Myrsini Sakarika ^1* Silvio Matassa ² José Carvajal ¹ Ramon Ganigue ¹

• ¹ Ghent University, Ghent, Belgium
• ² University of Naples Federico II, Naples, Campania, Italy

focuses on improving soy-processing enzyme production using Aspergillus niger. Their research demonstrates how microbial strains can enhance their enzyme yields by finetuning the bioreactor operation, which can reduce overall production costs and improve sustainability in industrial processes. These enzymes are critical not only in food processing, but also in other bio-based industries where they drive biochemical reactions.By leveraging the capabilities of microbes and biotechnological tools, enzyme production can become more efficient and environmentally friendly. This reflects a larger trend within microbial biorefineries, where innovations aim to streamline processes toward economic and environmental sustainability. Enhanced enzyme production, as discussed in this study, will undoubtedly play a critical role in expanding the functionality of microbial biorefineries by supporting more efficient bioconversion of feedstocks. The conversion of methane into valuable chemicals opens new opportunities to producing buildingblock chemicals from renewable resources. The work by Baldo et al. (2024) on methane biohydroxylation presents an innovative process where Methylosinus trichosporium OB3b converts methane into methanol, leveraging the methane monooxygenase (MMO) enzyme. The study successfully demonstrates that M. trichosporium OB3b can use formate as an auxiliary electron donor to improve methane-to-methanol conversion efficiency. This finding opens up new possibilities for enhancing the overall performance of methane biohydroxylation systems.However, as the study reveals, there are several challenges that need to be addressed for this process to become scalable. These include limitations in substrate availability, product inhibition, and the need for electron donors like formate to optimize the reaction. Overcoming these technical barriers is essential for the industrialization of methane-to-methanol conversion. The potential environmental and economic impacts of successfully implementing this technology on a large scale are significant, providing an environmentally-friendly alternative to conventional methanol production methods and positioning methane bioconversion as a cornerstone of future biorefineries. The production of biodegradable plastics, such as PHA, represents a promising solution to plastic pollution. In this context, Kim et al. (2023) investigated how methanotrophic microbial communities, enriched from wetlands, accumulate PHA depending on the availability of carbon and nitrogen. The study emphasizes the importance of optimizing the resource (carbon, energy, nutrients) availability to enhance PHA production, highlighting that resource-rich environments lead to more efficient PHA accumulation in Methylocystis species. On the other hand, reduced resource availability results in lower yields and microbial diversity.Understanding these dynamics is critical for scaling up PHA production in microbial biorefineries, particularly when using waste feedstocks, that have an inherent organic load. With growing interest in biodegradable plastics, optimizing microbial systems to efficiently convert waste streams into PHA will be essential in the global push for sustainable materials. Microalgae can be powerful contributors to microbial biorefineries due to their ability to convert CO2 into valuable compounds such as lipids, carbohydrates, and biofuels. Takagi et al. (2023) explored the potential of Parachlorella sp. BX1.5, a microalga capable of producing intracellular lipids and extracellular polysaccharides under a range of pH and dissolved CO2 levels. The study demonstrates that by adjusting the pH and CO2 concentrations, biomass production can be significantly increased in both indoor and outdoor systems. This research provides insights for large-scale microalgal biorefineries, where the challenge is to maintain high levels of productivity under varying environmental conditions. By fine-tuning these parameters, the efficiency of microalgal systems can be maximized, contributing to the development of bio-based products such as biofuels and bioplastics. The adaptability of Parachlorella to extreme pH environments also highlights its potential as a robust and versatile player in future biorefineries. The four articles in this collection demonstrate the versatility and potential of microbial processes to convert waste into valuable products. However, several challenges remain. The scalability of these processes, particularly methane biohydroxylation and methane-based PHA production, is a critical hurdle that must be addressed to enable their widespread industrial application. Furthermore, downstream processing -especially product recovery and purification -and its interaction with the upstream part of the process is often overlooked, and remains a bottleneck in many biorefinery systems.Looking ahead, the integration of life cycle assessments (LCA) and techno-economic analyses (TEA) will be essential to fully understand the environmental and economic benefits of microbial biorefineries. As these technologies advance, it will be crucial to balance resource inputs with product yields to ensure their sustainability. Innovations in the development of more resilient microbial strains or consortia, will also be key to improving the efficiency of microbial biorefineries.Moreover, expanding the range of products that can be derived from microbial processes, including high-value compounds, will improve the economic viability of biorefineries. These efforts will not only reduce reliance on fossil fuels and petrochemical processes but also help address pressing environmental issues like plastic pollution and greenhouse gas emissions.In conclusion, microbial biorefineries have the potential to play a transformative role in creating a circular bioeconomy. As research continues to overcome current limitations and scale up production processes, the potential for microbial systems to revolutionize sustainable production becomes increasingly clear.