Polyhydroxyalkanoate (PHA) production by a mixed microbial culture using sugar molasses: effect of the influent substrate concentration on culture selection

Bioaugmentation of Thauera mechernichensis TL1 for enhanced polyhydroxyalkanoate production in mixed microbial consortia for wastewater treatment

Polyhydroxyalkanoate (PHA) is a fully biodegradable bioplastic. To foster a circular economy, the integration of PHA production into wastewater treatment facilities can be accomplished using mixed microbial consortia. The effectiveness of this approach relies greatly on the enrichment of PHA-accumulating microorganisms. Hence, our study focused on bioaugmenting Thauera mechernichensis TL1 into mixed microbial consortia with the aim of enriching PHA-accumulating microorganisms and enhancing PHA production. Three sequencing batch reactors-SBRctrl, SBR2.5%, and SBR25%-were operated under feast/famine conditions. SBR2.5% and SBR25% were bioaugmented with T. mechernichensis TL1 at 2.5%w/w of mixed liquor volatile suspended solids (MLVSS) and 25%w/w MLVSS, respectively, while SBRctrl was not bioaugmented. SBR2.5% and SBR25% achieved maximum PHA accumulation capacities of 56.3 %gPHA/g mixed liquor suspended solids (MLSS) and 50.2 %gPHA/gMLSS, respectively, which were higher than the 25.4 %gPHA/gMLSS achieved by SBRctrl. The results of quantitative polymerase chain reaction targeting the 16S rRNA gene specific to T. mechernichensis showed higher abundances of T. mechernichensis in SBR2.5% and SBR25% compared with SBRctrl in the 3rd, 17th, and 31st cycles. Fluorescence in situ hybridization, together with fluorescent staining of PHA with Nile blue A, confirmed PHA accumulation in Thauera spp. The study demonstrated that bioaugmentation of T. mechernichensis TL1 at 2.5%w/w MLVSS is an effective strategy to enhance PHA accumulation and facilitate the enrichment of PHA-accumulating microorganisms in mixed microbial consortia. The findings could contribute to the advancement of PHA production from wastewater, enabling the transformation of wastewater treatment plants into water and resource recovery facilities.

Communication mediated interaction between bacteria and microalgae advances photogranulation

Recently, photogranules composed of bacteria and microalgae for carbon-negative nitrogen removal receive extensive attention worldwide, yet which type of bacteria is helpful for rapid formation of photogranules and whether they depend on signaling communication remain elusive. Varied signaling communication was analyzed using metagenomic method among bacteria and microalgae in via of two types of experimentally verified signaling molecule from bacteria to microalgae, which include indole-3-acetic acid (IAA) and N-acyl homoserine lactones (AHLs) during the operation of photo-bioreactors. Signaling communication is helpful for the adaptability of bacteria to survive with algae. Compared with non-signaling bacteria, signaling bacteria more easily adapt to the varied conditions, evidenced by the increased abundance in the operated reactors. Signaling bacteria are easier to enter the phycosphere, and they dominate the interactions between bacteria and algae rather than non-signaling bacteria. The co-abundance groups (CAGs) with signaling bacteria possess higher abundance than that without signaling bacteria (22.27 % and 6.67 %). Importantly, signaling bacteria accessibly interact with microalgae, which possess higher degree centralities and 32.50 % of them are keystone nodes in the network, in contrast to only 18.66 % of non-signaling bacteria. Thauera carrying both IAA and AHLs synthase genes are highly enriched and positively correlated with nitrogen removal rate. Our work not only highlights the essential roles of signaling communication between microalgae and bacteria in the development of photogranules, but also enriches our understanding of microbial sociobiology.

Polyhydroxyalkanoates production in purple phototrophic bacteria ponds: A breakthrough in outdoor pilot-scale operation

The versatile capacity of purple phototrophic bacteria (PPB) for producing valuable bioproducts has gathered renewed interest in the field of resource recovery and waste valorisation. However, greater knowledge regarding the viability of applying PPB technologies in outdoor, large-scale systems is required. This study assessed, for the first time, the upscaling of the phototrophic polyhydroxyalkanoate (PHA) production technology in a pilot-scale system operated in outdoor conditions. An integrated system composed of two up-flow anaerobic sludge blanket (UASB) reactors (for fermentation of wastewater with molasses), and two high-rate algal ponds retrofitted into PPB ponds, was operated in a wastewater treatment plant under outdoor conditions. UASB's adaptation to the outdoor temperatures involved testing different operational settings, namely hydraulic retention times (HRT) of 48 and 72 h, and molasses fermentation in one or two UASBs. Results have shown that the fermentation of molasses in both UASBs with an increased HRT of 72 h was able to ensure a suitable operation during colder conditions, achieving 3.83 ± 0.63 g CODFermentative Products/L, compared to the 3.73 ± 0.85 g CODFermentative Products/L achieved during warmer conditions (molasses fermentation in one UASB; HRT 48 h). Furthermore, the PPB ponds were operated under a light-feast/dark-aerated-famine strategy and fed with the fermented wastewater and molasses from the two UASBs. The best PHA production was obtained during the summer of 2018 and spring of 2019, attaining 34.7 % gPHA/gVSS with a productivity of 0.11 gPHA L-1 day-1 and 36 % gPHA/gVSS with a productivity of 0.14 gPHA L-1 day-1, respectively. Overall, this study showcases the first translation of phototrophic PHA production technology from an artificially illuminated laboratory scale system into a naturally illuminated, outdoor, pilot-scale system. It also addresses relevant process integration aspects with UASBs for pre-fermenting wastewater with molasses, providing a novel operational strategy to achieve photosynthetic PHA production in outdoor full-scale systems.

Hydraulic Retention Time as an Operational Tool for the Production of Short-Chain Carboxylates via Anaerobic Fermentation of Carbohydrate-Rich Waste

The carboxylate platform is a sustainable and cost-effective way to valorize wastes into biochemicals that replace those of fossil origin. Short-chain fatty acids (SCFAs) are intermediates generated during anaerobic fermentation (AF) and are considered high-value-added biochemicals among carboxylates. This investigation aimed to produce SCFAs through the AF of sugar beet molasses at 25 °C and semi-continuous feeding mode in completely stirred tank reactors. A particular focus was devoted to the role of hydraulic retention time (HRT) variation in SCFAs production and distribution profile. The highest SCFAs concentration (44.1 ± 2.3 gCOD/L) was reached at the HRT of 30 days. Caproic acid accounted for 32.5-35.5% (COD-concentration basis) at the long HRTs of 20 and 30 days due to the carbon chain elongation of shorter carboxylic acids. The findings of this study proved that HRT could be used to steer the anaerobic process toward the targeted SCFAs for specific uses. Furthermore, the successful operation at low-temperature conditions (i.e., 25 °C) makes the process economically promising.

Toward the use of mixed microbial cultures for the biological production of adipic and levulinic acid

Biological synthesis of high added-value compounds like adipic acid (AA), levulinic acid (LA), or polyhydroxybutyrate (PHB) using pure culture has been separately reported. However, pure culture requires sterile conditions and the use of specific carbon sources resulting in high operating costs. Different alternatives based on the use of mixed microbial cultures (MMC) have been explored to resolve this problem. MMC have been widely reported for the production of PHB, but scarcely reported for LA production and never for AA synthesis. This work presents a novel strategy for the co-production of AA LA, and PHB using MMC. The strategy consists in selecting an MMC producer of AA, LA and PHB from an inoculum obtained from a wastewater treatment plant, which is then subjected to the feast and famine culture strategy in a sequential batch reactor, coupled with a batch reactor step to enhance the accumulation of AA and LA. The results showed that the MMC could produce a 16 ± 2, 23 ± 1 and 5 ± %1 (g compound/g volatile solids) of AA, LA and PHB, respectively, using a non-fermented residual biomass rich in pentose, namely synthetic hemicellulose hydrolysate (SHH) as the carbon source. These results contribute to generating future research to better understand and optimise the biosynthesis of these compounds by MMC.

