Simultaneous biosynthesis of bacterial polyhydroxybutyrate (PHB) and extracellular polymeric substances (EPS): Process optimization and Scale-up

Polyhydroxyalkanoate production during electroactive biofilm formation and stabilization in wetland microbial fuel cells for petroleum hydrocarbon bioconversion

This study presented new insights into the sustainable conversion of total petroleum hydrocarbon (TPHC) into polyhydroxyalkanoates (PHAs) using wetland microbial fuel cells (WMFCs). The main innovations included the following two points: (1) The integration of bioelectricity generation with efficient PHA production further underscored the potential of electroactive biofilms as a sustainable platform for simultaneous TPHC biotransformation, bioelectricity recovery and PHA production. (2) The interactive dynamics of PHAs, metabolites, extracellular polymeric substances (EPS) and microorganisms during the formation and stabilization of electroactive biofilms provided novel insights into microbial strategies for carbon utilization. As the electroactive biofilm formed and stabilized, the current density enhanced significantly from 0 to 101 mA m- 2, then stabilized, and finally dropped to 3.51 mA m-2. Similarly, the power density showed a trend of increasing in the initial stage, maintaining in the middle stage, and then descending in the later stage. The production of six types of PHAs was identified: poly(3-hydroxybutyrate) [P(3HB)], poly(3-hydroxyvalerate) [P(3HV)], poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)], poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] [P(3HB-co-3HHX)], poly(3-hydroxyhexadecanoate) [P(3HHD)] and poly(3-hydroxyoctadecanoate) [P(3HOD)], highlighting the metabolic flexibility of electroactive biofilms. The total PHA content was initially undetectable (days 0-4), gradually increased (days 4-28), rose rapidly (days 28-48), gradually increased and descended (days 48-68). The maximum PHA content of 0.664 g g⁻1 DCW achieved highlighted the dual functionality of WMFCs in bioelectricity production and PHA biosynthesis, distinguishing it from conventional MFC applications. The TPHC biodegradation ratio demonstrated a gradual increase (days 0-28), with a more pronounced rise (days 28-48), and a gradual rise to 76.1 % (days 48-68). Throughout the process, the metabolite volatile fatty acids (VFAs) produced were primarily acetate, propionate, butyrate and valerate. The trend of VFA production from days 0-56 closely followed that of TPHC biodegradation. The trend of tyrosine/tryptophan proteins in EPS was aligned with that of biofilm thickness. The strong correlation between the increase in the biofilm thickness and the intensity and peak height of tyrosine/tryptophan proteins during the first 20 days suggested that these proteins were integral to the structural integrity of the biofilms, and from days 20-64, the minimal variation in their intensity and peak height indicated that the biofilms had reached a relatively stable state. The biofilms in turn provided a stable microbial substrate and energetic support for the subsequent efficient synthesis of PHA. During the early phase, the dual-function bacteria, such as Pseudomonas, Bacillus, Acinetobacter and Desulfosarcina, prioritized electron transfer and bioelectricity production using available carbon sources. As bioelectricity generation became less critical in the later phase, the bacteria shifted to intracellular PHA accumulation, transitioning from bioelectricity production to PHA biosynthesis. Finally, a comprehensive network connecting functional microorganisms with bioelectricity production, PHA content, TPHC biodegradation, VFA production and EPS peak height was established. Overall, these findings provided valuable insights into the dynamic interactions and metabolic strategies of electroactive biofilms in WMFCs, highlighting their potential for the efficient bioconversion of PHCs into PHAs.

Sustainable polyhydroxybutyrate (PHB) production from biowastes by Halomonas sp. WZQ-1 under non-sterile conditions

Polyhydroxyalkanoates (PHA) are promising candidates for replacing petroleum-derived plastics; however, their high production costs limit their commercialisation. In this study, we successfully isolated an efficient PHA-producing strain from a salt lake, which was subsequently identified as Halomonas sp. WZQ-1. Notably, Halomonas sp. WZQ-1 could serve as a promising cell-factory platform for polyhydroxybutyrate (PHB) production, achieving a comparatively high PHB productivity (7.64 ± 0.4 g L-1) under moderate salt stress (60 g L-1 NaCl). We further realised semi-continuous PHB production in a bench-scale fermenter at a steady state by irregularly replenishing the organic substrate. The maximum PHB concentration reached 12.13 g L-1. Finally, we realised the non-sterile conversion of typical biowastes (e.g. pomelo and cantaloupe residues) to PHB using Halomonas sp. WZQ-1. Encouragingly, 4.36 g L-1 PHB was directly obtained from the hydrolysate of pomelo residues with a characteristic melting temperature of 174.0 °C. Life cycle assessment was employed to systematically evaluate the environmental sustainability and potential challenges of biowaste-driven PHB biorefineries. Overall, our findings could serve as a pivotal step toward the commercialisation of PHB and provide a valuable reference for PHB biorefineries.

