Agroindustrial residues and energy crops for the production of hydrogen and poly-β-hydroxybutyrate via photofermentation

Photofermentative production of poly-β-hydroxybutyrate (PHB) by purple non-sulfur bacteria using olive oil by-products

This study evaluated the ability of six purple non-sulfur bacteria (PNSB) to convert olive oil by-products into poly-β-hydroxybutyrate (PHB). Strains were first independently cultivated in synthetic media with different carbon sources (acetic, lactic and malic acid) to assess their physiology and PHB production. Subsequently, their growth and PHB production using ingested pâté olive cake (IPOC) as a substrate were investigated. Transmission electron microscopy (TEM) observations were conducted on strains cultivated on IPOC to investigate their cell morphologies and inclusion bodies presence and size. Rhodopseudomonas palustris strains accumulated up to 6.8% w PHB/w cells with acetate and 0.86% w PHB/w cells with a daily productivity of 0.54 mg PHB L⁻1 culture d⁻1 on IPOC. In contrast, Cereibacter johrii and Cereibacter sphaeroides reached 58.64% w PHB/w cells and 65.45% w PHB/w cells with acetate, respectively, while C. sphaeroides achieved 21.48% w PHB/w cells and a daily productivity of 10.85 mg PHB L⁻1 culture d⁻1 when cultivated on IPOC. All strains exhibited growth and PHB accumulation in both synthetic media and IPOC substrate. Specifically, R. palustris strains 42OL, AV33 and CGA009 displayed growth capability in all substrates, while C. johrii strains 9Cis and PISA 7, and C. sphaeroides F17 showed promising PHB synthesis capabilities. TEM observations revealed that R. palustris strains, with smaller cell and inclusion body sizes, exhibited lower PHB accumulations, while C. johrii and C. sphaeroides strains, characterized by larger cells and inclusion bodies, demonstrated higher PHB production, recognizing them as promising candidates for PHB production using olive oil by-products. Further investigations under laboratory-scale conditions will be necessary to optimize operating parameters and develop integrated strategies for simultaneous PHB synthesis and the co-production of value-added products, thereby enhancing the economic feasibility of the process within a biorefinery framework.

Bioconversion of volatile fatty acids from organic wastes to produce high-value products by photosynthetic bacteria: A review

Anaerobic fermentation of organic waste to produce volatile fatty acids (VFAs) production is a relatively mature technology. VFAs can be used as a cheap and readily available carbon source by photosynthetic bacteria (PSB) to produce high value-added products, which are widely used in various applications. To better enhance the VFAs obtained from organic wastes for PSB to produce high value-added products, a comprehensive review is needed, which is currently not available. This review systematically summarizes the current status of microbial proteins, H2, poly-β-hydroxybutyrate (PHB), coenzyme Q10 (CoQ10), and 5-aminolevulinic acid (ALA) production by PSB utilizing VFAs as a carbon resource. Meanwhile, the metabolic pathways involved in the H2, PHB, CoQ10, and 5-ALA production by PSB were deeply explored. In addition, a systematic resource utilization pathway for PSB utilizing VFAs from anaerobic fermentation of organic wastes to produce high value-added products was proposed. Finally, the current challenges and priorities for future research were presented, such as the screening of efficient PSB strains, conducting large-scale experiments, high-value product separation, recovery, and purification, and the mining of metabolic pathways for the VFA utilization to generate high value-added products by PSB.

Polyhydroxyalkanoates production in biorefineries: A review on current status, challenges and opportunities

The need for a sustainable and circular bioeconomy model is imperative due to petroleum non-renewability, scarcity and environmental impacts. Biorefineries systems explore biomass to its maximum, being an important pillar for the development of circular bioeconomy. Polyhydroxyalkanoates (PHAs) can take advantage of biorefineries, as they can be produced using renewable feedstocks, and are potential substitutes for petrochemical plastics. The present work aims to evaluate the current status of the industrial development of PHAs production in biorefineries and PHAs contributions to the bioeconomy, along with future development points. Advancements are noticed when PHA production is coupled in wastewater treatment systems, when residues are used as substrate, and also when analytical methodologies are applied to evaluate the production process, such as the Life Cycle and Techno-Economic Analysis. For the commercial success of PHAs, it is established the need for dedicated investment and policies, in addition to proper collaboration of different society actors.

