Halobacteria as producers of polyhydroxyalkanoates

Bacteria for Bioplastics: Progress, Applications, and Challenges

Bioplastics are one of the answers that can point society toward a sustainable future. Under this premise, the synthesis of polymers with competitive properties using low-cost starting materials is a highly desired factor in the industry. Also, tackling environmental issues such as nonbiodegradable waste generation, high carbon footprint, and consumption of nonrenewable resources are some of the current concerns worldwide. The scientific community has been placing efforts into the biosynthesis of polymers using bacteria and other microbes. These microorganisms can be convenient reactors to consume food and agricultural wastes and convert them into biopolymers with inherently attractive properties such as biodegradability, biocompatibility, and appreciable mechanical and chemical properties. Such biopolymers can be applied to several fields such as packing, cosmetics, pharmaceutical, medical, biomedical, and agricultural. Thus, intending to elucidate the science of microbes to produce polymers, this review starts with a brief introduction to bioplastics by describing their importance and the methods for their production. The second section dives into the importance of bacteria regarding the biochemical routes for the synthesis of polymers along with their advantages and disadvantages. The third section covers some of the main parameters that influence biopolymers' production. Some of the main applications of biopolymers along with a comparison between the polymers obtained from microorganisms and the petrochemical-based ones are presented. Finally, some discussion about the future aspects and main challenges in this field is provided to elucidate the main issues that should be tackled for the wide application of microorganisms for the preparation of bioplastics.

Exploitation of wasted bread as substrate for polyhydroxyalkanoates production through the use of Haloferax mediterranei and seawater

The use of the halophile microorganism Haloferax mediterranei, able to synthesize poly(hydroxybutyrate-hydroxyvalerate) (PHBV), is considered as a promising tool for the industrial production of bioplastic through bioprocessing. A consistent supplementation of the growth substrate in carbohydrates and minerals is overall necessary to allow its PHBV production. In this work, wasted bread was used as substrate for bioplastic production by microbial fermentation. Instead of the consistent and expensive minerals supplement required for Hfx. mediterranei DSM1411 growth, microfiltered seawater was added to the wasted bread-derived substrate. The suitable ratio of wasted bread homogenate and seawater, corresponding to 40:60, was selected. The addition of proteases and amylase to the bread homogenate promoted the microbial growth but it did not correspond to the increase of bioplastic production by the microorganism, that reach, under the experimental conditions, 1.53 g/L. An extraction procedure of the PHBV from cells, based on repeated washing with water, followed or not by a purification through ethanol precipitation, was applied instead of the conventional extraction with chloroform. Yield of PHBV obtained using the different extraction methods were 21.6 ± 3.6 (standard extraction/purification procedure with CHCl3:H2O mixture), 24.8 ± 3.0 (water-based extraction), and 19.8 ± 3.3 mg PHAs/g of wasted bread (water-based extraction followed by ethanol purification). Slightly higher hydroxyvalerate content (12.95 vs 10.78%, w/w) was found in PHBV obtained through the water-based extraction compared to the conventional one, moreover, the former was characterized by purity of 100% (w/w). Results demonstrated the suitability of wasted bread, supplemented with seawater, to be used as substrate for bioplastic production through fermentation. Results moreover demonstrated that a solvent-free extraction, exclusively based on osmotic shock, could be used to recover the bioplastic from cells.

Combination of Hypotonic Lysis and Application of Detergent for Isolation of Polyhydroxyalkanoates from Extremophiles

Production of polyhydroxyalkanoates (PHA), microbial biopolyesters, employing extremophilic microorganisms is a very promising concept relying on robustness of such organisms against microbial contamination, which provides numerous economic and technological benefits. In this work, we took advantage of the natural susceptibility of halophilic and thermophilic PHA producers to hypotonic lysis and we developed a simple and robust approach enabling effective isolation of PHA materials from microbial cells. The method is based on the exposition of microbial cells to hypotonic conditions induced by the diluted solution of sodium dodecyl sulfate (SDS) at elevated temperatures. Such conditions lead to disruption of the cells and release of PHA granules. Moreover, SDS, apart from its cell-disruptive function, also solubilizes hydrophobic components, which would otherwise contaminate PHA materials. The purity of obtained materials, as well as the yields of recovery, reach high values (values of purity higher than 99 wt.%, yields close to 1). Furthermore, we also focused on the removal of SDS from wastewater. The simple, inexpensive, and safe technique is based on the precipitation of SDS in the presence of KCl. The precipitate can be simply removed by decantation or centrifugation. Moreover, there is also the possibility to regenerate the SDS, which would substantially improve the economic feasibility of the process.

