxanthomonasand the re-evaluation of the phenotype ofP. broegbernensisDSM 12573^Tnecessitates the emendation of its description.

A thermophilic terephthalate-degrading methanogenic consortium was successfully enriched for 272 days in an anaerobic hybrid reactor, and the microbial structure was characterized using terminal RFLPs, clone libraries and fluorescence in-situ hybridization with rRNA-targeted oligonucleotide probes. All the results suggested that Methanothrix thermophila-related methanogens, Desulfotomaculum-related bacterial populations in the Gram-positive low-G+C group, and OP5-related populations were the key members responsible for terephthalate degradation under thermophilic methanogenic conditions except during periods when the reactor experienced heat shock and pump failure. These perturbations caused a significant shift in bacterial population structure in sludge samples taken from the sludge bed but not from the surface of the packing materials. After system recovery, many other bacterial populations emerged, which belonged mainly to the Gram-positive low-G+C group and Cytophaga–Flexibacter–Bacteroides, as well as β-Proteobacteria, Planctomycetes and Nitrospira. These newly emerged populations were probably also capable of degrading terephthalate in the hybrid system, but were out-competed by those bacterial populations before perturbations.