h i g h l i g h t sConductive carbon cloth stimulated syntrophic metabolism in DIET co-cultures. Carbon cloth did not stimulate metabolism in a co-culture that relied on H 2 transfer. Non-conductive cotton cloth did not facilitate DIET. Carbon cloth restored DIET in Geobacter strains missing pili or OmcS cytochrome. a r t i c l e i n f o t r a c tThis study investigated the possibility that the electrical conductivity of carbon cloth accelerates direct interspecies electron transfer (DIET) in co-cultures. Carbon cloth accelerated metabolism of DIET co-cultures (Geobacter metallireducens-Geobacter sulfurreducens and G. metallireducens-Methanosarcina barkeri) but did not promote metabolism of co-cultures performing interspecies H 2 transfer (Desulfovibrio vulgaris-G. sulfurreducens). On the other hand, DIET co-cultures were not stimulated by poorly conductive cotton cloth. Mutant strains lacking electrically conductive pili, or pili-associated cytochromes participated in DIET only in the presence of carbon cloth. In co-cultures promoted by carbon cloth, cells were primarily associated with the cloth although the syntrophic partners were too far apart for cell-to-cell biological electrical connections to be feasible. Carbon cloth seemingly mediated interspecies electron transfer between the distant syntrophic partners. These results suggest that the ability of carbon cloth to accelerate DIET should be considered in anaerobic digester designs that incorporate carbon cloth.

Some acetogenic bacteria are capable of using solid electron donors, such as a cathode or metallic iron [Fe(0)]. Acetogens using a cathode as electron donor are of interest for novel applications such as microbial electrosynthesis, while microorganisms using Fe(0) as electron donor cause detrimental microbial induced corrosion. The capacity to use solid electron donors strongly differs between acetogenic strains, which likely relates to their extracellular electron transfer (EET) mechanism. Different EET mechanisms have been proposed for acetogenic bacteria, including a direct mechanism and a H 2 dependent indirect mechanism combined with extracellular hydrogenases catalyzing the H 2 evolution reaction on the cathode or Fe(0) surface. Interestingly, low H 2 partial pressures often prevail during acetogenesis with solid electron donors. Hence, an additional mechanism is here proposed: the maintenance of low H 2 partial pressures by microbial H 2 consumption, which thermodynamically favors the H 2 evolution reaction on the cathode or Fe(0) surface. This work elaborates how the H 2 partial pressure affects the H 2 evolution onset potential and the H 2 evolution rate on a cathode, as well as the free energy change of the anoxic corrosion reaction. In addition, the H 2 consumption characteristics, i.e., H 2 threshold (thermodynamic limit for H 2 consumption) and H 2 consumption kinetic parameters, of acetogenic bacteria are reviewed and evidence is discussed for strongly different H 2 consumption characteristics. Different acetogenic strains are thus expected to maintain different H 2 partial pressures on a cathode or Fe(0) surface, while those that maintain lower H 2 partial pressures (lower H 2 threshold, higher H 2 affinity) more strongly increase the H 2 evolution reaction. Consequently, I hypothesize that the different capacities of acetogenic bacteria to use solid electron donors are related to differences in their H 2 consumption characteristics. The focus of this work is on acetogenic bacteria, but similar considerations are likely also relevant for other hydrogenotrophic microorganisms.

The mechanism of electron transport across electroactive biofilms (EABs) is of high interest and still a matter of debate. Quantitative assessments of their redox conduction take considerable time and require non-turnover conditions (absence of substrate), which can be detrimental to EABs. Here, we measure the charge-transport parameters of Geobacter spp. dominated EABs with double potential step chronoamperometry (DPSC) with Cottrell analysis. The DPSC measurement is simpler and much faster than usual techniques and allows the determination of the charge-transport parameters even under turnover conditions. The electrochemical responses were well-described by a model of redox conduction driven only by electron diffusion within the EAB. The apparent diffusion coefficient for the electron (D app ) was measured as approximately 3.2 10 À7 cm 2 s À1 , a value similar to those recorded for pure Geobacter sulfurreducens EABs, or for some redox polymers with comparable redox center concentrations. This method will be valuable for assessing the impact of EAB characteristics and environmental factors on the charge-transport ability of the biofilm, and for determining the rate-limiting step(s) for current production.

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The acetogen Clostridium ljungdahlii is capable of syngas fermentation and microbial electrosynthesis. Biofilm formation could benefit both these applications, but was not yet reported for C. ljungdahlii. Biofilm formation does not occur under standard growth conditions, but attachment or aggregation could be induced by different stresses. The strongest biofilm formation was observed with the addition of sodium chloride. After 3 days of incubation, the biomass volume attached to a plastic surface was 20 times higher with than without the addition of 200 mM NaCl to the medium. The addition of NaCl also resulted in biofilm formation on glass, graphite and glassy carbon, the latter two being often used electrode materials for microbial electrosynthesis. Biofilms were composed of extracellular proteins, polysaccharides, as well as DNA, while pilus-like appendages were observed with, but not without, the addition of NaCl. A transcriptome analysis comparing planktonic (no NaCl) and biofilm (NaCl addition) cells showed that C. ljungdahlii coped with the salt stress by the upregulation of the general stress response, Na+ export and osmoprotectant accumulation. A potential role for poly-N-acetylglucosamines and D-alanine in biofilm formation was found. Flagellar motility was downregulated, while putative type IV pili biosynthesis genes were not expressed. Moreover, the gene expression analysis suggested the involvement of the transcriptional regulators LexA, Spo0A and CcpA in stress response and biofilm formation. This study showed that NaCl addition might be a valuable strategy to induce biofilm formation by C. ljungdahlii, which can improve the efficacy of syngas fermentation and microbial electrosynthesis applications.