Functional Group Distribution of the Carrier Surface Influences Adhesion of Methanothermobacter thermautotrophicus.

Various support carriers are used for high-density retention of methanogenic archaea in anaerobic wastewater treatment systems. Although the physicochemical properties of carrier materials and microorganisms influence the adhesion of methanogenic archaea, details about the underlying mechanism remain poorly characterized. We applied seven types of chemical surface modifications to carbon felts to clarify the adhesion properties of Methanothermobacter thermautotrophicus, a representative thermophilic hydrogenotrophic methanogen. The relationship between carrier surface properties and methanogen adhesion was evaluated. M. thermautotrophicus adhesion was significantly increased up to 2.6 times in comparison with control on carbon felts treated with NaOH, HCl, H2SO4, or Na2HPO4. Treated carbon felts showed a lower water contact angle, but no correlation between the carrier surface contact angle and methanogen adhesion was observed. On the other hand, at the surface of the carrier that showed improved adhesion of methanogens, the ratio of -COOH : -OH was 1 : 0.65. Such a ratio was not observed with treated carriers for which methanogen adhesion was not improved. Therefore, in the adhesion of M. thermautotrophicus, the functional group abundance was important as well as physical surface properties such as the hydrophobicity. Hydrogenotrophic methanogens are involved in active methanation during the startup of anaerobic digestion. Additionally, these methanogenic archaea function as methanogenic cathode catalysts. Therefore, anaerobic digestion performance will greatly improve by controlling the adhesion of hydrogenotrophic methanogens such as M. thermautotrophicus.

Semi-wet methanogen cathode composed of oak white charcoal for developing sustainable microbial fuel cells.

The present study aimed to evaluate a semi-wet biocathode composed of oak white charcoal and agarose gel as an alternative to the standard carbon felt biocathodes used in microbial fuel cells (MFCs). The MFC containing the oak white charcoal cathode (Oak-MFC) recorded a higher current value than that of the MFC containing a carbon felt cathode (CF-MFC). The Oak-MFC produced approximately 4.0-fold more electrons in the external circuit and 1.7-fold more methane (CH4) than the CF-MFC. A real-time PCR targeting mcrA showed that the number of methanogens adhering to the oak white charcoal cathode was approximately 15-fold that adhering to the carbon felt cathode. These results suggest that the methanogens attached to the cathode of both MFCs received electrons and CH4 was produced from carbon dioxide (CO2). Furthermore, Oak-MFC performed better than CF-MFC, thereby suggesting that oak white charcoal bound by agarose gel can be used as an alternative methanogen cathode. The propionic acid degradation rate of Oak-MFC was faster than that of CF-MFC suggesting that the cathodic reaction may affect the anodic reaction. The use of oak-derived electrode as a methanogen cathode also could contribute to sustainable forest management and promote regular thinning of oak trees. Further, its use will enable carbon fixation and efficient energy conversion from CO2 to CH4, thus contributing to sustainable energy use.