Impacts of conductive materials on microbial community during syntrophic propionate oxidization for biomethane recovery

Abstract

Propionate is one of the most important intermediates in anaerobic digestion, and its degradation requires a syntrophic partnership between propionate-oxidizing bacteria and hydrogenotrophic methanogens. Anaerobic digestion efficiency can be improved by direct interspecies electron transfer (DIET) through conductive materials. This study aimed to investigate the effects of DIET on syntrophic propionate oxidization under room temperature (20°C) and reveal the syntrophic partners. Firstly, conventional anaerobic consortium and conductive material-enriched consortium were tested for DIET under high H 2 partial pressure. The latter supplemented with granular activated carbon (GAC) can mitigate H 2 inhibition through DIET. Secondly, a DIET consortium was enriched for testing GAC and magnetite, both showed DIET facilitation. Microbial communities in GAC-and magnetite-supplemented reactors were similar. Syntrophic propionate-oxidizing bacteria, for example, Smithella (3.9%-9.9%) and a genus from the family Syntrophaceae (1.9%-3.6%) and methanogens Methanobacterium (30.3%-75.2%), Methanolinea (8.5%-25.2%), Methanosaeta (11.4%-36.7%), and Candidatus Methanofastidiosum (3.6%-6.6%), were predominant. Functional genes for cell mobility and membrane transport (3.3% and 9.5% in control reactor) increased with GAC (3.7% and 11.1%, respectively) and magnetite (3.7% and 10.9%, respectively) addition. Syntrophic propionate-oxidizing bacteria and methanogenesis partners were revealed by co-occurrence network, for example, Methanobacterium with Smithella, Syntrophobacter, Dechloromonas, and Trichococcus, signifying the importance of the syntrophic partnership in DIET environment.

Sparking Anaerobic Digestion: Promoting Direct Interspecies Electron Transfer to Enhance Methane Production

“…Once interspecies electrical connections are established, electron transfer via DIET should be faster than diffusive electron exchange via soluble electron shuttles. DIET is also less sensitive to the accumulation of H 2 ( Cruz et al., 2014 ; Guo et al., 2020 ; Xu et al., 2016 ; Zhang et al., 2020c ; Zhao et al., 2017c ). Thus, DIET is expected to be especially beneficial when rates of organic loading are high and more rapid metabolism of fermentable substrates releases H 2 at elevated levels.…”

“…For example, the metabolism of non-fermentable substrates such as propionate are less susceptible to inhibition by high H 2 concentrations in the presence of conductive materials, suggesting that interspecies electron transfer is proceeding via DIET rather than interspecies hydrogen transfer ( Cruz et al., 2014 ; Guo et al., 2020 ; Xu et al., 2016 ; Zhang et al., 2020c ; Zhao et al., 2017c ). In some instances the enhancement of methane production following the addition of conductive materials is associated with the enrichment of microbial populations for which pure culture representatives are known to participate in DIET ( Guo et al., 2020 ; Lei et al., 2016 ; Lei et al., 2019 ; Lin et al., 2017 ; Ma et al., 2020a ; Mei et al., 2018 ; Sun et al., 2020 ; Tian et al., 2017 ; Wang et al., 2018a , 2018b , 2019b ; Xing et al., 2020 ; Xu et al., 2016 ; Yang et al., 2017 ; Zhang et al., 2018b , 2020a , 2020c ; Zhao et al., 2016a , 2017c ). However, in very few instances has the in situ physiological state of the microbial community been investigated in sufficient detail to verify the predominance of DIET ( Rotaru et al., 2014a ), and in those studies the impact of conductive materials was not investigated.…”