Preparative Coupled Enzymatic Synthesis of L-Homophenylalanine and 2-Hydroxy-5-oxoproline with Direct In Situ Product Crystallization and Cyclization

A continuous in situ crystallization concept is presented for the coupled preparative synthesis of L-homophenylalanine and 2-hydroxy-5-oxoproline (a cyclized form of α-ketoglutarate) using the α-transaminase from Megasphaera elsdenii. The process consists of a spontaneous reactive crystallization step of the enantiopure amino acid itself and a parallel spontaneous cyclization of the deaminated cosubstrate in solution. In parallel, these effects significantly improve the overall productivity of the biocatalytic reaction. Batch, repetitive, and fed-batch processes were investigated, and the fed-batch option proved to be the most viable option. The fed-batch process was subsequently used for a coupled synthesis approach at the gram scale. In total, >18 g of chemically pure L-homophenylalanine and >9 g of 2-hydroxy-5-oxoproline were isolated. This optimized process allows for the design of effective transaminase-catalyzed reactions at a preparative scale utilizing standard (fed-)batch-mode crystallizers.

Optimized batch cultivation and scale-up of Bacillus thuringiensis for high-yield production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)

Addition of statistically optimized concentration of electron acceptor, propionic acid (1.2 g/L) at different cultivation times (0 h, 14.86 h and 19 h) during batch cultivation of B. thuringiensis in mixed substrate (glucose and glycerol) featured production of 8 g/L of biomass and 3.57 g/L of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) containing 0.805 g/L of 3-hydroxyvalerate concentration. Successful scale up of batch cultivation from 7 L to a 70 L bioreactor was, thereafter, achieved using power/volume (P/V) criteria with maximum PHBV and biomass concentration of 3.57 g/L and 7.15 g/L respectively. Characterization of PHBV so produced was carried out using NMR, FTIR, DSC and TGA to elucidate its structure, thermal properties and stability to map their applications in society. These findings highlight the potential of the optimized batch cultivation and scale-up process in producing PHBV emphasizing its relevance in sustainable biopolymer production.

Biodegradation of bisphenol-A in water using a novel strain of Xenophilus sp. embedded onto biochar: Elucidating the influencing factors and degradation pathway

Bisphenol-A (BPA) is an emerging hazardous contaminant, which is ubiquitous in the environment and can cause endocrine disruptor and cancer risks. Therefore, biodegradation of BPA is an essential issue to mitigate the associated human health. In this work, a bacterial strain enables of degrading BPA, named BPA-LRH8 (identified as Xenophilus sp.), was newly isolated from activated sludge and embedded onto walnut shell biochar (WSBC) to form a bio-composite (BCM) for biodegradation of BPA in water. The Langmuir maximum adsorption capacity of BPA by WSBC was 21.7 mg g-1. The free bacteria of BPA-LRH8 showed high BPA degradation rate (∼100 %) at pH 5-11, while it was lower (＜20 %) at pH 3. The BCM eliminated all BPA (∼100 %) at pH 3-11 and 25-45 °C when the BPA level was ≤ 25 mg L-1. The spectrometry investigations suggested two possible degradation routes of BPA by Xenophilus sp. In one route, BPA (C15H16O3) was oxidized to C15H16O3, and then broken into C9H12O3 through chain scission. In another route, BPA was likely hydroxylated, oxidized, and cleaved into C9H10O4P4, which was further metabolized into CO2 and H2O in the TCA cycle. This study concluded that the novel isolated bacteria (BPA-LRH8) embedded onto WSBC is a promising and new method for the effective removal of BPA and similar hazardous substances from contaminated water under high concentrations and wide range of pH and temperature.