Anammox bacteria adapt to long-term light irradiation in photogranules

Photogranules composed of algae, nitrifiers, and anammox bacteria are promising for nitrogen removal from wastewater with reduced aeration and carbon emissions. However, it is difficult to be achieved as the potential inhibition of anammox bacteria by light. In this study, a syntrophic algal-partial nitrification/anammox granular sludge process was developed, with a nitrogen removal rate of 294.5 mg N/(L·d). We found the symbiosis in the community promoted the adaptation of anammox bacteria under light, and cross-feeding played an important role. Microalgae in the outer layers of photogranules sheltered most of the light and supplied cofactors and amino acids to promote nitrogen removal. In particular, Myxococcota MYX1 degraded the extracellular proteins produced by microalgae, providing amino acids to the entire bacterial community, which helped anammox bacteria save metabolic energy and adapt to light. Notably, the anammox bacteria Candidatus Brocadia exhibited unique light-sensing potential and adaptations to light irradiation compared with Candidatus Jettenia, including diverse DNA repair, scavenging of reactive oxygen species, cell movement. The phytochrome-like proteins encoded by Candidatus Brocadia further facilitated their spatial positioning and niche partitioning in photogranules. This study provides insights into the response of anammox bacteria in the algae-bacteria symbiosis system and suggests its potential application for carbon-negative nitrogen removal.

Effect of organic loading rate on the production of Polyhydroxyalkanoates from sewage sludge

The aim of this work was to study the effect of organic loading rate on the production of Polyhydroxyalkanoates (PHA) from sewage sludge. Synthesis of PHA using sewage sludge as platform was achieved in this work. Three pilot-scale selection-sequencing batch reactors (S-SBR) were used for obtaining a culture able to accumulate PHA following a strategy of aerobic dynamic feeding (ADF) at different volumetric organic-loading-rate (vOLR): 1.3, 1.8 and 0.8 g COD L^-1 d^-1 for S-SBR 1, S-SBR 2 and S-SBR 3, respectively. Decreasing the vOLR enhanced the general performance of the process as for organic matter removal (from 99.2% ± 0.3% in S-SBR-3 to 92 ± 2 in S-SBR-2) while the opposite trend was recorded for PHA production (6.0 PHA % w/w in S-SBR-3 vs 13.7 PHA % w/w in S-SBR-2 at the end of the feast phase). Furthermore, indirect and direct emissions, as N2O, were evaluated during the process for the first time. Finally, three accumulation tests were performed achieving 24% w/w.

Production of polyhydroxyalkanoates from renewable resources: a review on prospects, challenges and applications

Bioplastics replace synthetic plastics of petrochemical origin, which contributes challenge to both polymer quality and economics. Novel polyhydroxyalkanoates (PHA)-composite materials, with desirable product quality, could be developed, thus targeting the global plastics market, in the coming years. It is possible that PHA can be a greener substitute for their petroleum-based competitors since they are simply decomposed, which may lessen the pressure on municipal and industrial waste management systems. PHA production has proven to be the bottleneck in industrial application and commercialization because of the high price of carbon substrates and downstream processes required to achieve reliability. Bacterial PHA production by these municipal and industrial wastes, which act as a cheap, renewable carbon substrate, eliminates waste management hassles and acts as an efficient substitute for synthetic plastics. In the present review, challenges and opportunities related to the commercialization of polyhydroxyalkanoates are discussed and presented. Moreover, it discusses critical steps of their production process, feedstock evaluation, optimization strategies, and downstream processes. This information may provide us the complete utilization of bacterial PHA during possible applications in packaging, nutrition, medicine, and pharmaceuticals.

Biopolymers Produced by Lactic Acid Bacteria: Characterization and Food Application

Plants, animals, bacteria, and food waste are subjects of intensive research, as they are biological sources for the production of biopolymers. The topic links to global challenges related to the extended life cycle of products, and circular economy objectives. A severe and well-known threat to the environment, the non-biodegradability of plastics obliges different stakeholders to find legislative and technical solutions for producing valuable polymers which are biodegradable and also exhibit better characteristics for packaging products. Microorganisms are recognized nowadays as exciting sources for the production of biopolymers with applications in the food industry, package production, and several other fields. Ubiquitous organisms, lactic acid bacteria (LAB) are well studied for the production of exopolysaccharides (EPS), but much less as producers of polylactic acid (PLA) and polyhydroxyalkanoates (PHAs). Based on their good biodegradability feature, as well as the possibility to be obtained from cheap biomass, PLA and PHAs polymers currently receive increased attention from both research and industry. The present review aims to provide an overview of LAB strains' characteristics that render them candidates for the biosynthesis of EPS, PLA, and PHAs, respectively. Further, the biopolymers' features are described in correlation with their application in different food industry fields and for food packaging. Having in view that the production costs of the polymers constitute their major drawback, alternative solutions of biosynthesis in economic terms are discussed.

Polyhydroxyalkanoates production from ethanol- and lactate-rich fermentate of confectionary industry effluents

Polyhydroxyalkanoate (PHA) production has been the focus of considerable research to increase productivities and reduce production costs. In this study, a fermented confectionary industry wastewater was used as feedstock for mixed microbial culture PHA production. The feedstock was dominated by lactate and ethanol (60-90 % of all soluble fermentation products). The culture selection reactor was inoculated with municipal activated sludge and was operated at an organic loading rate (OLR) of 100 Cmmol·L^-1·d^-1, achieving a robust PHA-accumulating enrichment, which produced up to 52.6 ± 0.4 wt% of PHA in accumulation assays. An OLR increase in the culture selection stage to 150 Cmmol·L^-1·d^-1 led to a PHA content of 59.1 ± 0.6, a yield of 0.93 ± 0.01 Cmol-PHA·Cmol-S^-1 and a productivity of 0.93 ± 0.01 g-PHA L^-1·h^-1. A correlation analysis of the impact of ethanol concentrations from 3.19 to 20.3 Cmmol·L^-1 in the reactor showed that ethanol inhibited PHA production rate and yield and the consumption of other carbon sources available. Microbial community analysis revealed the increase of Amaricoccus genus during the bioreactor operation time, a known PHA accumulator. The produced polymer was poly(3-hydroxybutyrate) with an average molecular weight of 4.3 × 10⁵ Da and a polydispersity index of 1.88.

Trends and challenges in the valorization of kitchen waste to polyhydroxyalkanoates

Kitchen waste (KW) is frequently available for free or with a negative cost due to its huge production. It contains a large proportion of organic substances, especially fermentable sugars, which can be used for bioplastic (polyhydroxyalkanoates or PHA) synthesis. Nevertheless, due to the difficulties in processing, various pre-treatments of KW are being investigated to enhance the concentration of simple sugars released during its hydrolysis. The effective use of KW will help in minimizing the issues of its inappropriate disposal. However, the review on KW to bioplastic synthesis is rarely reported in the literature. Hence, this particular review provides a comprehensive summary of the updated research developments in KW valorization and its potency as a feedstock for PHAs synthesis. Additionally, the impacts of KW, its availability, the necessary pre-treatments for the biopolymerization process, as well as the prospects and challenges for industrially generating sustainable PHAs, are critically discussed.

Formation of polyhydroxyalkanoates using agro and industrial waste as a substrate - a review

In the present scenario, rising environmental concerns of non-biodegradable plastic pollution and depletion of petroleum based raw materials lead to the development of biopolymers. The biodegradability of biopolymers gives them a specific advantage for the environmental concerns. Polyhydroxyalkanoates (PHAs) are a type of biopolymers which are synthesized by microorganisms. Although there are different substrates available in pure forms which are currently used in the production of PHA, 40% of production cost depends on the expensive substrate which is a major disadvantage and make it far from many applications. The use of an inexpensive carbon source which is high in organic matter content such as waste streams of process industries can make this process viable and diminish PHA production cost. This study explores the current research initiatives on various agricultural and industrial waste feedstocks, formulations and processing conditions for producing PHA in a way that is both inexpensive and beneficial to the environment. The creation of fermentation conditions and metabolic engineering techniques for promoting microbial growth and PHA synthesis were also discussed in the review.