Characteristics and Application of Rhodopseudomonas palustris as a Microbial Cell Factory

Rhodopseudomonas palustris, a purple nonsulfur bacterium, is a bacterium with the properties of extraordinary metabolic versatility, carbon source diversity and metabolite diversity. Due to its biodetoxification and biodegradation properties, R. palustris has been traditionally applied in wastewater treatment and bioremediation. R. palustris is rich in various metabolites, contributing to its application in agriculture, aquaculture and livestock breeding as additives. In recent years, R. palustris has been engineered as a microbial cell factory to produce valuable chemicals, especially photofermentation of hydrogen. The outstanding property of R. palustris as a microbial cell factory is its ability to use a diversity of carbon sources. R. palustris is capable of CO₂ fixation, contributing to photoautotrophic conversion of CO₂ into valuable chemicals. R. palustris can assimilate short-chain organic acids and crude glycerol from industrial and agricultural wastewater. Lignocellulosic biomass hydrolysates can also be degraded by R. palustris. Utilization of these feedstocks can reduce the industry cost and is beneficial for environment. Applications of R. palustris for biopolymers and their building blocks production, and biofuels production are discussed. Afterward, some novel applications in microbial fuel cells, microbial electrosynthesis and photocatalytic synthesis are summarized. The challenges of the application of R. palustris are analyzed, and possible solutions are suggested.

A review of biopolymer (Poly-β-hydroxybutyrate) synthesis in microbes cultivated on wastewater

The large quantities of non-degradable single use plastics, production and disposal, in addition to increasing amounts of municipal and industrial wastewaters are among the major global issues known today. Biodegradable plastics from biopolymers such as Poly-β-hydroxybutyrates (PHB) produced by microorganisms are potential substitutes for non-degradable petroleum-based plastics. This paper reviews the current status of wastewater-cultivated microbes utilized in PHB production, including the various types of wastewaters suitable for either pure or mixed culture PHB production. PHB-producing strains that have the potential for commercialization are also highlighted with proposed selection criteria for choosing the appropriate PHB microbe for optimization of processes. The biosynthetic pathways involved in producing microbial PHB are also discussed to highlight the advancements in genetic engineering techniques. Additionally, the paper outlines the factors influencing PHB production while exploring other metabolic pathways and metabolites simultaneously produced along with PHB in a bio-refinery context. Furthermore, the paper explores the effects of extraction methods on PHB yield and quality to ultimately facilitate the commercial production of biodegradable plastics. This review uniquely discusses the developments in research on microbial biopolymers, specifically PHB and also gives an overview of current commercial PHB companies making strides in cutting down plastic pollution and greenhouse gases.

Advances in nanomaterials induced biohydrogen production using waste biomass

Recent advances on biohydrogen production using different types of waste biomass with the implementation of nanomaterials are summarized. Inspired by exceptional physicochemical and catalytic properties of nanomaterials, the present review focuses on several approaches including impact of nanomaterials on cellulosic biohydrogen production, possible pretreatment technology, as well as improved enzyme & sugar production in order to enhance the biohydrogen yield. Particularly, impacts of nanomaterial are elaborated in detail on different pathways of biohydrogen production (e.g. dark fermentation, photo-fermentation and hybrid-fermentation) using variety of waste biomass. Additionally, emphases are made on the feasibility of nanomaterials for making the biohydrogen production process more economical and sustainable and hence to develop advanced techniques for biohydrogen production using waste biomass.

Sustainable utilization of crop residues for energy generation: A life cycle assessment (LCA) perspective

Modernization in the crop cultivation and development of high yielding varieties resulted in increased crop residues. A large portion of crop residues is not handled appropriately, which leads to environmental burden on society. The crop residues are rich in organic substances, which can be better utilized for various purposes, including energy generation. The utilization of crop residues for energy generation has partially contributed to resolve the inappropriate handling practices, thus reducing their environmental impacts. Life cycle assessment (LCA) is used as a tool to investigate environmental sustainability and can be explored to integrate with social and economic effects to quantify environmental impacts for energy generation from crop residues. This review will provide a comprehensive understanding on LCA inference for decision support to policy-makers and different relevant choices to various applications for sustainable energy generation from crop residues.

Photo-fermentative hydrogen production from crop residue: A mini review

Photofermentative hydrogen production from crop residues, if feasible, can lead to complete conversion of organic substances to hydrogen (and carbon dioxide). This mini review lists the studies on photofermentative hydrogen production using crop residues as feedstock. Pretreatment methods, substrate structure, mechanism of photosynthetic bacteria growth and metabolism were discussed. Photofermentative hydrogen production from pure culture, consortia and mutants, and the geometry, light sources, mass transfer resistances and the operational strategies of the photo-bioreactor were herein reviewed. Future studies of regulation mechanism of photosynthetic bacteria, such as highly-efficient strain breeding and gene reconstruction, and development of new-generation photo-bioreactor were suggested.