Nowadays, sustainable and renewable energy production is a global priority. Over the past decade, several Power-to-X (PtX) technologies have been proposed to store and convert the surplus of renewable energies into chemical bonds of chemicals produced by different processes. CO₂ is a major contributor to climate change, yet it is also an undervalued source of carbon that could be recycled and represents an opportunity to generate renewable energy. In this context, PtX technologies would allow for CO₂ valorization into renewable fuels while reducing greenhouse gas (GHG) emissions. With this work we want to provide an up-to-date overview of biomethanation as a PtX technology by considering the biological aspects and the main parameters affecting its application and scalability at an industrial level. Particular attention will be paid to the concept of CO₂-streams valorization and to the integration of the process with renewable energies. Aspects related to new promising technologies such as in situ, ex situ, hybrid biomethanation and the concept of underground methanation will be discussed, also in connection with recent application cases. Furthermore, the technical and economic feasibility will be critically analyzed to highlight current options and limitations for implementing a sustainable process.

H₂ produced from renewable energies will play a central role in both greenhouse gas reduction and decarbonization by 2050. Nonetheless, to improve H₂ diffusion and utilization as a fuel, large storage capacity systems are needed. Underground storage of natural gas in depleted reservoirs, aquifers and salt caverns is a well-established technology. However, new challenges arise when it comes to storing hydrogen due to the occurrence and activity of indigenous microbial populations in deep geological formations. In a previous study, four Italian natural gas reservoirs were characterized both from a hydro-chemical and microbiological point of view, and predictive functional analyses were carried out with the perspective of underground hydrogen storage (UHS). In the present work, formation waters from the same reservoirs were used as inoculant during batch cultivation tests to characterize microbial activity and its effects on different gas mixtures. Results evidence a predominant acidogenic/acetogenic activity, whilst methanogenic and sulfate reducing activity were only marginal for all tested inoculants. Furthermore, the microbial activation of tested samples is strongly influenced by nutrient availability. Obtained results were fitted and screened in a computational model which would allow deep insights in the study of microbial activity in the context of UHS.

Depleted gas reservoirs are a valuable option for underground hydrogen storage (UHS). However, different classes of microorganisms, which are capable of using free H ₂ as a reducing agent for their metabolism, inhabit deep underground formations and can potentially affect the storage. This study integrates metagenomics based on Illumina‐NGS sequencing of bacterial and archaeal 16S rRNA and dsrB and mcrA functional genes to unveil the composition and the variability of indigenous microbial populations of four Italian depleted reservoirs. The obtained mcrA sequences allow us to implement the existing taxonomic database for mcrA gene sequences with newly classified sequences obtained from the Italian gas reservoirs. Moreover, the KEGG and COG predictive functional annotation was used to highlight the metabolic pathways potentially associated with hydrogenotrophic metabolisms. The analyses revealed the specificity of each reservoir microbial community, and taxonomic and functional data highlighted the presence of an enriched number of taxa, whose activity depends on both reservoir hydrochemical composition and nutrient availability, of potential relevance in the context of UHS. This study is the very first to address the profiling of the microbial population and allowed us to perform a preliminary assessment of UHS feasibility in Italy.

E-materials become e-waste once they have been discarded without the intent of reuse. Due to its rich content of metals, among which many are Critical Raw Materials (CRMs), e-waste can be considered an urban mine to exploit and valorise. Common metal refining is performed by energy-intensive processes frequently based on the use of fossil fuel. Bio-metallurgy is a promising alternative for e-waste valorisation based on biological routes of specialised microorganisms able to leach solid-containing metals. Because of the physiology of these microorganisms, microbial leaching can be economically feasible, besides being an environmentally sustainable process. Like Bacteria and Fungi, Archaea are also capable of metal leaching activity, though their potential is underestimated. Among them, the extremophiles are the most studied and applied in the field of metal recovery, while mesophilic species are less common but still of high interest. Here we provide the state of industrial application of bio-metallurgy and report on the state of the art of Archaea exploitation in metal recovery from e-waste. Moreover, we give a special highlight to methanogenic archaea, which are able to convert CO₂ into methane in order to highlight the potential for the valorisation of CO₂-rich industrial streams generated by key processes (i.e., anaerobic digestion, concrete, and steel production) in CH₄ for gas grid distribution, while making metals content in e-waste available again as raw material.

To successfully scale-up the production of bio-based building blocks through CO₂ and H₂-based gas fermentation, it is crucial to deeply understand and control the microbial catalyst response to the bioreactor environment. This study investigates the effects of key process parameters, such as CO₂ and H₂ partial pressures, gas feeding strategies, and mixture composition, on the production pathways of an evolved Clostridium carboxidivorans strain. The ultimate goal is to optimize 1-hexanol production in elevated-pressure stirred-tank reactors. Continuous gas feeding enhanced acetogenic and solventogenic metabolisms, while gas-limited conditions promoted chain elongation to caproic acid. An optimized process, combining an initial gas-limited step followed by a continuous gas phase, increased 1-hexanol production, achieving a maximum biomass-specific productivity of 0.9 g g_CDW^-1 day^-1. In-situ product extraction improved 1-hexanol carbon selectivity to an unprecedented 60 %. These findings demonstrate the potential of CO₂ and H₂-fed fermentation to produce high-value chemicals other than ethanol and acetate.