Thauera sp. Sel9, a new bacterial strain for polyhydroxyalkanoates production from volatile fatty acids

Thauera is one of the main genera involved in polyhydroxyalkanoate (PHA) production in microbial mixed cultures (MMCs) from volatile fatty acids (VFAs). However, no Thauera strains involved in PHA accumulation have been obtained in pure culture so far. This study is the first report of the isolation and characterization of a Thauera sp. strain, namely Sel9, obtained from a sequencing batch reactor (S-SBR) set up for the selection of PHA storing biomass. The 16S rRNA gene evidenced a high sequence similarity with T. butanivorans species. Genome sequencing identified all genes involved in PHA synthesis, regulation and degradation. The strain Sel9 was able to grow with an optimum of chemical oxygen demand-to-nitrogen (COD:N) ratio ranging from 4.7 to 18.9. Acetate, propionate, butyrate and valerate were used as sole carbon and energy sources: a lag phase of 72 h was observed in presence of propionate. Final production of PHAs, achieved with a COD:N ratio of 75.5, was 60.12 ± 2.60 %, 49.31 ± 0.7 %, 37.31 ± 0.43 % and 18.06 ± 3.81 % (w/w) by using butyrate, acetate, valerate and propionate as substrates, respectively. Also, the 3-hydroxybutyrate/3-hydroxyvalerate ratio reflected the type of carbon sources used: 12.30 ± 0.82 for butyrate, 3.56 ± 0.02 for acetate, 0.93 ± 0.03 for valerate and 0.76 ± 0.02 for propionate. The results allow a better elucidation of the role of Thauera in MMCs and strongly suggest a possible exploitation of Thauera sp. Sel9 for a cost-effective and environmentally friendly synthesis of PHAs using VFAs as substrate.

Valorization of Brewery Waste through Polyhydroxyalkanoates Production Supported by a Metabolic Specialized Microbiome

Simple Summary

Raw brewers’ spent grain, a by-product of beer production, is produced at a large scale and is usually used as animal feed or is landfilled. However, its composition shows that this feedstock has the potential for other applications, such as bioplastics production (e.g., polyhydroxyalkanoates). In this way, the aim of this work was to assess the use of raw brewers’ spent grain for polyhydroxyalkanoates production, adding new value to this feedstock. The results confirm the potential of raw brewers’ spent grain to produce polyhydroxyalkanoates, as the population was enriched in the microorganisms able to accumulate these biopolymers. These results will contribute to society’s knowledge and competence via the development of a treatment process for brewery waste of both environmental (productive waste treatment) and economic interest (production of biopolymers), which will certainly attract its application to the brewery industry worldwide.

Abstract

Raw brewers’ spent grain (BSG), a by-product of beer production and produced at a large scale, presents a composition that has been shown to have potential as feedstock for several biological processes, such as polyhydroxyalkanoates (PHAs) production. Although the high interest in the PHA production from waste, the bioconversion of BSG into PHA using microbial mixed cultures (MMC) has not yet been explored. This study explored the feasibility to produce PHA from BSG through the enrichment of a mixed microbial culture in PHA-storing organisms. The increase in organic loading rate (OLR) was shown to have only a slight influence on the process performance, although a high selectivity in PHA-storing microorganisms accumulation was reached. The culture was enriched on various PHA-storing microorganisms, such as bacteria belonging to the Meganema, Carnobacterium, Leucobacter, and Paracocccus genera. The enrichment process led to specialization of the microbiome, but the high diversity in PHA-storing microorganisms could have contributed to the process stability and efficiency, allowing for achieving a maximum PHA content of 35.2 ± 5.5 wt.% (VSS basis) and a yield of 0.61 ± 0.09 Cmmol_PHA/Cmmol_VFA in the accumulation assays. Overall, the production of PHA from fermented BSG is a feasible process confirming the valorization potential of the feedstock through the production of added-value products.

Polyhydroxyalkanoate recovery from newly screened Bacillus sp. LPPI-18 using various methods of extraction from Loktak Lake sediment sample

BACKGROUND

Nowadays, the conventional plastic wastes are very challenging to environments and its production cost also creates an economic crisis due to petrochemical-based plastic. In order to solve this problem, the current studies were aimed at screening and characterizing these polyhydroxyalkanoate (PHA)-producing isolates and evaluating the suitability of some carbon source for newly screened PHA-producing isolates.

MATERIAL AND METHODS

Some carbon sources such as D-fructose, glucose, molasses, D-ribose and sucrose were evaluated for PHA production. Data were analyzed using SPSS version 20. The 16SrRNA gene sequence of these isolates was performed. These newly isolated taxa were related to Bacillus species. It was designated as Bacillus sp. LPPI-18 and affiliated Bacillus cereus ATCC 14577^T (AE01687) (99.10%). Paenibacillus sp. 172 (AF273740.1) was used as an outgroup.

RESULTS

Bacillus sp. LPPI-18 is a gram-positive, rod-shaped, endospore former, and citrate test positive. This isolate showed positive for amylase, catalase, pectinase, and protease test. They produced intracellular PHA granules when this isolate was stained with Sudan Black B (SBB) and Nile blue A (NBA) preliminary and specific staining dyes, respectively. Both temperature and pH used to affect polyhydroxyalkanoates (PHA) productivity. Bacteria are able to reserve PHA in the form of granules during stress conditions. This isolate produces only when supplied with carbon sources. More PHA contents (PCs) were obtained from glucose, molasses, and D-fructose. In this regard, the maximum mean value of PC was obtained from glucose (40.55±0.7%) and the minimum was obtained from D-ribose (12.4±1.4%). Great variations (P≤0.05) of PCs were observed among glucose and sucrose, molasses and sucrose, and D-fructose and sucrose carbon sources for PHA productivity (PP) of cell dry weight (CDW) g/L. After extraction, PHA film was produced for this typical isolate using glucose as a sole carbon source. Fourier transform infrared spectrum was performed for this isolate and showed the feature of polyester at 1719.64 to 1721.16 wavelengths for these extracted samples. The peak of fingerprinting (band of carboxylic acid group) at this wavelength is a characteristic feature of polyhydroxybutyrate (PHB) and corresponds to the ester functional group (C=O).

CONCLUSION

In this study, newly identified Bacillus sp. LPPI-18 is found to be producing biodegradable polymers that are used to replace highly pollutant conventional plastic polymers. This isolate is also used to employ certain cost-effective carbon sources for the production of PHA polymers.

Influent carbon to phosphorus ratio drives the selection of PHA-storing organisms in a single CSTR

Enriching a biomass with a high fraction of polyhydroxyalkanoate-storing organisms (PHA-storers) represents an essential step in the production of PHAs (bioplastics) from municipal wastewater using mixed microbial cultures. A major challenge is however to create selective growth conditions that are favourable to PHA-storers. Our study thus investigates to what extent the influent COD to phosphorus (COD:P) ratio can be used as a tool for the robust selection of PHA-storers in a single continuous-flow stirred-tank reactor (CSTR). Therefore, we operated five CSTRs in parallel, fed with synthetic wastewater (50% acetate - 50% propionate) with different COD:P ratios (200-1000 gCOD gP^-1), and performed a detailed analysis of the microbial communities over long-term (30-70 solid retention times). Our study demonstrates that efficient and robust selection of PHA-storers can be achieved in a single CSTR at high influent COD:P ratios. The selective advantage for PHA-storers increases with the influent COD:P ratio, but only if growth conditions remain limited by both C-substrate and P. In contrast, selection performance deteriorates when COD:P ratios are too high and growth conditions are limited by P only. At an optimal COD:P ratio of 800 gCOD gP^-1, a stable microbial community consisting of >90% PHA-storers and dominated by Pannonibacter sp. was selected in the long-term. Finally, our results suggest that high COD:P ratios provide a selective advantage to microorganisms with low cellular P requirements, explaining why different PHA-storers (i.e., Xanthobacter sp. vs. Pannonibacter sp.) were selected depending on the influent COD:P ratio (i.e., 200 vs. 800 gCOD gP^-1). Overall, our results provide relevant insights for the development of a new approach for selecting PHA-storers, based on the use of a single CSTR and control of the influent COD:P ratio.