PHA and EPS production from industrial wastewater by conventional activated sludge, membrane bioreactor and aerobic granular sludge technologies: A comprehensive comparison

The present study has focused on the mainstream integration of polyhydroxyalkanoate (PHA) production with industrial wastewater treatment by exploiting three different technologies all operating in sequencing batch reactors (SBR): conventional activated sludge (AS-SBR), membrane bioreactor (AS-MBR) and aerobic granular sludge (AGS). A full aerobic feast/famine strategy was adopted to obtain enrichment of biomass with PHA-storing bacteria. All the systems were operated at different organic loading (OLR) rate equal to 1-2-3 kgCOD/m3∙d in three respective experimental periods. The AS-MBR showed the better and stable carbon removal performance, whereas the effluent quality of the AS-SBR and AGS deteriorated at high OLR. Biomass enrichment with PHA-storing bacteria was successfully obtained in all the systems. The AS-MBR improved the PHA productivity with increasing OLR (max 35% w/w), whereas the AS-SBR reduced the PHA content (max 20% w/w) above an OLR threshold of 2 kgCOD/m3∙d. In contrast, in the AGS the increase of OLR resulted in a significant decrease in PHA productivity (max 14% w/w) and a concomitant increase of extracellular polymers (EPS) production (max 75% w/w). Results demonstrated that organic carbon was mainly driven towards the intracellular storage pathway in the AS-SBR (max yield 51%) and MBR (max yield 61%), whereas additional stressors in AGS (e.g., hydraulic selection pressure, shear forces) induced bacteria to channel the COD into extracellular storage compounds (max yield 50%) necessary to maintain the granule's structure. The results of the present study indicated that full-aerobic feast/famine strategy was more suitable for flocculent sludge-based technologies, although biofilm-like systems could open new scenarios for other biopolymers recovery (e.g., EPS). Moreover, the AS-MBR resulted the most suitable technology for the integration of PHA production in a mainstream industrial wastewater treatment plant, considering the greater process stability and the potential reclamation of the treated wastewater.

Polyhydroxyalkanoates and exopolysaccharides: An alternative for valuation of the co-production of microbial biopolymers

Polyhydroxyalkanoates (PHAs) and exopolysaccharides (EPSs) belong to a class of abundant biopolymers produced by various fermenting microorganisms. These biocompounds have high value-added potential and can be produced concurrently. Co-production of PHAs and EPSs is a strategy employed by researchers to reduce costs associated with large-scale production. EPSs and PHAs are non-toxic, biocompatible, and biodegradable, making them suitable for various industrial sectors, including packaging and the medical and pharmaceutical industries. These biopolymers can be derived from agro-industrial residues, thus contributing to the bioeconomy by producing high-value-added products. This review investigates approaches for simultaneously synthesizing PHAs and EPSs using different carbon sources and microorganisms.

Enhanced removal of tetrabromobisphenol A by Burkholderia cepacian Y17 immobilized on biochar

Biochar-immobilized bacteria have been widely used to remove organic pollutants; however, the enhanced effect of biochar-immobilized bacteria on tetrabromobisphenol A (TBBPA) removal has not been fully investigated and the removal mechanism remains unclear. In this study, a bacterial strain with high TBBPA degradation ability, Burkholderia cepacian Y17, was isolated from an e-waste disassembly area, immobilized with biochar, and used for the removal of TBBPA. Comparisons were performed of the factors affecting the immobilization and TBBPA removal efficiency, including the biochar preparation temperature, immobilization temperature, and pH. The highest 7-day TBBPA removal efficiency by immobilized bacteria was observed with the most suitable biochar preparation temperature (BC500) and an immobilization pH and temperature of 7 and 35 °C, respectively. The TBBPA removal efficiency reached 59.37%, which was increased by 30.23% and 15.88% compared to that of free and inactivated immobilized Y17, respectively. The suitable biochar preparation temperature BC500, immobilization temperature of 35 °C, and neutral pH of 7 increased the bacterial population and extracellular polymer concentration, which facilitated bacterial immobilization on biochar and promoted TBBPA removal. In this case, the high immobilized bacteria concentration (4.62 × 108 cfu∙g-1) and protein and polysaccharide contents (28.43 and 16.16 mg·g-1) contributed to the removal of TBBPA by facilitating TBBPA degradation. The main TBBPA degradation processes by BC500-immobilized Y17 involved debromination, β-scission, demethylation, O-methylation, hydroxylation, and hydroxyl oxidation. This study proposes a method for preparing immobilized bacteria for TBBPA removal and enriches the microbial degradation technology for TBBPA.