Metabolic potential of the moderate halophile Yangia sp. ND199 for co-production of polyhydroxyalkanoates and exopolysaccharides

Yangia sp. ND199 is a moderately halophilic bacterium isolated from mangrove samples in Northern Vietnam, which was earlier reported to grow on several sugars and glycerol to accumulate poly(hydroxyalkanoates) (PHA). In this study, the potential of the bacterium for co‐production of exopolysaccharides (EPS) and PHA was investigated. Genome sequence analysis of the closely related Yangia sp. CCB‐M3 isolated from mangroves in Malaysia revealed genes encoding enzymes participating in different EPS biosynthetic pathways. The effects of various cultivation parameters on the production of EPS and PHA were studied. The highest level of EPS (288 mg/L) was achieved using sucrose and yeast extract with 5% NaCl and 120 mM phosphate salts but with modest PHA accumulation (32% of cell dry weight, 1.3 g/L). Growth on fructose yielded the highest PHA concentration (85% of CDW, 3.3 g/L) at 90 mM phosphate and higher molecular weight EPS at 251 mg/L yield at 120 mM phosphate concentration. Analysis of EPS showed a predominance of glucose, followed by fructose and galactose, and minor amounts of acidic sugars.

Optimising PHBV biopolymer production in haloarchaea via CRISPRi-mediated redirection of carbon flux

The haloarchaeon Haloferax mediterranei is a potential strain for poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) production, yet the production yield and cost are the major obstacles hindering the use of this archaeal strain. Leveraging the endogenous type I-B CRISPR-Cas system in H. mediterranei, we develop a CRISPR-based interference (CRISPRi) approach that allows to regulate the metabolic pathways related to PHBV synthesis, thereby enhancing PHBV production. Our CRISPRi approach can downregulate the gene expression in a range of 25% to 98% depending upon the target region. Importantly, plasmid-mediated CRISPRi downregulation on the citrate synthase genes (citZ and gltA) improves the PHBV accumulation by 76.4% (from 1.78 to 3.14 g/L). When crRNA cassette integrated into chromosome, this further shortens the PHBV fermentation period and enhances PHA productivity by 165%. Our transcriptome analysis shows that repression of citrate synthase genes redirects metabolic flux from the central metabolic pathways to PHBV synthesis pathway. These findings demonstrate that the CRISPRi-based gene regulation is a transformative toolkit for fine-tuning the endogenous metabolic pathways in the archaeal system, which can be applied to not only the biopolymer production but also many other applications.

Lin et al. investigate the use of CRISPRi technology in haloarchaea to regulate the metabolic pathways related to PHBV synthesis to increase PHBV production in H. mediterranei. The authors report that repression of citrate synthase genes redirects metabolic flux and increases production of this degradable bioplastic, which could be used as an alternative to chemical synthetic plastic.

Haloarchaea as Cell Factories to Produce Bioplastics

Plastic pollution is a worldwide concern causing the death of animals (mainly aquatic fauna) and environmental deterioration. Plastic recycling is, in most cases, difficult or even impossible. For this reason, new research lines are emerging to identify highly biodegradable bioplastics or plastic formulations that are more environmentally friendly than current ones. In this context, microbes, capable of synthesizing bioplastics, were revealed to be good models to design strategies in which microorganisms can be used as cell factories. Recently, special interest has been paid to haloarchaea due to the capability of some species to produce significant concentrations of polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), and polyhydroxyvalerate (PHV) when growing under a specific nutritional status. The growth of those microorganisms at the pilot or industrial scale offers several advantages compared to that of other microbes that are bioplastic producers. This review summarizes the state of the art of bioplastic production and the most recent findings regarding the production of bioplastics by halophilic microorganisms with special emphasis on haloarchaea. Some protocols to produce/analyze bioplastics are highlighted here to shed light on the potential use of haloarchaea at the industrial scale to produce valuable products, thus minimizing environmental pollution by plastics made from petroleum.