“…In some instances, enhanced methane production is associated with an increased enrichment of microbes closely related to microbes that are available in pure culture that are known to participate in DIET, such as Geobacter , Syntrophus , Methnothrix , and Methanoarcina species ( Baek et al., 2016 ; Barua and Dhar, 2017 ; Guo et al., 2020 ; Kato et al., 2012a ; Lei et al., 2019 ; Lin et al., 2020 ; Ma et al., 2020a ; Mei et al., 2018 ; Shen et al, 2020 ; Tian et al., 2017 ; Wang et al., 2018a , 2018b , 2019b ; Xing et al., 2020 ; Xu et al., 2015 , 2016 ; Yan et al., 2018 ; Yang et al., 2017 ; Zhang et al., 2018b , 2020c ; Zhao et al., 2016a , 2017c ; Zhuang et al., 2015b ). However, in many other studies the addition of conductive materials to samples from anaerobic digesters or other methanogenic environments has led to the enrichment of other bacteria and methanogens ( Baek et al., 2015 ; Barua et al., 2018 ; Fu et al., 2018 ; Guo et al., 2018 , 2020 ; Hu et al., 2017 ; Im et al., 2019 ; Jing et al., 2017 ; Lei et al., 2016 , 2018 ; Lin et al., 2018 ; Luo et al., 2015 ; Ma et al., 2020b ; Mostafa et al., 2020 ; Ren et al., 2020 ; Rotaru et al., 2018 ; Sun et al., 2020 ; Tan et al., 2015 ; Usman et al., 2019 ; Wang et al., 2019a , 2020a ; Xia et al., 2019 ; Xu et al., 2018 ; Yamada et al., 2015 ; Yang et al., 2017 ; Yan et al., 2018 ; Yang et al., 2015 , 2016 , 2020 ; …”

“…Known characteristics of some of the enriched microbes further support the possibility that they are involved in DIET. For example, Syntrophus , Smithella , Syntrophobacter , Syntrophorhabdus , Syntrophomonas , Desulfatibacillum , Syntrophaceticus , Flexistipes , Desulfurivibrio , and Calditerrivibrio ( Walker et al., 2018 , 2020 ) have genes likely to yield e-pili, and several of these microbes were enriched in studies in which conductive materials were added to methanogenic samples ( Baek et al., 2016 ; Capson-Tojo et al, 2018 ; Fu et al., 2018 ; Guo et al., 2020 ; Guo et al., 2020 , 2020 ; Lei et al., 2016 ; Lim et al., 2020 ; Luo et al., 2015 ; Ma et al., 2020a , 2020b ; Mostafa et al., 2020 ; Wang et al., 2019b , 2020a ; Xia et al., 2019 ; Xing et al., 2020 ; Xu et al., 2018 ; Zhang et al., 2018b , 2020c ; Zhang and Lu, 2016 ) ( Table S2 ). However, as previously discussed in detail ( Lovley, 2017c ), the mechanisms for electron exchange via DIET may be diverse, and more studies are required to further identify the diversity of bacteria capable of functioning as electron-donating partners for DIET.…”

“…In a similar manner, the diversity of methanogens capable of DIET probably extends well beyond the Methanothrix and Methanosarcina species, which have been shown to participate in DIET in defined co-culture studies ( Rotaru et al., 2014a , 2014b ; Yee and Rotaru, 2020 ). A broad range of methanogenic genera, including Methanobacterium , Methanospirillum , Methanocella , Methanoregula , Methanolinea , Methanosphaerula , Methanomassiliicoccus , Methanothermobacter , and Methanoculleus species, were enriched with the addition of electrically conductive materials and inferred to be capable of accepting electrons via DIET ( Capson-Tojo et al, 2018 ; Chowdhury et al., 2019 ; Fu et al., 2018 ; Guo et al., 2018 , 2020 ; Im et al., 2019 ; Jing et al., 2017 ; Lei et al., 2016 ; Lei et al., 2016 ; Lin et al., 2017 , 2018 ; Ma et al., 2020b ; Tian et al., 2017 ; Xia et al., 2019 ; Yang et al., 2016 ; Zhang and Lu, 2016 ; Zhang et al., 2018b ; Zhuang et al., 2015a ) ( Table S2 ). The inability of Methanobacterium formicum and Methanospirillium hungateti to grow in defined co-culture with G .…”