Methane Production and Conductive Materials: A Critical Review Gilberto Martins,* Andreia F. Salvador, Luciana Pereira, and M. Madalena Alves Centre of Biological Engineering, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal * S Supporting Information ABSTRACT: Conductive materials (CM) have been exten- sively reported to enhance methane production in anaerobic digestion processes. The occurrence of direct interspecies electron transfer (DIET) in microbial communities, as an alternative or complementary to indirect electron transfer (via hydrogen or formate), is the main explanation given to justify the improvement of methane production. Not disregarding that DIET can be promoted in the presence of certain CM, it surely does not explain all the reported observations. In fact, in methanogenic environments DIET was only unequivocally demonstrated in cocultures of Geobacter metallireducens with Methanosaeta harundinacea or Methanosarcina barkeri and frequently Geobacter sp. are not detected in improved methane production driven systems. Furthermore, conductive carbon nanotubes were shown to accelerate the activity of methanogens growing in pure cultures, where DIET is not expected to occur, and hydrogenotrophic activity is ubiquitous in full-scale anaerobic digesters treating for example brewery wastewaters, indicating that interspecies hydrogen transfer is an important electron transfer mechanism in those systems. This paper presents an overview of the effect of several iron-based and carbon-based CM in bioengineered systems, focusing on the improvement in methane production and in microbial communities’ changes. Control assays, as fundamental elements to support major conclusions in reported experiments, are critically revised and discussed. 1. INTRODUCTION Methane is a renewable energy source that can be produced in controlled bioengineered systems from a wide range of organic substrates including diluted industrial wastewater, animal manure or the organic fraction of municipal solid waste, through a process generally called anaerobic digestion (AD). Fundamental knowledge and technology developments of AD processes have evolved significantly and in parallel in the last decades. The fact that the process relies on the activity of slow growing anaerobic microorganisms 1 results in low nutrient requirements and low amounts of sludge produced, which are strong advantages of the anaerobic treatment process. These microorganisms grow slowly because the energy gain from the anaerobic metabolism is low and has to be divided by different trophic groups, that is, the bacteria performing hydrolytic, acidogenesis, and acetogenesis reactions, and the methanogens converting intermediary degradation products into methane. 2 Syntrophic interactions between bacteria and methanogens are the basis to maintain an AD system working efficiently. These microorganisms, with distinct, but complementary metabolic capabilities, exchange electrons for energy purposes, normally through the transfer of small soluble chemical compounds, such as hydrogen or formate, that act as electron shuttles. This interspecies hydrogen/formate transfer process is very important since the overall thermodynamics depends on the capacity of the microbial communities to maintain a low hydrogen partial pressure. 3 Thus, diffusion limitations of these metabolites, between anaerobic bacteria and methanogenic archaea, can be important bottlenecks in the anaerobic conversion process. 4,5 Recent studies proposed that interspecies electron transfer (IET) can also be performed directly between bacteria and methanogenic archaea, or with the aid of conductive materials (CM), being potentially a more energy conserving approach, and thus improving the rate of methanogenesis. 6,7 However, clear evidence of direct interspecies electron transfer (DIET) was only observed in cocultures of electroactive bacteria, namely Geobacter species, and in cocultures of G. metalliredu- cens with Methanosaeta harundinacea 3,7 or Methanosarcina barkeri. 8 Moreover, DIET seems to require outer membrane c-type cytochromes and pili, 6,7 but traditional syntrophic fatty acid-degrading bacteria (e.g., Syntrophomonas wolfei and Syntrophus aciditrophicus) 9 and most methanogens (e.g., all members of the orders Methanopyrales, Methanococcales, Methanobacteriales and Methanomicrobiales) 10 lack the genes for these cell components. Another indication that not all syntrophic bacteria are capable of DIET is the case of Pelobacter carbinolicus, a known syntrophic ethanol oxidizing bacterium, that could only establish syntrophic interactions Received: April 11, 2018 Revised: August 10, 2018 Accepted: August 17, 2018 Published: August 17, 2018 Critical Review pubs.acs.org/est Cite This: Environ. Sci. Technol. 2018, 52, 10241-10253 © 2018 American Chemical Society 10241 DOI: 10.1021/acs.est.8b01913 Environ. Sci. Technol. 2018, 52, 10241-10253