The metabolic pathways of polyhydroxyalkanoates and exopolysaccharides synthesized by Haloferax mediterranei in response to elevated salinity

How polymer synthesis is mobilized or activated as a biological response of Haloferax mediterranei against hypertonic conditions remains largely unexplored. This study investigated the protein expression of H. mediterranei in response to high salinity by using isobaric tags for relative and absolute quantitation (iTRAQ)-based proteomic analysis. The microbes were harvested at end of fermentation at the NaCl salinity of 75 and 250 g L^-1. Among the identified 2123 proteins, 170 proteins were differentially expressed. Gene ontology annotation revealed that the highest number of proteins was annotated in biological process category, which was responsible for metabolic process, cellular component and catalytic activity. Differentially expressed proteins were belonged to the class of response to stimulus as well as catalytic activity and binding. Under high salinity conditions, three pathways were established as key responses of PHA and EPS production to hypertonic pressure. Two overexpressed proteins, beta-ketoacyl-ACP reductase and 3-hydroxyacyl-CoA dehydrogenase, enhanced the synthesis of PHAs. The serine-pyruvate transaminase and serine-glyoxylate transaminase were upregulated, thereby increasing the conversion of glucose to PHA. Downregulated levels of sulfate-adenylyl transferase and adenylyl-sulfate kinase could cause diminished EPS synthesis. This study could contribute to better understanding of the proteomic mechanisms of the synthesized polymers in defending against salt stress. SIGNIFICANCE: Haloferax mediterranei, a family member of halophilic archaea, is well known for its fermentative production of poly-β-hydroxyalkanoates (PHAs). PHAs are natural polymers that exhibit great potential in a wide range of applications such as a good alternative to petroleum-based plastics and the biocompatible material. For decades, the functional role of PHAs synthesized by H. mediterranei is deemed to be carbon and energy reservations. The finding proved that differential production of PHA and EPS in H. mediterranei exposed to elevated salinity was caused by differential protein expression. This is the first report on how PHA and EPS synthesized by H. mediterranei is mobilized as the response of increased salinity, contributing to the understanding of halophilic archaea's response to hypertonic stress and the precise control of fermentation production. Despite its advantages as a PHA cell factory, H. mediterranei synthesized EPS simultaneously, thereby lowering the maximum yield of PHA production. Overall, salinity can be used as a vital microbial fermentation parameter to obtain the highest harvest of PHA, as well as the lowest EPS synthesis in industrial fermentation.

Analysis of Zobellella denitrificans ZD1 draft genome: Genes and gene clusters responsible for high polyhydroxybutyrate (PHB) production from glycerol under saline conditions and its CRISPR-Cas system

Polyhydroxybutyrate (PHB) is biodegradable and renewable and thus considered as a promising alternative to petroleum-based plastics. However, PHB production is costly due to expensive carbon sources for culturing PHB-accumulating microorganisms under sterile conditions. We discovered a hyper PHB-accumulating denitrifying bacterium, Zobellella denitrificans ZD1 (referred as strain ZD1 hereafter) capable of using non-sterile crude glycerol (a waste from biodiesel production) and nitrate to produce high PHB yield under saline conditions. Nevertheless, the underlying genetic mechanisms of PHB production in strain ZD1 have not been elucidated. In this study, we discovered a complete pathway of glycerol conversion to PHB, a novel PHB synthesis gene cluster, a salt-tolerant gene cluster, denitrifying genes, and an assimilatory nitrate reduction gene cluster in the ZD1 genome. Interestingly, the novel PHB synthesis gene cluster was found to be conserved among marine Gammaproteobacteria. Higher levels of PHB accumulation were linked to higher expression levels of the PHB synthesis gene cluster in ZD1 grown with glycerol and nitrate under saline conditions. Additionally, a clustered regularly interspaced short palindromic repeat (CRISPR)-Cas type-I-E antiviral system was found in the ZD1 genome along with a long spacer list, in which most of the spacers belong to either double-stranded DNA viruses or unknown phages. The results of the genome analysis revealed strain ZD1 used the novel PHB gene cluster to produce PHB from non-sterile crude glycerol under saline conditions.

