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Climate change is mainly related to the release of CO2 (and other greenhouse gases) to the atmosphere. Concentrated streams of CO2 are normally generated and emitted by a number of different human activities. These include both biogenic sources (e.g. organic waste and wastewater ...

Climate change is mainly related to the release of CO2 (and other greenhouse gases) to the atmosphere. Concentrated streams of CO2 are normally generated and emitted by a number of different human activities. These include both biogenic sources (e.g. organic waste and wastewater treatment plants, biogas plants, landfills, waste or biomass combustion facilities, etc.) and fossil-carbon sources (e.g. centralized fossil-based energy production facilities, engines, etc.).

In light of the possibility to efficiently utilize such concentrated CO2 streams, their dispersion in the atmosphere is a waste of a potentially valuable resource. In fact, only photosynthetic organisms are able to utilize CO2 at atmospheric concentrations (around 400 ppm). However, both plants and photosynthetic microorganisms are known to significantly increase their growth rates, under higher CO2 concentrations. CO2 fertilization in microalgae production facilities is widely recognized as a strategy to improve biomass yields and synthetize different types of bioproducts and food ingredients.

Alternative paths enabling utilization of concentrated CO2 streams to synthesize organic molecules have recently been developed, using microorganisms as catalysts. Gas fermentations are processes where microorganisms can fix CO2 , if sources of reducing power and metabolic energy are available. Hydrogen (H2 ) generated by electrochemical water splitting is an example of energy-rich electron carrier, that can be utilized in co-fermentation with CO2 . Bio-syngas streams coming from biomass gasification or pyrolysis are rich in mainly H2 , CO, CH4 and CO2. Their efficient utilization by gas fermentation have already been demonstrated at pilot-scales, for the production of high value biocommodities (e.g. succinate, 2,3-butanediol, lactate, acetone).

More recently, microbial electrochemical technologies (MET) were proposed as new strategy to furnish electrons and metabolic energy for carbon fixation. In microbial electrosynthesis and in electrofermentation, renewable electricity stimulates the metabolism of selected electro-active microbial communities, to produce organic molecules (short chain fatty acids, alcohols, etc). These molecules can further undergo carbon chain-elongation by heterotrophic communities to synthesize higher value biocommodities and biopolymers.

An interesting synergy between METs and syngas fermentation, is that the bio-char resulting from biomass pyrolysis can have interesting properties for the fabrication of bio-electrodes, such as electrical conductivity and high surface area for microbial biofilm growth.

In all abovementioned biotechnologies, a multidisciplinary approach is needed to develop new metabolic pathways and to optimize existing processes. Synthetic biology and microbial community selection should play a major role in constructing strains or communities for commercial operations. Metatranscriptomics, metabolomics and proteomics, as well as metabolic engineering are fundamental tools to understand and enhance microbial catalysis. At the same time, bioreactor engineering and material science are crucial to study scalable process architectures and to optimize microbial biofilm growth, gas solubilization, product recovery.

In this Research Topic we look forward to collect basic studies as well as applied experiences on this field. We encourage contributions (original research papers, review/mini-review papers, book reviews, short communications, and case reports/case studies) about all the aspects and disciplines that concur to develop applicable microbial processes for the utilization CO2 and other gaseous streams:

• Gas-fermentation process studies
• MES or gas-electrofermentation reactors configurations and design
• Electrode - anode and cathode - new materials, including biochar
• Microbial community and biofilm analysis
• Metabolic studies, modeling and engineering
• New engineered microbial strains
• Electron transfer mechanism/pathways
• Operational/microbial/materials Optimization
• Process efficiency
• Downstream processing
• Product separation and recovery
• Life-cycle and techno-economic analysis
• Integrated technologies

Keywords: Gas Fermentation, Bioelectrochemical Systems, Microbial Electrosynthesis, Electrofermentation, Biosynthesis

Important Note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.

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