Clostridium pasteurianum Bioprocessing: Pioneering Circular Bioeconomy Advancements through Sustainable Resource Utilization †

: The transition towards a circular bioeconomy represents a paradigm shift in sustainable resource management, aiming to synergize biological processes, waste valorization, and renewable product synthesis. A key contender in this transformative landscape is the versatile anaerobic bacterium Clostridium pasteurianum . With its unique metabolic capabilities, C. pasteurianum holds substantial promise as a cornerstone of bioprocessing strategies within the circular bioeconomy framework. As we navigate the path towards a sustainable and regenerative future, continued research and innovation are imperative to unlock the full potential of C. pasteurianum .


Introduction
The transition towards a circular bioeconomy marks a profound departure from the traditional linear economic model [1,2].In the conventional "take-make-dispose" linear economy, resources are extracted, processed, used, and finally discarded as waste.This linear model has long been associated with overconsumption, resource depletion, environmental degradation, and an unsustainable approach to economic development.Waste generation is one of the repercussions of this typical linearly productive method.Moreover, economic growth has expedited the way that primary resources are extracted, produced, used, and disposed of, which has increased the amount of trash produced [3][4][5].In contrast, the circular bioeconomy introduces a fundamental paradigm shift in how we conceptualize and manage resources.
The concept of the circular bioeconomy represents a paradigm shift in how modern society views and manages resources.It underscores the need to break away from the linear "take-make-dispose" model and instead foster a regenerative system where resources are used efficiently, waste is minimized, and environmental impact is curtailed.At the heart of this paradigm lies the aspiration to create a closed-loop system where biological processes are harnessed to their fullest potential and the value of resources is continually preserved and enhanced.The circular bioeconomy is founded on several key principles: resource efficiency, biological processes, waste valorization, renewable product synthesis, closed-loop systems, and environmental stewardship [1,2,[6][7][8].
One of the core tenets of the circular bioeconomy is the seamless integration of biological processes with waste valorization and renewable product synthesis.This synergy is pivotal in achieving the overarching goals of sustainability and resource efficiency [1,9].Biological processes/systems, such as the bacterium Clostridium pasteurianum (full scientific name: Clostridium pasteurianum Winogradsky, 1895), are key agents in this synergy.C. pasteurianum possesses enhanced abilities to convert organic materials into valuable products with high efficiency and minimal environmental impact.This paper aims to shed light on the multifaceted potential of C. pasteurianum within the circular bioeconomy framework.By delving into its metabolic capabilities, its proficiency in acetone-butanol-ethanol (ABE) fermentation (acetone, butanol, and ethanol are produced in a ratio of 3:6:1), and its compatibility with biorefinery strategies, this review seeks to elucidate how C. pasteurianum can advance the circular bioeconomy's goals of waste minimization, resource efficiency, and reduced environmental impact.Furthermore, it acknowledges the challenges that lie ahead in harnessing C. pasteurianum's full potential and underscores the importance of continued research and innovation in this burgeoning field.

Clostridium pasteurianum: A Microbial Architect of Circular Bioeconomy
The genus Clostridium encompasses a diverse group of anaerobe bacteria with unique metabolic capabilities that hold significant potential within the circular bioeconomy.Among the many species within this genus, C. pasteurianum stands out as an exemplar due to its preeminent attributes and suitability for sustainable resource utilization.One of its most remarkable features is its ability to ferment an impressive array of feedstocks, ranging from lignocellulosic biomass to agricultural residues, and organic waste.Alongside ABE fermentation, C. pasteurianum exhibits a non-biphasic metabolism, alternatively producing 1,3-propanediol-butanol-ethanol (PBE fermentation).This metabolic flexibility grants C. pasteurianum the capacity to transform otherwise underutilized and often discarded resources into valuable biofuels and bio-based chemicals.C. pasteurianum has the ability to convert carbohydrates (e.g., glycerol, glucose, other hexoses, and biopolymers in the form of cellulose fibers and starches) into acetates, butanol, ethanol, and 1,3-propanediol (1,3-PDO) while releasing gaseous compounds (molecular hydrogen and carbon dioxide) [10][11][12][13].The metabolism of this anaerobe bacterium can be switched from alcohols and reduced acids to volatile fatty acids (VFAs; for example, acetic, propionic, and butyric acids) in order to maximize the yield of molecular hydrogen from reduced fermentation end-products (e.g., butanol, ethanol, and lactate) [14][15][16][17].The fermentation end-products of C. pasteurianum depend on the growth conditions (Table 1).RS Residual sugars: C. pasteurianum was unable to efficiently use those sugars as compared to glucose.Bolded and underlined products: main products of the fermentation process; SM: secondary metabolites; NgA: negligible amounts; PVA: polyvinyl alcohol; glycerol 01 and 02: crude glycerol samples (89.55% and 52.66% glycerol content, respectively); 68 RB: 68 repeated batches (results are showed for the last 56th-68th batches, as the process duration gradually decreased until the 56th batch and was quite stable for the rest of them); 6.3X 6.3 times more butanol was produced from glycerol by PVA-entrapped cells than by free cells; butanol yield is calculated as final butanol concentration divided by concentration of utilized glycerol.
In summary, the data in Table 1 demonstrate the versatility of C. pasteurianum in utilizing various feedstocks (rice straw, crude glycerol) to produce a range of valuable gaseous (hydrogen) and liquid biofuels (butanol and ethanol) or organic chemicals (1,3-PDO).It is noteworthy that the generation of butanol and ethanol simultaneously declines as 1,3-PDO is produced.The bacterium's ability to adapt to different growth conditions and efficiently convert these feedstocks underscores its potential in the circular bioeconomy, where waste materials and renewable resources can be harnessed for sustainable resource utilization and biofuel production.

Challenges and Future Directions
As we explore the potential of Clostridium pasteurianum within the circular bioeconomy, several challenges and opportunities emerge, paving the way for future research and innovation.
A promising avenue for advancing C. pasteurianum's capabilities lies in genetic engineering.By manipulating its metabolic pathways and optimizing enzyme expression, researchers can potentially enhance the bacterium's substrate utilization efficiency [21], product yields [21,22], and tolerance to inhibitory compounds [23].Genetic modifications can also be employed to tailor C. pasteurianum for specific feedstocks or target products, thereby increasing its versatility in different circular bioeconomy applications [24,25].
The transition from laboratory-scale experiments to industrial applications presents a substantial challenge.Scaling up C. pasteurianum fermentation processes to meet commercial demands requires careful consideration of bioreactor design, process control, and the optimization of parameters such as pH, temperature, and nutrient supply.Addressing these challenges is crucial for achieving cost-effective and efficient large-scale production.
The concept of integrated biorefineries, where multiple valuable products are generated from the same feedstock, can be further explored.C. pasteurianum's ability to produce both ABE and PBE fermentation products makes it an attractive candidate for the development of sustainable and multifunctional biorefinery systems.
In conclusion, while Clostridium pasteurianum offers immense potential within the circular bioeconomy, addressing challenges related to genetic engineering, scale-up, regulatory compliance, and safety is imperative for its successful industrial adoption.Additionally, leveraging the complementary PBE fermentation pathway and exploring novel feedstocks can unlock new possibilities for sustainable resource utilization and product diversification.These future directions hold promise for a more sustainable and regenerative bioeconomy driven by the metabolic prowess of C. pasteurianum.

Table 1 .
The fermentation end-products of various C. pasteurianum strains.