Polyhydroxyalkanoate Biosynthesis at the Edge of Water Activitiy-Haloarchaea as Biopolyester Factories

Haloarchaea, the extremely halophilic branch of the Archaea domain, encompass a steadily increasing number of genera and associated species which accumulate polyhydroxyalkanoate biopolyesters in their cytoplasm. Such ancient organisms, which thrive in highly challenging, often hostile habitats characterized by salinities between 100 and 300 g/L NaCl, have the potential to outperform established polyhydroxyalkanoate production strains. As detailed in the review, this optimization presents due to multifarious reasons, including: cultivation setups at extreme salinities can be performed at minimized sterility precautions by excluding the growth of microbial contaminants; the high inner-osmotic pressure in haloarchaea cells facilitates the recovery of intracellular biopolyester granules by cell disintegration in hypo-osmotic media; many haloarchaea utilize carbon-rich waste streams as main substrates for growth and polyhydroxyalkanoate biosynthesis, which allows coupling polyhydroxyalkanoate production with bio-economic waste management; finally, in many cases, haloarchaea are reported to produce copolyesters from structurally unrelated inexpensive substrates, and polyhydroxyalkanoate biosynthesis often occurs in parallel to the production of additional marketable bio-products like pigments or polysaccharides. This review summarizes the current knowledge about polyhydroxyalkanoate production by diverse haloarchaea; this covers the detection of new haloarchaea producing polyhydroxyalkanoates, understanding the genetic and enzymatic particularities of such organisms, kinetic aspects, material characterization, upscaling and techno-economic and life cycle assessment.

Macroalgal biomass subcritical hydrolysates for the production of polyhydroxyalkanoate (PHA) by Haloferax mediterranei

Non-conventional carbon sources, such as macroalgae, are sustainable alternatives for large-scale production of biopolymers. The present study examined macroalgae-derived carbohydrates, as carbon sources for the production of polyhydroxyalkanoates (PHAs) by Haloferax mediterranei. Simulants of the hydrolysates of seven different macroalgal biomasses were prepared and the PHA production was studied. A maximum biomass concentration with maximum PHA content was detected in medium prepared from green macroalgae. The highest cell dry weight and PHA concentrations were 3.8 ± 0.2 g·L^-1 and 2.2 ± 0.12 g·L^-1 respectively when Haloferax mediterranei was grown in 25% (w/w) of Ulva sp. hydrolysate, at 42 °C temperature and initial pH of 7.2. Poly(3-hydroxy-butyrate-co-3-hydroxyvalerate was the major PHA constituent. The present study demonstrated that Ulva sp. is a promising feedstock for PHA production.

Biolytic extraction of poly(3-hydroxybutyrate) from Bacillus megaterium Ti3 using the lytic enzyme of Streptomyces albus Tia1

The applicability of Streptomyces sp. cell lytic enzymes for devising a simple and competent biological polyhydroxyalkanoate (PHA) recovery approach from Bacillus megaterium cells was investigated. B. megaterium strain Ti3 produced 50% (w/w) PHA using glucose as carbon source. The intracellular PHA was recovered employing a non-PHA accumulating actinomycetes (Tia1) identified as Streptomyces albus, having potent lytic activity against living and heat inactivated B. megaterium. Interestingly, maximum biomass (2.53 ± 0.6 g/L by 24 h) of the lytic actinomycete was obtained in PHA production medium itself thus circumventing the prior actinomycete acclimatization just by co-inoculation with B. megaterium as an inducer. Maximum lytic activity was observed at pH 6.0, 40 °C, 220 mg of biomass and 33.3 mL of concentrated culture filtrate in a 100 mL reaction mixture. Preliminary biochemical investigations confirmed the proteolytic and caseinolytic nature of the lytic enzyme. PHA yield of 0.55 g/g by co-inoculation extraction approach was comparable with the conventional sodium hypochlorite based extraction method. Interestingly, S. albus also demonstrated a broad spectrum lytic potential against varied Gram-negative and Gram-positive PHA producers highlighting the extensive applicability of this biolytic PHA recovery approach. The lytic enzyme retained almost 100% relative activity on storage at -20 °C upto two months. 1H Nuclear magnetic resonance analysis of the extracted polymer confirmed it as a homopolymer composed of 3-hydroxybutyrate monomeric units. This is the first report on Streptomyces sp. based biological and eco-friendly, intracellular PHA recovery from Bacillus spp.