Cyanobacteria as a Promising Alternative for Sustainable Environment: Synthesis of Biofuel and Biodegradable Plastics

With the aim to alleviate the increasing plastic burden and carbon footprint on Earth, the role of certain microbes that are capable of capturing and sequestering excess carbon dioxide (CO2) generated by various anthropogenic means was studied. Cyanobacteria, which are photosynthetic prokaryotes, are promising alternative for carbon sequestration as well as biofuel and bioplastic production because of their minimal growth requirements, higher efficiency of photosynthesis and growth rates, presence of considerable amounts of lipids in thylakoid membranes, and cosmopolitan nature. These microbes could prove beneficial to future generations in achieving sustainable environmental goals. Their role in the production of polyhydroxyalkanoates (PHAs) as a source of intracellular energy and carbon sink is being utilized for bioplastic production. PHAs have emerged as well-suited alternatives for conventional plastics and are a parallel competitor to petrochemical-based plastics. Although a lot of studies have been conducted where plants and crops are used as sources of energy and bioplastics, cyanobacteria have been reported to have a more efficient photosynthetic process strongly responsible for increased production with limited land input along with an acceptable cost. The biodiesel production from cyanobacteria is an unconventional choice for a sustainable future as it curtails toxic sulfur release and checks the addition of aromatic hydrocarbons having efficient oxygen content, with promising combustion potential, thus making them a better choice. Here, we aim at reporting the application of cyanobacteria for biofuel production and their competent biotechnological potential, along with achievements and constraints in its pathway toward commercial benefits. This review article also highlights the role of various cyanobacterial species that are a source of green and clean energy along with their high potential in the production of biodegradable plastics.

The Hyperproduction of Polyhydroxybutyrate Using Bacillus mycoides ICRI89 through Enzymatic Hydrolysis of Affordable Cardboard

Bioplastics are contemplated as remarkable substitutes for conventional plastics to accommodate green technological advancements. However, their industrial production has not been fully implemented owing to the cost of carbon resources. From another perspective, valorizing different paper mill wastes has become a prominent research topic. These materials may serve as an affording sustainable feedstock for bioplastic production. Adjustment of cardboard waste hydrolysate as suitable fermentation media for production of bacterial polyhydroxyalkanoates (PHAs) has been investigated. Cardboard samples were defibered and dried before enzymatic hydrolysis. The enzymatic degradation of commercial cellulase was monitored over 15 days. Interestingly, 18.2 ± 0.2 g/L glucose yield was obtained from 50 g cardboard samples using a 1.5% (v/v) enzyme concentration. The samples exhibited maximum weight loss values of 69-73%. Meanwhile, five soil samples were collected from local sites in Lodz, Poland. A total of 31 bacterial isolates were screened and cultured on Nile blue plates. Analysis of the 16S rRNA gene sequence of the most potent producer revealed 100% similarity to Bacillus mycoides. Cardboard hydrolysates whole medium, modified MSM with cardboard hydrolysate and nitrogen depleted MSM with cardboard hydrolysate were utilized for PHA production, followed by PHA productivity and cell dry weight (CDW) estimation compared to glucose as a standard carbon source. An impressive PHA accumulation of 56% CDW was attained when the waste hydrolysate was used as a carbon source. FTIR and NMR analysis of the isolated PHA indicated that functional groups of the polymer were related to PHB (polyhydroxybutyrate). Thermal analysis demonstrates that PHB and PHB-CB (PHB produced from cardboard hydrolysate) have degradation temperatures of 380 and 369 °C, respectively, which reflect the high thermal stability and heat resistance compared to the same properties for a standard polymer. This is the first demonstration of full saccharification of corrugated cardboard paper waste for high-level production of PHA. In addition, the attained PHB productivity is one of the highest levels achieved from a real lignocellulosic waste.

From Organic Wastes and Hydrocarbons Pollutants to Polyhydroxyalkanoates: Bioconversion by Terrestrial and Marine Bacteria

The use of fossil-based plastics has become unsustainable because of the polluting production processes, difficulties for waste management sectors, and high environmental impact. Polyhydroxyalkanoates (PHA) are bio-based biodegradable polymers derived from renewable resources and synthesized by bacteria as intracellular energy and carbon storage materials under nutrients or oxygen limitation and through the optimization of cultivation conditions with both pure and mixed culture systems. The PHA properties are affected by the same principles of oil-derived polyolefins, with a broad range of compositions, due to the incorporation of different monomers into the polymer matrix. As a consequence, the properties of such materials are represented by a broad range depending on tunable PHA composition. Producing waste-derived PHA is technically feasible with mixed microbial cultures (MMC), since no sterilization is required; this technology may represent a solution for waste treatment and valorization, and it has recently been developed at the pilot scale level with different process configurations where aerobic microorganisms are usually subjected to a dynamic feeding regime for their selection and to a high organic load for the intracellular accumulation of PHA. In this review, we report on studies on terrestrial and marine bacteria PHA-producers. The available knowledge on PHA production from the use of different kinds of organic wastes, and otherwise, petroleum-polluted natural matrices coupling bioremediation treatment has been explored. The advancements in these areas have been significant; they generally concern the terrestrial environment, where pilot and industrial processes are already established. Recently, marine bacteria have also offered interesting perspectives due to their advantageous effects on production practices, which they can relieve several constraints. Studies on the use of hydrocarbons as carbon sources offer evidence for the feasibility of the bioconversion of fossil-derived plastics into bioplastics.

An Eco-Friendly Conversion of Aquaculture Suspended Solid Wastes Into High-Quality Fish Food by Improving Poly-β-Hydroxybutyrate Production

The aquaculture industry is vital in providing a valuable protein food source for humans, but generates a huge amount of solid and dissolved wastes that pose great risks to the environment and aquaculture sustainability. Suspended solids (in short SS), one of the aquaculture wastes, are very difficult to be treated due to their high organic contents. The bioconversion from wastewater, food effluents, and activated sludge into poly-β-hydroxybutyrate (PHB) is a sustainable alternative to generate an additional income and could be highly attractive to the agricultural and environmental management firms. However, little is known about its potential application in aquaculture wastes. In the present study, we first determined that 7.2% of SS was PHB. Then, the production of PHB was increased two-fold by the optimal fermentation conditions of wheat bran and microbial cocktails at a C/N ratio of 12. Also, the PHB-enriched SS showed a higher total ammonia nitrogen removal rate. Importantly, we further demonstrated that the PHB-enriched SS as a feed could promote fish growth and up-regulate the expression of the immune-related genes. Our study developed an eco-friendly and simple approach to transforming problematic SS wastes into PHB-enriched high-quality food for omnivorous fish, which will increase the usage efficiency of SS and provide a cheaper diet for aquatic animals.

Polyhydroxyalkanoate bio-production and its rise as biomaterial of the future

The first observation of a polyhydroxyalkanoate (PHA) aggregate was in 1888 by Beijenrinck. Despite polyhydroxybutyrate (PHB) being the first type of PHA discovered, it was not extracted and characterized until 1925 by Maurice Lemoigne in France, even before the concept of "macromolecules" was known. After more than 30 years, in 1958, Wilkinson and co-workers rediscovered PHB and its metabolic role in the cells as storage compound. PHB started to be appealing to the industry in the 1980s, when a few companies started to commercialize microbially produced PHAs. During the 1990 s, the focus was on reducing production costs to make PHA production economically feasible, for instance by genetically modified microorganisms and even plants. Since then, many advances have been made: diverse wastes as feedstock, different production processes, and tailored design of biopolymers. This paper summarizes the scientific and technological development of PHAs from their discovery in 1888 until their latest applications and current commercial uses. Future perspectives have been devised too based on the current bottlenecks.

Ecological insight into incompatibility between polymer storage and floc settling in polyhydroxyalkanoate producer selection using complex carbon sources

Polyhydroxyalkanoate (PHA) producer selection is a key step in mixed culture (MC) production. This study focused on incompatibility between PHA storage and floc settling of MCs in the selection process. In a selector using fermented waste activated sludge as substrate under varying organic loading, average maximum PHA content obtained in batch assays increased by ∼ 22 wt% and biomass concentration increased by ∼ 34% with the increasing of organic loading. However, poor floc settling occurred, causing decreased batch PHA production and costly downstream process. A flank community which can corporately use non-VFA organics existed in the selector. When organic loading increased, PHA producers had stronger negative interactions, but not cooperation with the flank community members. Thus, high PHA storage of MCs was bounded to the domination of core PHA producer. But the domination of Thauera bacteria under high organic loading indirectly induced a bloom of filamentous bacteria.