Characteristics of EPS from Pseudomonas aeruginosa and Alcaligenes faecalis under Cd(II) stress: changes in chemical components and adsorption performance

EPS (extracellular polymeric substance) production is a self-protection mechanism by which microorganisms slow or eliminate adverse effects in unfavorable environments. In this study, Pseudomonas aeruginosa and Alcaligenes faecalis were selected to explore changes in EPS components, especially protein components, under stress caused by different concentrations of Cd(II). The results showed that the protein content in EPS was the highest. The two strains achieved maximum EPS production levels of 109.17 and 214.96 mg/g VSS at Cd(II) stress concentrations of 20 and 50 mg/L, which were increased by 52.07% and 409.69% compared with the levels exhibited before stress, respectively. The protein content correlated very well with data from adsorption experiments. Furthermore, FTIR, 3D-EEM, and XPS results illustrated that after Cd(II) stress, C-N, C=O/-COOH, and R-NO₂^- moieties were formed in substantial quantities, and the stress effects of Pseudomonas aeruginosa were significantly higher than those of Alcaligenes faecalis. The results of this study showed that addition of Cd(NO₃)₂ effectively regulated the components of EPS, especially the protein content, and improved the adsorption capacity, which has application prospects for prevention and control of heavy metals.

Comparative appraisal of nutrient recovery, bio-crude, and bio-hydrogen production using Coelestrella sp. in a closed-loop biorefinery

A closed loop algal-biorefinery was designed based on a three-stage integration of dairy wastewater (DWW) treatment, hydrothermal liquefaction (HTL) of defatted algal biomass, and acidogenic process in a semi-synthetic framework. Initially, Coelestrella sp SVMIICT5 was grown in a 5 L photo-bioreactor and scaled up to a 50 L flat-panel photo-bioreactor using DWW. The microalgal growth showed higher photosynthetic efficiency, resulting in a biomass growth of 3.2 g/L of DCW with 87% treatment efficiency. The biomolecular composition showed 26% lipids with a good fatty acid profile (C12-C21) as well as carbohydrate (24.9%) and protein (31.8%) content. In the second stage, the de-oiled algal biomass was valorized via HTL at various temperatures (150°C, 200°, and 250°C) and reaction atmospheres (N2 and H2). Among these, the 250°C (H2) condition showed a 52% bio-crude fraction and an HHV of ∼29.47 MJ/kg (bio-oil) with a saturated hydrocarbon content of 64.3% that could be further upgraded to jet fuels. The energy recovery (73.01%) and elemental enrichment (carbon; 65.67%) were relatively greater in H2 compared to N2 conditions. Finally, dark fermentation of the complex-structured HTL-AF stream resulted in a total bio-H2 production of 231 ml/g of TOC with a 63% treatment efficiency. Life cycle analysis (LCA) was also performed for the mid-point and damage categories to assess the sustainability of the integrated process. Thus, the results of this study demonstrated comprehensive wastewater treatment and valorization of de-oiled algal biomass for chemical/fuel intermediates in the biorefinery context by low-carbon processes.

Potential Applications of an Exopolysaccharide Produced by Bacillus xiamenensis RT6 Isolated from an Acidic Environment

The Bacillus xiamenensis RT6 strain was isolated and identified by morphological, biochemical and molecular tests from an extreme acidic environment, Rio Tinto (Huelva). Optimisation tests for exopolysaccharide (EPS) production in different culture media determined that the best medium was a minimal medium with glucose as the only carbon source. The exopolymer (EPS_t) produced by the strain was isolated and characterised using different techniques (GC-MS, HPLC/MSMS, ATR-FTIR, TGA, DSC). The molecular weight of EPS_t was estimated. The results showed that the average molecular weight of EPS_t was approximately 2.71 × 10⁴ Da and was made up of a heteropolysaccharide composed of glucose (60%), mannose (20%) and galactose (20%). The EPS_t showed antioxidant capabilities that significantly improved cell viability. Metal chelation determined that EPS_t could reduce the concentration of transition metals such as iron at the highest concentrations tested. Finally, the emulsification study showed that EPS_t was able to emulsify different natural polysaccharide oils, reaching up to an 80% efficiency (olive and sesame oil), and was a good candidate for the substitution of the most polluting emulsifiers. The EPS_t was found to be suitable for pharmaceutical and industrial applications.

