Relevance of extracellular electron uptake mechanisms for electromethanogenesis applications.

Electromethanogenesis has emerged as a biological branch of Power-to-X technologies that implements methanogenic microorganisms, as an alternative to chemical Power-to-X, to convert electrical power from renewable sources, and CO2 into methane. Unlike biomethanation processes where CO2 is converted via exogenously added hydrogen, electromethanogenesis occurs in a bioelectrochemical set-up that combines electrodes and microorganisms. Thereby, mixed, or pure methanogenic cultures catalyze the reduction of CO2 to methane via reducing equivalents supplied by a cathode. Recent advances in electromethanogenesis have been driven by interdisciplinary research at the intersection of microbiology, electrochemistry, and engineering. Integrating the knowledge acquired from these areas is essential to address the specific challenges presented by this relatively young biotechnology, which include electron transfer limitations, low energy and product efficiencies, and reactor design to enable upscaling. This review approaches electromethanogenesis from a multidisciplinary perspective, putting emphasis on the extracellular electron uptake mechanisms that methanogens use to obtain energy from cathodes, since understanding these mechanisms is key to optimize the electrochemical conditions for the development of these systems. This work summarizes the direct and indirect extracellular electron uptake mechanisms that have been elucidated to date in methanogens, along with the ones that remain unsolved. As the study of microbial corrosion, a similar bioelectrochemical process with Fe0 as electron source, has contributed to elucidate different mechanisms on how methanogens use solid electron donors, insights from both fields, biocorrosion and electromethanogenesis, are combined. Based on the repertoire of mechanisms and their potential to convert CO2 to methane, we conclude that for future applications, electromethanogenesis should focus on the indirect mechanism with H2 as intermediary. By summarizing and linking the general aspects and challenges of this process, we hope that this review serves as a guide for researchers working on electromethanogenesis in different areas of expertise to overcome the current limitations and continue with the optimization of this promising interdisciplinary technology.

A Win-Loss Interaction on Fe0 Between Methanogens and Acetogens From a Climate Lake.

Diverse physiological groups congregate into environmental corrosive biofilms, yet the interspecies interactions between these corrosive physiological groups are seldom examined. We, therefore, explored Fe0-dependent cross-group interactions between acetogens and methanogens from lake sediments. On Fe0, acetogens were more corrosive and metabolically active when decoupled from methanogens, whereas methanogens were more metabolically active when coupled with acetogens. This suggests an opportunistic (win-loss) interaction on Fe0 between acetogens (loss) and methanogens (win). Clostridia and Methanobacterium were the major candidates doing acetogenesis and methanogenesis after four transfers (metagenome sequencing) and the only groups detected after 11 transfers (amplicon sequencing) on Fe0. Since abiotic H2 failed to explain the high metabolic rates on Fe0, we examined whether cell exudates (spent media filtrate) promoted the H2-evolving reaction on Fe0 above abiotic controls. Undeniably, spent media filtrate generated three- to four-fold more H2 than abiotic controls, which could be partly explained by thermolabile enzymes and partly by non-thermolabile constituents released by cells. Next, we examined the metagenome for candidate enzymes/shuttles that could catalyze H2 evolution from Fe0 and found candidate H2-evolving hydrogenases and an almost complete pathway for flavin biosynthesis in Clostridium. Clostridial ferredoxin-dependent [FeFe]-hydrogenases may be catalyzing the H2-evolving reaction on Fe0, explaining the significant H2 evolved by spent media exposed to Fe0. It is typical of Clostridia to secrete enzymes and other small molecules for lytic purposes. Here, they may secrete such molecules to enhance their own electron uptake from extracellular electron donors but indirectly make their H2-consuming neighbors-Methanobacterium-fare five times better in their presence. The particular enzymes and constituents promoting H2 evolution from Fe0 remain to be determined. However, we postulate that in a static environment like corrosive crust biofilms in lake sediments, less corrosive methanogens like Methanobacterium could extend corrosion long after acetogenesis ceased, by exploiting the constituents secreted by acetogens.

Baltic Sea methanogens compete with acetogens for electrons from metallic iron.

Microbially induced corrosion of metallic iron (Fe0)-containing structures is an environmental and economic hazard. Methanogens are abundant in low-sulfide environments and yet their specific role in Fe0 corrosion is poorly understood. In this study, Sporomusa and Methanosarcina dominated enrichments from Baltic Sea methanogenic sediments that were established with Fe0 as the sole electron donor and CO2 as the electron acceptor. The Baltic-Sporomusa was phylogenetically affiliated to the electroactive acetogen S. silvacetica. Baltic-Sporomusa adjusted rapidly to growth on H2. On Fe0, spent filtrate enhanced growth of this acetogen suggesting that it was using endogenous enzymes to retrieve electrons and produce acetate. Previous studies have proposed that acetate produced by acetogens can feed commensal acetoclastic methanogens such as Methanosarcina. However, Baltic-methanogens could not generate methane from acetate, plus the decrease or absence of acetogens stimulated their growth. The decrease in numbers of Sporomusa was concurrent with an upsurge in Methanosarcina and increased methane production, suggesting that methanogens compete with acetogens for electrons from Fe0. Furthermore, Baltic-methanogens were unable to use H2 (1.5 atm) for methanogenesis and were inhibited by spent filtrate additions, indicating that enzymatically produced H2 is not a favorable electron donor. We hypothesize that Baltic-methanogens retrieve electrons from Fe0 via a yet enigmatic direct electron uptake mechanism.