The impact of biomass withdrawal strategy on the biomass selection and polyhydroxyalkanoates accumulation of mixed microbial cultures

The production of polyhydroxyalkanoates (PHA) by mixed microbial cultures (MMC) has been studied as an alternative to pure cultures in order to reduce the price of PHA through use of open systems and low-cost substrates, such as agro-industrial sub-products. However, the widespread applicability of this process depends on the optimization of operational factors impacting PHA productivity. This study addresses the impact of biomass withdrawal strategy on the performance of MMC selection reactors and consequently on biomass productivity and global PHA productivity. Two selection reactors were operated in parallel under similar conditions, except for the timing of biomass withdrawal, at the end of the famine phase (Reactor 1, R1) versus at the end of the feast phase (Reactor 2, R2) at an organic loading rate of 100 Cmmol.L^-1.d^-1 and solids retention time of 4 days. The biomass selected in both conditions had similar PHA storing capacity as shown by the similar yields of PHA per substrate obtained in the accumulation assays; however, R1 reached a higher biomass productivity (about 4-fold higher than R2). This study demonstrated that removing the excess biomass at the end of the famine phase resulted in a much higher global PHA productivity and that the key parameter affecting the global PHA productivity of the 2-stage system was the volumetric biomass productivity. Results obtained provide important insight into how MMC systems can be best operated to maximize PHA productivity.

Insightful Advancement and Opportunities for Microbial Bioplastic Production

Impetuous urbanization and population growth are driving increased demand for plastics to formulate impeccable industrial and biomedical commodities. The everlasting nature and excruciating waste management of petroleum-based plastics have catered to numerous challenges for the environment. However, just implementing various end-of-life management techniques for assimilation and recycling plastics is not a comprehensive remedy; instead, the extensive reliance on finite resources needs to be reduced for sustainable production and plastic product utilization. Microorganisms, such as bacteria and algae, are explored substantially for their bioplastic production repertoire, thus replacing fossil-based plastics sooner or later. Nevertheless, the utilization of pure microbial cultures has led to various operational and economical complications, opening the ventures for the usage of mixed microbial cultures (MMCs) consisting of bacteria and algae for sustainable production of bioplastic. The current review is primarily focuses on elaborating the bioplastic production capabilities of different bacterial and algal strains, followed by discussing the quintessence of MMCs. The present state-of-the-art of bioplastic, different types of bacterial bioplastic, microalgal biocomposites, operational factors influencing the quality and quantity of bioplastic precursors, embracing the potential of bacteria-algae consortia, and the current global status quo of bioplastic production has been summarized extensively.

Integrating mechanistic and deep learning models for accurately predicting the enrichment of polyhydroxyalkanoates accumulating bacteria in mixed microbial cultures

The enrichment of polyhydroxyalkanoates (PHA) accumulating bacteria (PAB) in mixed microbial cultures (MMC) is extremely difficult to be predicted and optimized. Here we demonstrate that mechanistic and deep learning models can be integrated innovatively to accurately predict the dynamic enrichment of PAB. Well-calibrated activated sludge models (ASM) of the PAB enrichment process provide time-dependent data under different operating conditions. Recurrent neural network (RNN) models are trained and tested based on the time-dependent dataset generated by ASM. The accurate prediction performance is achieved (R² > 0.991) for three different PAB enrichment datasets by the optimized RNN model. The optimized RNN model can also predict the equilibrium concentration of PAB (R² = 0.944) and corresponding time, which represents the end of the PAB enrichment process. This study demonstrates the strength of integrating mechanistic and deep learning models to predict long-term variations of specific microbes, helping to optimize their selection process for PHA production.

Enhanced polyhydroxybutyrate production from acid whey through determination of process and metabolic limiting factors

To sustainably produce biodegradable polyhydroxybutyrate (PHB), this study investigated effects of process and metabolic limiting factors during bioconversion of acid whey (AW) to PHB, offering economic and environmental advantages for dairy industry. Recombinant Escherichia coli LSBJ was used to achieve high PHB yields by utilizing both lactose and lactic acid as carbon source. Up to 85% PHB accumulation was achieved during growth on the synthetic AW. Growth on raw AW had the highest PHB yield of 4 g/L and a high substrate utilization efficiency (95%). Notably, ratios of lactate: lactose and C/N impacted metabolic flux and PHB yields. Maintaining the fermentation pH enhanced PHB production. Furthermore, additives of inorganic nitrogen sources, minerals and trace metals promoted PHB production from AW. The study improves the understanding of factors affecting utilization of AW and demonstrated the high PHB yields using recombinant E. coli that could be leveraged to design a sustainable process.

On-Line Control of Feast/Famine Cycles to Improve PHB Accumulation during Cultivation of Mixed Microbial Cultures in Sequential Batch Reactors

Production of polyhydroxyalkanoates (PHA) has generated great interest as building blocks for bioplastic production. Their production using mixed microbial cultures represents an interesting alternative, since it enables the use of organic wastes as a carbon source. Feast/famine strategy is a common way to promote selection of microorganisms with PHA accumulation capacity. However, when using waste sources, changes in substrate concentration are expected, that may affect performance and efficiency of the process. This study showed how the dissolved oxygen level can be used for online control of the cycle time, ensuring that the desired feast/famine ratio is effectively applied. An operation strategy is presented and validated, using sequential batch reactors fed with acetate as the carbon source. Production of polyhydroxybutyrate (PHB) was studied, which is the expected type of PHA to be synthetized when using acetate as substrate. Two reactors were operated by applying the proposed control strategy, to provide F/F ratios of 0.2 and 0.6, respectively. A third reactor was operated with a fixed cycle time, for comparison purposes. Results showed that the reactor that operated at an F/F ratio of 0.6 promoted higher biomass productivity and PHB content, as a result of a better use of available time, preventing unnecessary long famine times. The application of the tested strategy is a simple a reliable way to promote a better performance of feast/famine-based bioreactors involving mixed microbial cultures for PHB production.

Dependence of poly-β-hydroxybutyrate accumulation in sludge on biomass concentration in SBRs

The combination of wastewater treatment with polyhydroxyalkanoate production has attracted increasing interest in the context of the circular economy. Recent studies have thus attempted to optimize the conditions for polyhydroxyalkanoate accumulation in sludge when treating wastewater. The effects of biomass concentration and sludge morphologies in reactors on PHB storage, however, were neglected in the literature. Therefore, in this study settling time and organic loading rate were manipulated to adjust sludge morphology and biomass concentration in sequential batch reactors (SBRs) to investigate their influence on PHB storage in the feast phase. Our study shows that reducing settling times in SBRs from 10 to 0 min under organic loading rate of 3 g L^-1 d^-1 resulted in the decrease in biomass concentration at steady states from 4.2 to 1.0 g L^-1 and the change of sludge morphology from well-settled granules to poorly settled pinpoint flocs, but PHB content in sludge at the end of feast phase increased from 7.7 to 26.7%. The well-fitted regression lines between PHB content, SRT, feast/famine and food/microorganisms ratios and biomass concentration under different settling times suggest that PHB was highly dependent on biomass concentration but independent on sludge morphology. Under settling time of 0 min, the increase in OLR from 3 to 7.5 g L^-1 d^-1 resulted in an increased biomass concentration from 1.0 to 2.1 g L^-1 and an increase in PHB content from 26.7 to 33.8%. The batch and fed-batch experiments with different biomass concentrations also showed the influence of biomass concentration on PHB accumulation in sludge. The conclusion of the dependence of PHB content on biomass concentration under a fixed OLR and varied OLRs drawn from this study enables sludge PHB content as high as possible by adjusting biomass concentration in SBRs apart from the selective enriching strategies for PHB accumulating organisms when treating VFA-rich wastewater.