Haloarchaea: worth exploring for their biotechnological potential

Halophilic archaea are unique microorganisms adapted to survive under high salt conditions and biomolecules produced by them may possess unusual properties. Haloarchaeal metabolites are stable at high salt and temperature conditions that are useful for industrial applications. Proteins and enzymes of this group of archaea are functional under salt concentrations at which bacterial counterparts fail to be active. Such properties makes haloarchaeal enzymes suitable for salt-based applications and their use under dehydrating conditions. For example, bacteriorhodopsin or the purple membrane protein present in halophilic archaea has the most recognizable applications in photoelectric devices, artificial retinas, holograms etc. Haloarchaea are also useful for bioremediation of polluted hypersaline areas. Polyhydroxyalkanoates and exopolysccharides produced by these microorganisms are biodegradable and have the potential to replace commercial non-degradable plastics and polymers. Moreover, halophilic archaea have excellent potential to be used as drug delivery systems and for nanobiotechnology by virtue of their gas vesicles and S-layer glycoproteins. Despite of possible applications of halophilic archaea, laboratory-to-industrial transition of these potential candidates is yet to be established.

Genomic analysis reveals the biotechnological and industrial potential of levan producing halophilic extremophile, Halomonas smyrnensis AAD6T

Halomonas smyrnensis AAD6T is a gram negative, aerobic, and moderately halophilic bacterium, and is known to produce high levels of levan with many potential uses in foods, feeds, cosmetics, pharmaceutical and chemical industries due to its outstanding properties. Here, the whole-genome analysis was performed to gain more insight about the biological mechanisms, and the whole-genome organization of the bacterium. Industrially crucial genes, including the levansucrase, were detected and the genome-scale metabolic model of H. smyrnensis AAD6T was reconstructed. The bacterium was found to have many potential applications in biotechnology not only being a levan producer, but also because of its capacity to produce Pel exopolysaccharide, polyhydroxyalkanoates, and osmoprotectants. The genomic information presented here will not only provide additional information to enhance our understanding of the genetic and metabolic network of halophilic bacteria, but also accelerate the research on systematical design of engineering strategies for biotechnology applications.

Synthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) from unrelated carbon sources in engineered Rhodospirillum rubrum

Different genes encoding pyridine nucleotide transhydrogenases (pntAB, udhA) and acetoacetyl-CoA reductases (phaB) were heterologously overexpressed in Rhodospirillum rubrum S1. A recombinant strain, which harbored the gene encoding the membrane-bound transhydrogenase PntAB from Escherichia coli MG1655 and the phaB1 gene coding for an NADPH-dependent acetoacetyl-CoA reductase from Ralstonia eutropha H16, accumulated poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [Poly(3HB-co-3HV)] with a 3HV fraction of up to 13 mol% from fructose. This was a 13-fold increase of the 3HV content when compared to the wild-type strain. Higher contents of 3HV are known to reduce the brittleness of this polymer, which is advantageous for most applications. The engineered R. rubrum strain was also able to synthesize this industrially relevant copolymer from CO2 and CO from artificial synthesis gas (syngas) with a 3HV content of 56 mol%. The increased incorporation of 3HV was attributed to an excess of propionyl-CoA, which was generated from threonine and related amino acids to compensate for the intracellular redox imbalance resulting from the transhydrogenase reaction. Thereby, our study presents a novel, molecular approach to alter the composition of bacterial PHAs independently from external precursor supply. Moreover, this study also provides a promising production strain for syngas-derived second-generation biopolymers.

Preparation of poly(3-hydroxybutyrate-co-hydroxyvalerate) films from halophilic archaea and their potential use in drug delivery

Halophilic archaea offer a potential source for production of polyhydroxyalkanoates (PHAs). Hence, the experiments were carried out with five extremely halophilic archaeal isolates to determine the highest PHA-producing strain. PHA production of each isolates was separately examined in cheap carbon sources such as corn starch, sucrose, whey, apple, melon and tomato wastes. Corn starch was found to be a fairly effective substrate for PHA production. Among the strains studied here, the strain with the highest capability for PHA biosynthesis was found to be 1KYS1. Phylogenetic analysis based on 16S rRNA gene sequence comparison showed that 1KYS1 closely related to species of the genus Natrinema. The closest phylogenetic similarity was with the strain of Natrinema pallidum JCM 8980 (99 %). PHA content of 1KYS1 was about 53.14 % of the cell dry weight when starch was used as a carbon source. The formation of large and uniform PHA granules was confirmed by transmission electron microscopy and the biopolymer was identified as poly(3-hydroxybutyrate-co-hydroxyvalerate) (PHBV). PHBV produced by 1KYS1 was blended with low molar mass polyethylene glycol (PEG 300) to prepare biocompatible films for drug delivery. Rifampicin was used as a model drug and its release from PHBV films was investigated at pH 7.4, 37 °C. It was found that PHBV films obtained from 1KYS1 were very effective for drug delivery. In conclusion, PHBV of 1KYS1 may have a potential usage in drug delivery applications.