Denitrification strategy of Pantoea sp. MFG10 coupled with microbial dissimilatory manganese reduction: Deciphering the physiological response based on extracellular secretion

In this study, the manganese (Mn) reduction-coupled denitrification strategy of dissimilatory Mn reducing bacteria was insightfully investigated. Different parameters (MnO₂ level, pH, and temperature) were optimized by kinetic fitting to improve denitrification and Mn reduction effects. The 300 mg L^-1 MnO₂ addition achieved 98.72% NO₃^--N removal in 12 h, which was 54.62% higher than blank group without MnO₂. Scale-up studies showed that the metabolic activity of the bacteria was effectively enhanced by the addition of MnO₂. Besides the deepening of humification in the system, tryptophan-like protein and polysaccharide as potential electron donor precursors revealed remarkable contributions to the extracellular secretion-dependent denitrification process of DMRB. The effect of EPS on Mn reduction depends mainly on the capture of MnO₂ by the LB-EPS layer versus its dissolution in the TB-EPS layer. Ultimately, the EPS possess a dual effect of accelerated denitrification and Mn reduction efficiency due to the enhanced EET process.

Genomic analysis, simultaneous production, and process optimization of extracellular polymeric substances and polyhydroxyalkanoates by Methylobacterium sp. ISTM1 by utilizing molasses

In the current study, the isolated Methylobacterium sp. ISTM1 simultaneously produced both extracellular polymeric substances (EPS) and polyhydroxyalkanoates (PHA) in a single-step process. The yield of biopolymers (EPS and PHA) was enhanced by optimizing the process parameters of EPS and PHA production. Methylobacterium sp. ISTM1 was able to produce 7.18 ± 0.04 g L^-1 EPS and 1.41 ± 0.04 g L^-1 PHA simultaneously at optimized culture conditions i.e., 9% molasses and pH 7. The genomic analysis of the strain has identified the involved genes and pathways in the production of EPS and PHA. Both the biopolymers were found non-toxic according to the cytotoxicity analysis. The results of the current study present the potential of the bacterium Methylobacterium sp. ISTM1 produces non-toxic biopolymers by utilizing agro-industrial waste (molasses) that can be harnessed sustainably for various applications.

Polyhydroxybutyrate production by Chlorella sorokiniana SVMIICT8 under Nutrient-deprived mixotrophy

Polyhydroxybutyrates (PHBs) are naturally occurring biopolymeric compounds that accumulate in a variety of microorganisms, including microalgae as energy and carbon storage sources. The present study was designed to evaluate nature-based PHB production using microalgae (Chlorella sorokiniana SVMIICT8) in biphasic (growth (GP) and stress phase (SP)) nutritional mode of cultivation. Microalgal PHB accumulation was driven by nutrient constraint, with a maximal production of 29.5% of PHB from 0.94 gm L^-1 of biomass. Fluorescence microscopy revealed PHB granules in the cell cytoplasm, while NMR (¹H and ¹³C), XRD and TGA analysis confirmed the structure. The biopolymer obtained was homopolymer of PHB with carbonyl (C=O) stretch of the aliphatic ester moiety. In GC-MS analysis, major peak representing butyric acid methyl ester also confirmed the PHB. Chlorophyll a fluorescence transients inferred through OJIP, exhibited significant variation in photosynthetic process during growth and nutrient limiting conditions. Mining of bio-based products from microalgae cultivation embrace nature-based approach addressing climate change and sustainability inclusively.

Micro/nano-plastics occurrence, identification, risk analysis and mitigation: challenges and perspectives

Micro/nanoplastics (MP/NPs) are emerging global pollutants that garnered enormous attention due to their potential threat to the ecosystem in virtue of their persistence and accumulation. Notably, United Nations Environment Programme (UNEP) yearbook in 2014 proposed MPs as one among ten emergent issues that the Earth is facing today. MP/NPs can be found in most regularly used products (primary microplastics) or formed by the fragmentation of bigger plastics (secondary microplastics) and are inextricably discharged into the environment by terrestrial and land-based sources, particularly runoff. They are non-degradable, biologically incompatible, and their presence in the air, soil, water, and food can induce ecotoxicological issues and also a menace to the environment. Due to micro size and diverse chemical nature, MP/NPs easily infiltrate wastewater treatment processes. This communication reviews the current understanding of MP/NPs occurrence, mobility, aggregation behavior, and degradation/assimilation in terrestrial, aquatic (fresh & marine), atmospheric depositions, wetlands and trophic food chain. This communication provide current perspectives and understanding on MP/NPs concerning (1) Source, occurrence, distribution, and properties (2) Impact on the ecosystem and its services, (3) Techniques in detection and identification and (4) Strategies to manage and mitigation.