Research and economic perspectives on an integrated biorefinery approach for the simultaneous production of polyhydroxyalkanoates and biohydrogen

Alarming environmental impacts have been resulted across the globe due to the recovery and consumption of fossil fuels. The elevated global carbon footprint has paved the way to an alternative to combat the prevalent pollution. On the other hand, the fossil-based plastics produced from the byproducts of petroleum remain intact in the environment leading to pollution. Fossil abated bioproducts are in high demand due to the increase in pollution. This call to utilize feedstock for simultaneous production of biologically useful products through carbon capture utilisation where the leftover carbon-rich substrate is converted into usable chemicals like bioplastics, methanol, urea and various other industrially essential components. The present review extensively focuses on the research and economic perspectives of an integrated biorefinery and addresses technical breaches, bottlenecks, and efficient strategies for the simultaneous production of biohydrogen and polyhydroxyalkanoates.

Bioconversion of renewable lignocellulosic biomass into multicomponent substrate via pressurized hot water pretreatment for bioplastic polyhydroxyalkanoate accumulation

The pretreatment of lignocellulosic biomass (LB) has become an important process to reduce the cost of polyhydroxyalkanoate (PHA) production. In this study, an economical and effective pressurized hot water pretreatment was used to investigate on bioconversion of four typical LB (rubber wood, sugarcane bagasse, sorghum stalk, cassava stalk) into reducing sugar, then as feedstock to accumulate PHA by mixed microbial cultures. The results showed that pretreatment changed the surface morphology of LB and further improved saccharification efficiency. The maximum reducing sugar yield of 620.7 mg·g^-1 (438.7 mg·g^-1 glucose and 178.0 mg·g^-1 xylose) was obtained by rubber wood. The conversion of cellulose and hemicellulose reached 83.10% and 78.22%. Moreover, PHA content reached the maximum of 773.2 mg COD·L^-1 in the operation cycle of 24 h. The results demonstrated that hot water pretreatment was an effective physical process to improve the saccharification efficiency of LB for reducing the cost of PHA.

Beyond PHA: Stimulating intracellular accumulation of added-value compounds in mixed microbial cultures

This review compiled and analyzed the operational conditions (dissolved oxygen, feast and famine ratio, sequential batch reactor cycle length, organic loading rate (OLR), pH, C/N, and temperature) established during the feast and famine culture strategy for the mixed microbial cultures (MMC) selection to understand how these variables could affect the synthesis of polyhydroxyalkanoates, polyglucose, triacylglycerides, levulinic acid and adipic acid from non-fermented substrates. According to the reported information, the dissolved oxygen has a greater impact on the type and amount of produced compound. In a lesser extent, the OLR and the cycle length were identified to have an impact on the accumulation of polyhydroxyalkanoates, whose accumulation was favored at lower OLR and longer cycle lengths. Thereby, the information of this work will allow the design of future strategies for the simultaneous accumulation of compounds of interest other than the polyhydroxyalkanoates or understand the operational conditions that would optimize the polyhydroxyalkanoates production.

Bioconversion of Organic Pollutants in Fish-Canning Wastewater into Volatile Fatty Acids and Polyhydroxyalkanoate

The wastewater from the cookers of a tuna-canning plant was used as feedstock for the process. It was acidified in a continuous stirred tank reactor (CSTR) of 1.5 L to produce a mixture of volatile fatty acids (VFAs). The effluent contained 28.3 ± 8.7 g COD_S/L and 25.0 ± 4.6 g COD_VFA/L, 4.4 ± 1.6 g NH₄⁺/L, and 10.9 ± 4.0 g Na⁺/L, which corresponds to about 28 g NaCl/L approximately. This was used to feed a PHA production system. The enriched MMC presented a capacity to accumulate PHAs from the fermented tuna wastewater. The maximum PHA content of the biomass in the fed-batch (8.35 wt% PHA) seemed very low, possibly due to the variable salinity (from 2.2 up to 12.3 g NaCl/L) and the presence of ammonium (which promoted the biomass growth). The batch assay showed a PHA accumulation of 5.70 wt% PHA, but this is a much better result if the productivity of the reactor is taken into account. The fed-batch reactor had a productivity of 10.3 mg PHA/(L h), while the batch value was about five times higher (55.4 mg PHA/(L h)). At the sight of the results, it can be seen that the acidification of fish-canning wastewater is possible even at high saline concentrations (27.7 g NaCl/L). On the other hand, the enrichment and accumulation results show us promising news and which direction has to be followed: PHAs can be obtained from challenging substrates, and the feeding mode during the accumulation stage has an important role to play when it comes to inhibition.

Combined Strategies to Prompt the Biological Reduction of Chlorinated Aliphatic Hydrocarbons: New Sustainable Options for Bioremediation Application

Groundwater remediation is one of the main objectives to minimize environmental impacts and health risks. Chlorinated aliphatic hydrocarbons contamination is prevalent and presents particularly challenging scenarios to manage with a single strategy. Different technologies can manage contamination sources and plumes, although they are usually energy-intensive processes. Interesting alternatives involve in-situ bioremediation strategies, which allow the chlorinated contaminant to be converted into non-toxic compounds by indigenous microbial activity. Despite several advantages offered by the bioremediation approaches, some limitations, like the relatively low reaction rates and the difficulty in the management and control of the microbial activity, can affect the effectiveness of a bioremediation approach. However, those issues can be addressed through coupling different strategies to increase the efficiency of the bioremediation strategy. This mini review describes different strategies to induce the reduction dechlorination reaction by the utilization of innovative strategies, which include the increase or the reduction of contaminant mobility as well as the use of innovative strategies of the reductive power supply. Subsequently, three future approaches for a greener and more sustainable intervention are proposed. In particular, two bio-based materials from renewable resources are intended as alternative, long-lasting electron-donor sources (e.g., polyhydroxyalkanoates from mixed microbial cultures) and a low-cost adsorbent (e.g., biochar from bio-waste). Finally, attention is drawn to novel bio-electrochemical systems that use electric current to stimulate biological reactions.

Effects of the Sludge Retention Time and Carbon Source on Polyhydroxyalkanoate-Storing Biomass Selection under Aerobic-Feast and Anoxic-Famine Conditions

Polyhydroxyalkanoates (PHAs) are versatile biodegradable polymers produced by bacteria and are suitable for many downstream applications. They can be produced inexpensively from mixed microbial cultures under feast and famine conditions in the presence of biobased volatile fatty acids (VFAs). Here, we investigated the effect of changing the sludge retention time (SRT) and the addition of fermented cellulosic primary sludge (CPS) as a carbon source on the selection of PHA-storing biomass when applying the feast and famine strategy under aerobic and anoxic conditions, respectively. Increasing the SRT from 5 to 7-10 days enhanced PHA yields under feast conditions from 0.18 gCOD_PHA/gCOD_VFA (period 1) to 0.40 gCOD_PHA/gCOD_VFA (period 2). The use of fermented CPS as a carbon source (period 3) increased PHA yields to 0.62 gCOD_PHA/gCOD_VFA despite the presence of biodegradable non-VFA fractions. Microbial characterization by denaturing gradient gel electrophoresis and fluorescence in situ hybridization revealed high microbial speciation during the three experimental periods. In period 3, the dominant genera were Thauera, Paracoccus, and Azoarcus, which accounted for ∼95% of the total microbial biomass.