[Molecular characterization of non-vaccine Streptococcus pneumoniae serotypes 11A, 15 B/C and 23A recovered from invasive isolates in Colombia].

INTRODUCTION: A total of 192 invasive Streptococcus pneumoniae isolates, from serotypes 11A, 15B/C and 23A (not included in the conjugated vaccines), were collected in Colombia between 1994 and 2014 as part of the activities of the Network surveillance system for the causative agents of pneumonia and meningitis (SIREVA II). OBJECTIVE: To determine the molecular characteristics of invasive S. pneumoniae isolates from serotypes 11A, 15B/C and 23A in Colombia from 1994 to 2014. MATERIALS AND METHODS: The molecular characterization of the isolates was carried out through Pulse-Field Gel Electrophoresis (PFGE) and Multilocus Sequence Typing (MLST). RESULTS: Serotype 11A showed one clonal group represented by ST62. Serotype 15B/C was composed of three groups associated with Netherlands15B-37 ST199 (28.75%), ST8495 (18.75%), and SLV (Single-Locus Variant) of ST193 (21.25%). Isolates from serotype 23A were gathered in three clonal groups, with 70.21% closely related to ST42, 17.02% to Colombia23F-ST338, and 6.38% to Netherlands15B-37 ST199. CONCLUSION: Clones Colombia23F-ST338 and Netherlands15B-ST199 covered more serotypes than those previously found by other authors, including serotype 23A. These analyses reveal the importance of capsular switching in the spreading of successful clones among non-vaccine serotypes causing invasive pneumococcal disease.

Molecular characterization of non-vaccine Streptococcus pneumoniae serotypes 11A, 15 B/C and 23A recovered from invasive isolates in Colombia / Caracterización molecular de los serotipos no vacunales 11A, 15 B/C y 23A de Streptococcus pneumoniae recuperados de aislamientos invasivos en Colombia

Resumen Introduction: A total of 192 invasive Streptococcus pneumoniae isolates, from serotypes 11A, 15B/C and 23A (not included in the conjugated vaccines), were collected in Colombia between 1994 and 2014 as part of the activities of the Network surveillance system for the causative agents of pneumonia and meningitis (SIREVA II). Objective: To determine the molecular characteristics ofinvasive S. pneumoniaeisolates from serotypes 11A, 15B/C and 23A in Colombia from 1994 to 2014. Materials and methods: The molecular characterization of the isolates was carried out through Pulse-Field Gel Electrophoresis (PFGE) and Multilocus Sequence Typing (MLST). Results: Serotype 11A showed one clonal group represented by ST62. Serotype 15B/C was composed of three groups associated with Netherlands15B-37 ST199 (28.75%), ST8495 (18.75%), and SLV (Single-Locus Variant) of ST193 (21.25%). Isolates from serotype 23A were gathered in three clonal groups, with70.21% closely related toST42, 17.02% to Colombia23F-ST338, and6.38% to Netherlands15B-37 ST199. Conclusion: Clones Colombia23F-ST338 andNetherlands15B-ST199 covered more serotypes than those previously found by other authors, including serotype 23A. These analyses reveal the importance of capsular switching in the spreading of successful clones among non-vaccine serotypes causing invasive pneumococcal disease.

Abstract Introducción. En Colombia se recolectaron 192 aislamientos invasivos de Streptococcus pneumoniae de los serotipos 11A, 15B/C y 23A (no incluidos en las vacunas conjugadas) entre 1994 y 2014, como parte de las actividades del Sistema de Redes de Vigilancia de los Agentes Responsables de Neumonías y MeningitisBacterianas (SIREVA II). Objetivo. Determinar las características moleculares de aislamientosinvasivos de los serotipos11A, 15B/C y 23A de S. pneumoniae recolectados en Colombia entre 1994 y 2014. Materiales y métodos. La caracterización molecular de los aislamientos se hizo medianteelectroforesis en gel de campo pulsado (Pulse-Field Gel Electrophoresis, PFGE) y por tipificación de secuencias multilocus (Multilocus Sequence Typing, MLST). Resultados. El serotipo 11A mostró un grupo clonal representadopor el ST62, en tanto que el serotipo15B/C se distribuyó en tres grupos asociados conlos clones Netherlands15B-37 ST199 (28,75 %), ST8495 (18,75 %) y SLV (variante en un solo locus) de ST193 (21,25 %). Los aislamientos con serotipo 23A se agruparon en tres gruposclonales; 70,21 % de ellos estaban estrechamente relacionadoscon elST42, 17,02 % con elColombia23F-ST338, y 6,38 % con el Netherlands15B-37 ST199. Conclusión. Los clones Colombia23F-ST338 y Netherlands15B-ST199 encontrados en este estudio abarcaronmás serotipos de los reportados previamente por otros autores, incluido el serotipo23A. Estos análisis revelan laimportancia de la conmutación(switching) capsular en la expansión de clones exitosos entre los serotipos no vacunales como causa de enfermedad invasiva neumocócica.