Igarashi Kensuke, Miyako Eijiro, Kato Souichiro
Bioproduction Research Institute, National Institute of Advanced Industrial Science and Technology, Sapporo, Japan.
Nanomaterials Research Institute, AIST, Tsukuba, Japan.
Front Microbiol. 2020 Jan 17;10:3068. doi: 10.3389/fmicb.2019.03068. eCollection 2019.
Conductive materials are known to promote direct interspecies electron transfer (DIET) by electrically bridging microbial cells. Previous studies have suggested that supplementation of graphene oxide (GO) based materials, including GO, and reduced GO (rGO), to anaerobic microbial communities, can promote DIET. This promotion mechanism is thought to be involved in electron transfer via rGO or biologically formed rGO. However, concrete evidence that rGO directly promotes DIET is still lacking. Furthermore, the effects of the physicochemical properties of GO-based materials on DIET efficiency have not been elucidated. In the current work, we investigated whether chemically and biologically reduced GO compounds can promote DIET in a defined model coculture system, and also examined the effects of surface properties on DIET-promoting efficiency. Supplementation of GO to a defined DIET coculture composed of an ethanol-oxidizing electron producer and a methane-producing electron consumer promoted methane production from ethanol. X-ray photoelectron spectroscopy revealed that GO was reduced to rGO during cultivation by activity. The stoichiometry of methane production from ethanol and the isotope labeling experiments clearly showed that biologically reduced GO induced DIET-mediated syntrophic methanogenesis. We also assessed the DIET-promoting efficiency of chemically reduced GO and its derivatives, including hydrophilic amine-functionalized rGO (rGO-NH) and hydrophobic octadecylamine-functionalized rGO (rGO-ODA). While all tested rGO derivatives induced DIET, the rGO derivatives with higher hydrophilicity showed higher DIET-promoting efficiency. Optical microscope observation revealed that microbial cells, in particular, , more quickly adhered to more hydrophilic GO-based materials. The superior ability to recruit microbial cells is a critical feature of the higher DIET-promoting efficiency of the hydrophilic materials. This study demonstrates that biologically and chemically reduced GO can promote DIET-mediated syntrophic methanogenesis. Our results also suggested that the surface hydrophilicity (i.e., affinity toward microbial cells) is one of the important determinants of the DIET-promoting efficiencies. These observations will provide useful guidance for the selection of conductive particles for the improvement of methanogenesis in anaerobic digesters.
已知导电材料可通过电桥接微生物细胞来促进种间直接电子转移（DIET）。先前的研究表明，向厌氧微生物群落中添加基于氧化石墨烯（GO）的材料，包括GO和还原氧化石墨烯（rGO），可以促进DIET。这种促进机制被认为与通过rGO或生物形成的rGO进行的电子转移有关。然而，仍然缺乏rGO直接促进DIET的具体证据。此外，基于GO的材料的物理化学性质对DIET效率的影响尚未阐明。在当前的工作中，我们研究了化学还原和生物还原的GO化合物是否能在定义的模型共培养系统中促进DIET，还研究了表面性质对DIET促进效率的影响。向由乙醇氧化电子产生菌和产甲烷电子消耗菌组成的定义的DIET共培养物中添加GO促进了乙醇产生甲烷。X射线光电子能谱显示，在培养过程中，GO通过 活性被还原为rGO。乙醇产生甲烷的化学计量和同位素标记实验清楚地表明，生物还原的GO诱导了DIET介导的互营产甲烷作用。我们还评估了化学还原的GO及其衍生物的DIET促进效率，包括亲水性胺官能化rGO（rGO-NH）和疏水性十八烷基胺官能化rGO（rGO-ODA）。虽然所有测试的rGO衍生物都诱导了DIET，但亲水性较高的rGO衍生物显示出更高的DIET促进效率。光学显微镜观察表明，微生物细胞，特别是 ，更快地附着在亲水性更高的基于GO的材料上。招募微生物细胞的卓越能力是亲水性材料具有更高DIET促进效率的关键特征。这项研究表明，生物还原和化学还原的GO可以促进DIET介导的互营产甲烷作用。我们的结果还表明，表面亲水性（即对微生物细胞的亲和力）是DIET促进效率的重要决定因素之一。这些观察结果将为选择导电颗粒以改善厌氧消化器中的产甲烷作用提供有用的指导。