Development and Characterization of Electrospun Biopapers of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Derived from Cheese Whey with Varying 3-Hydroxyvalerate Contents

In the present study, three different newly developed copolymers of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with 20, 40, and 60 mol % contents in 3-hydroxyvalerate (3HV) were produced by the biotechnological process of mixed microbial cultures (MMCs) using cheese whey (CW), a by-product from the dairy industry, as feedstock. The CW-derived PHBV copolyesters were first purified and then processed by solution electrospinning, yielding fibers of approximately 2 μm in cross-section in all cases. The resultant electrospun PHBV mats were, thereafter, post-processed by annealing at different temperatures, below their maximum of melting, selected according to their 3HV content in order to obtain continuous films based on coalesced fibers, so-called biopapers. The resultant PHBV films were characterized in terms of their morphology, crystallinity, and mechanical and barrier properties to assess their potential application in food packaging. The CW-derived PHBV biopapers showed high contact transparency but a slightly yellow color. The fibers of the 20 mol % 3HV copolymer were seen to contain mostly poly(3-hydroxybutyrate) (PHB) crystals, the fibers of the 40 mol % 3HV copolymer a mixture of PHB and poly(3-hydroxyvalerate) (PHV) crystals and lowest crystallinity, and the fibers of the 60 mol % 3HV sample were mostly made of PHV crystals. To understand the interfiber coalesce process undergone by the materials during annealing, the crystalline morphology was also assessed by variable-temperature both combined small-angle and wide-angle X-ray scattering synchrotron and Fourier transform infrared experiments. From these experiments and, different from previously reported biopapers with lower 3HV contents, all samples were inferred to have a surface energy reduction mechanism for interfiber coalescence during annealing, which is thought to be activated by a temperature-induced decrease in molecular order. Due to their reduced crystallinity and molecular order, the CW-derived PHBV biopapers, especially the 40 mol % 3HV sample, were found to be more ductile and tougher. In terms of barrier properties, the three copolymers performed similarly to water and limonene, but to oxygen, the 40 mol % sample showed the highest relative permeability. Overall, the materials developed, which are compatible with the Circular Bioeconomy organic recycling strategy, can have an excellent potential as barrier interlayers or coatings of application interest in food packaging.

Resource recovery in aerobic granular sludge systems: is it feasible or still a long way to go?

Lately, wastewater treatment plants are much often being designed as wastewater-resource factories inserted in circular cities. Among biological treatment technologies, aerobic granular sludge (AGS), considered an evolution of activated sludge (AS), has received great attention regarding its resource recovery potential. This review presents the state-of-the-art concerning the influence of operational parameters on the recovery of alginate-like exopolysaccharides (ALE), tryptophan, phosphorus, and polyhydroxyalkanoates (PHA) from AGS systems. The carbon to nitrogen ratio was identified as a parameter that plays an important role for the optimal production of ALE, tryptophan, and PHA. The sludge retention time effect is more pronounced for the production of ALE and tryptophan. Additionally, salinity levels in the bioreactors can potentially be manipulated to increase ALE and phosphorus yields simultaneously. Some existing knowledge gaps in the scientific literature concerning the recovery of these resources from AGS were also identified. Regarding industrial applications, tryptophan has the longest way to go. On the other hand, ALE production/recovery could be considered the most mature process if we take into account that existing alternatives for phosphorus and PHA production/recovery are optimized for activated sludge rather than granular sludge. Consequently, to maintain the same effectiveness, these processes likely could not be applied to AGS without undergoing some modification. Therefore, investigating to what extent these adaptations are necessary and designing alternatives is essential.

Process optimization, metabolic engineering interventions and commercialization of microbial polyhydroxyalkanoates production - A state-of-the art review

Microbial polyhydroxyalkanoates (PHAs) produced using renewable resources could be the best alternative for conventional plastics. Despite their incredible potential, commercial production of PHAs remains very low. Nevertheless, sincere attempts have been made by researchers to improve the yield and economic viability of PHA production by utilizing low-cost agricultural or industrial wastes. In this context, the use of efficient microbial culture or consortia, adoption of experimental design to trace ideal growth conditions, nutritional requirements, and intervention of metabolic engineering tools have gained significant attention. This review has been structured to highlight the important microbial sources for PHA production, use of conventional and non-conventional substrates, product optimization using experimental design, metabolic engineering strategies, and global players in the commercialization of PHA in the past two decades. The challenges about PHA recovery and analysis have also been discussed which possess indirect hurdle while expanding the horizon of PHA-based bioplastics. Selection of appropriate microorganism and substrate plays a vital role in improving the productivity and characteristics of PHAs. Experimental design-based bioprocess, use of metabolic engineering tools, and optimal product recovery techniques are invaluable in this dimension. Optimization strategies, which are being explored in isolation, need to be logically integrated for the successful commercialization of microbial PHAs.

Emergent Approaches to Efficient and Sustainable Polyhydroxyalkanoate Production

Petroleum-derived plastics dominate currently used plastic materials. These plastics are derived from finite fossil carbon sources and were not designed for recycling or biodegradation. With the ever-increasing quantities of plastic wastes entering landfills and polluting our environment, there is an urgent need for fundamental change. One component to that change is developing cost-effective plastics derived from readily renewable resources that offer chemical or biological recycling and can be designed to have properties that not only allow the replacement of current plastics but also offer new application opportunities. Polyhydroxyalkanoates (PHAs) remain a promising candidate for commodity bioplastic production, despite the many decades of efforts by academicians and industrial scientists that have not yet achieved that goal. This article focuses on defining obstacles and solutions to overcome cost-performance metrics that are not sufficiently competitive with current commodity thermoplastics. To that end, this review describes various process innovations that build on fed-batch and semi-continuous modes of operation as well as methods that lead to high cell density cultivations. Also, we discuss work to move from costly to lower cost substrates such as lignocellulose-derived hydrolysates, metabolic engineering of organisms that provide higher substrate conversion rates, the potential of halophiles to provide low-cost platforms in non-sterile environments for PHA formation, and work that uses mixed culture strategies to overcome obstacles of using waste substrates. We also describe historical problems and potential solutions to downstream processing for PHA isolation that, along with feedstock costs, have been an Achilles heel towards the realization of cost-efficient processes. Finally, future directions for efficient PHA production and relevant structural variations are discussed.

From Residues to Added-Value Bacterial Biopolymers as Nanomaterials for Biomedical Applications

Bacterial biopolymers are naturally occurring materials comprising a wide range of molecules with diverse chemical structures that can be produced from renewable sources following the principles of the circular economy. Over the last decades, they have gained substantial interest in the biomedical field as drug nanocarriers, implantable material coatings, and tissue-regeneration scaffolds or membranes due to their inherent biocompatibility, biodegradability into nonhazardous disintegration products, and their mechanical properties, which are similar to those of human tissues. The present review focuses upon three technologically advanced bacterial biopolymers, namely, bacterial cellulose (BC), polyhydroxyalkanoates (PHA), and γ-polyglutamic acid (PGA), as models of different carbon-backbone structures (polysaccharides, polyesters, and polyamides) produced by bacteria that are suitable for biomedical applications in nanoscale systems. This selection models evidence of the wide versatility of microorganisms to generate biopolymers by diverse metabolic strategies. We highlight the suitability for applied sustainable bioprocesses for the production of BC, PHA, and PGA based on renewable carbon sources and the singularity of each process driven by bacterial machinery. The inherent properties of each polymer can be fine-tuned by means of chemical and biotechnological approaches, such as metabolic engineering and peptide functionalization, to further expand their structural diversity and their applicability as nanomaterials in biomedicine.

Effect of inoculum and organic loading on mixed culture polyhydroxyalkanoate production using crude glycerol as the substrate

Two different sources of activated sludge were inoculated to select and enrich polyhydroxyalkanoate (PHA) producing culture from crude glycerol. The results showed that the sludge taken from the wastewater treatment plant with higher microbial diversity could enrich PHA producing culture with higher PHA synthesis capacity (25.93%) and specific PHA storage rate (0.27 mg COD/(mg·h)) in a short enrichment time, comparing to the sludge taken from the enriched PHA-producing culture using VFAs as the substrate. The enrichment performance under different organic loadings were investigated and similar microbial community composition, good operating stability and high PHA accumulation (SBR#1, 36.59%; SBR#2, 36.33%) was observed at 2000 mg COD/(L·d) when crude glycerol was used as the substrate. The maximum content of PHA was affected by the concentration of glycerol. Gardnerella was for the first time found to be the dominant genus in the PHA production system using crude glycerol. The research would guide the application of using crude glycerol resources for PHA production.

Effects of chemical oxygen demand concentration, pH and operation cycle on polyhydroxyalkanoates synthesis with waste sludge

To reduce the cost of polyhydroxyalkanoate (PHA) production and dispose the amount of waste sludge simultaneously, chemical oxygen demand (COD) concentration, pH and operation cycle were investigated to find the optimal PHA synthesis conditions with waste sludge in this study. The maximum PHA content (31.3% of the cell dry weight (CDW)), as well as the highest PHA conversion rate (0.30 mg COD/mg COD) and PHA-specific synthesis rate (6.12 mg COD/mg CDW·h), was achieved with initial COD concentration, pH value and operation cycle: 6000 mg/L, 8.5 and 24 h. In order to further investigate the process of PHA synthesis, COD removal rate and CDW were also introduced. This study could provide valuable information for increasing the production of PHA with waste sludge.

Cultivation processes to select microorganisms with high accumulation ability

The microbial ability to accumulate biomolecules is fundamental for different biotechnological applications aiming at the production of biofuels, food and bioplastics. However, high accumulation is a selective advantage only under certain stressful conditions, such as nutrient depletion, characterized by lower growth rate. Conventional bioprocesses maintain an optimal and stable environment for large part of the cultivation, that doesn't reward cells for their accumulation ability, raising the risk of selection of contaminant strains with higher growth rate, but lower accumulation of products. Here in this work the physiological responses of different microorganisms (microalgae, bacteria, yeasts) under N-starvation and energy starvation are reviewed, with the aim to furnish relevant insights exploitable to develop tailored bioprocesses to select specific strains for their higher accumulation ability. Microorganism responses to starvation are reviewed focusing on cell cycle, biomass production and variations in biochemical composition. Then, the work describes different innovative bioprocess configurations exploiting uncoupled nutrient feeding strategies (feast-famine), tailored to maintain a selective pressure to reward the strains with higher accumulation ability in mixed microbial populations. Finally, the main models developed in recent studies to describe and predict microbial growth and intracellular accumulation upon N-starvation and feast-famine conditions have been reviewed.

Bio-based conversion of volatile fatty acids from waste streams to polyhydroxyalkanoates using mixed microbial cultures

Production of polyhydroxyalkanoates is an important field in the biorefinery as bio-alternative to conventional plastics. However, its commercialization is still limited by high production cost. In this study, a process with the potential to reduce the production cost of polyhydroxyalkanoates was proposed. Mixed cultures accumulated polyhydroxyalkanoates using volatile fatty acid-rich effluents from waste streams, without pH and temperature control. In addition, the impact of two types of carbon sources was investigated by analyzing the microbial community as well as the polyhydroxyalkanoate accumulation capacity. Mixed cultures successfully adapted to different substrates, consuming the volatile fatty acids in their totality. The phyla Proteobacteria, Bacteroidetes and Firmicutes dominated the bacterial community. The highest polyhydroxyalkanoate content was 43.5% w/w, which is comparable to contents reported from mixed cultures using synthetic carbon sources. The biopolymer consisted of (R)-3-hydroxybutyrate 94.8 ± 1.7% w/w and (R)-3-hydroxyvaletare 5.2 ± 1.7% w/w.

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.

Characterization of Polyhydroxyalkanoates Produced at Pilot Scale From Different Organic Wastes

Polyhydroxyalkanoates (PHAs) production at pilot scale has been recently investigated and carried out exploiting different process configurations and organic wastes. More in detail, three pilot platforms, in Treviso (North-East of Italy), Carbonera (North-East of Italy) and Lisbon, produced PHAs by open mixed microbial cultures (MMCs) and different organic waste streams: organic fraction of municipal solid waste and sewage sludge (OFMSW-WAS), cellulosic primary sludge (CPS), and fruit waste (FW), respectively. In this context, two stabilization methods have been applied, and compared, for preserving the amount of PHA inside the cells: thermal drying and wet acidification of the biomass at the end of PHA accumulation process. Afterward, polymer has been extracted following an optimized method based on aqueous-phase inorganic reagents. Several PHA samples were then characterized to determine PHA purity, chemical composition, molecular weight, and thermal properties. The polymer contained two types of monomers, namely 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) at a relative percentage of 92.6-79.8 and 7.4-20.2 w/w, respectively, for Treviso and Lisbon plants. On the other hand, an opposite range was found for 3HB and 3HV monomers of PHA from Carbonera, which is 44.0-13.0 and 56.0-87.0 w/w, respectively. PHA extracted from wet-acidified biomass had generally higher viscosity average molecular weights (M v ) (on average 424.8 ± 20.6 and 224.9 ± 21.9 KDa, respectively, for Treviso and Lisbon) while PHA recovered from thermally stabilized dried biomass had a three-fold lower M v .

Polyhydroxyalkanoate nanoparticles produced by marine bacteria cultivated on cost effective Mediterranean algal hydrolysate media

Algae are omnipresent in all seas and oceans, which make thema target for many applications such as bio-fertilizers, fish feeding and removal of heavy metals. In the present study, different algal species were examined as sustainable alternatives substrates for PHA production by Halomonas sp. Several media simulations were utilized to achieve high polymer productivity. The maximum poly(3-hydroxybutyrate) (PHB) concentrations were determined by using Corallina mediterranea hydrolysates as a carbon and nitrogen source. The isolates Halomonas pacifica ASL10 and Halomonas salifodiane ASL11 were found to be able to produce PHA by 67 % wt and 63 % wt CDW, respectively. PHB nanoparticles (NPs) had high zeta potential values and small particle sizes. These properties make it suitable for several drug delivery and pharmaceutical applications. Interestingly, NPs showed a potent antibacterial activity against several reference strains. The antibacterial efficacy of PHA-NPs has not been previously studied, thus this study opens a promising use of PHA-NPs.

Optimization of Polyhydroxybutyrate Production by Amazonian Microalga Stigeoclonium sp. B23

The present work established the optimization and production of biodegradable thermoplastic polyhydroxybutyrate (PHB) from Amazonian microalga Stigeoclonium sp. B23. The optimization was performed in eight different growth media conditions of Stigeoclonium sp. B23, supplemented with sodium acetate and sodium bicarbonate and total deprivation of sodium nitrate. B23 was stained with Nile Red, and PHB was extracted and quantified by correlating the amount of fluorescence and biopolymer concentration through spectrofluorimetry and spectrophotometry, respectively. Our results detected the production of PHB in Stigeoclonium sp. B23 and in all modified media. Treatment with increased acetate and bicarbonate and without nitrate gave the highest concentration of PHB, while the treatment with only acetate gave the lowest among supplemented media. Our results showed a great potential of Stigeoclonium sp. B23, the first Amazonian microalga reported on PHB production. The microalga was isolated from a poorly explored and investigated region and proved to be productive when compared to other cyanobacterial and bacterial species. Additionally, microalga biomass changes due to the nutritional conditions and, reversely, biopolymer is well-synthetized. This great potential could lead to the pursuit of new Amazonian microalgae species in the search for alternative polyesters.

Influence of inoculum variation and nutrient availability on polyhydroxybutyrate production from activated sludge

Carbon recovery through polyhydroxybutyrate (PHB) production can create a value-added waste management system. Activated sludge as inoculum enables PHB production using cheap and renewable carbons source, bringing PHB at par to conventional plastics. The PHB accumulating potential of activated sludge needs to be improved to realize the objective. The interaction between the origin of activated sludge, petroleum refinery sludge and brewery sludge, and nitrogen availability was explored to effect culture enrichment, improve PHB accumulation, and polymer characteristics through aerobic dynamic feeding. Consequently, nitrogen excess and limitation enrichment of both sludges produced mix microbial culture with adequate PHB storage of 7.8 ± 0.05%, 14.4 ± 0.04%, 14.4 ± 0.04%, 13.4 ± 0.02% respectively. Batch accumulation revealed higher PHB accumulation of 76.1 ± 0.03% and 71.7 ± 0.05% under nitrogen limitation for PRS and BS enriched under nitrogen excess condition compared to any other combination. The higher decomposition temperature of 285 °C, 293 °C, and a lower melting point of 168 °C, 165 °C with a higher molecular weight of 4.3x10⁵g/mol and semi-crystalline arrangement indicates the potential applications for extracted PHB. PHB production enhanced under nitrogen limited conditions with culture enriched under nitrogen excess condition. However, similar PHB storage, physiochemical property, and overlapping microbial community show an insignificant effect of sludge origin on PHB production.