Biotechnological strategies to improve production of microbial poly-(3-hydroxybutyrate): a review of recent research work

Poly-(3-hydroxybutyrate) [P(3HB)] is a polyester synthesized as a carbon and energy reserve material by a wide number of bacteria. This polymer is characterized by its thermo-plastic properties similar to plastics derived from petrochemical industry, such as polyethylene and polypropylene. Furthermore, P(3HB) is an inert, biocompatible and biodegradable material which has been proposed for several uses in medical and biomedical areas. Currently, only few bacterial species such as Cupriavidus necator, Azohydromonas lata and recombinant Escherichia coli have been successfully used for P(3HB) production at industrial level. Nevertheless, in recent years, several fermentation strategies using other microbial models such as Azotobacter vinelandii, A. chroococcum, as well as some methane-utilizing species, have been developed in order to improve the P(3HB) production and also its mean molecular weight.

Halophiles, coming stars for industrial biotechnology

Industrial biotechnology aims to produce chemicals, materials and biofuels to ease the challenges of shortage on petroleum. However, due to the disadvantages of bioprocesses including energy consuming sterilization, high fresh water consumption, discontinuous fermentation to avoid microbial contamination, highly expensive stainless steel fermentation facilities and competing substrates for human consumption, industrial biotechnology is less competitive compared with chemical processes. Recently, halophiles have shown promises to overcome these shortcomings. Due to their unique halophilic properties, some halophiles are able to grow in high pH and high NaCl containing medium under higher temperature, allowing fermentation processes to run contamination free under unsterile conditions and continuous way. At the same time, genetic manipulation methods have been developed for halophiles. So far, halophiles have been used to produce bioplastics polyhydroxyalkanoates (PHA), ectoines, enzymes, and bio-surfactants. Increasing effects have been made to develop halophiles into a low cost platform for bioprocessing with advantages of low energy, less fresh water consumption, low fixed capital investment, and continuous production.

Accumulation of polyhydroxyalkanoates by halophilic archaea isolated from traditional solar salterns of India

Extremely halophilic archaeal isolates obtained from brine and sediment samples of solar salterns of Goa and Tamil Nadu, India were screened for accumulation of polyhydroxyalkanoates (PHA). Seven polymer accumulating haloarchaeal strains (TN4, TN5, TN6, TN7, TN9, TN10 and BBK2) were selected based on their growth and intensity of fluorescence when grown on 20 % NaCl synthetic medium supplemented with 2 % glucose and incorporated with Nile red dye. The polymer was quantified by conversion of PHA to crotonic acid which gave a characteristic absorption maxima at 235 nm. On the basis of phenotypic and genotypic characterization the cultures TN4, TN5, TN6, TN7, TN10 and BBK2 were grouped under genus Haloferax whereas isolate TN9 was grouped under the genus Halogeometricum. Growth kinetics and polymer accumulation studies revealed that the culture Halogeometricum borinquense strain TN9 accumulates PHA maximally at the mid-log phase, i.e. 5th day of growth (approx. 14 wt% PHA of CDW). Analysis of the polymer by IR, (1)H NMR and (13)C NMR confirmed it to be a homopolymer of 3-hydroxybutyrate.

Industrial and environmental applications of halophilic microorganisms

In comparison with the thermophilic and the alkaliphilic extremophiles, halophilic microorganisms have as yet found relatively few biotechnological applications. Halophiles are involved in centuries-old processes such as the manufacturing of solar salt from seawater and the production of traditional fermented foods. Two biotechnological processes involving halophiles are highly successful: the production of beta-carotene by the green alga Dunaliella and the production of ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidinecarboxylic acid), used as a stabilizer for enzymes and now also applied in cosmetic products, from moderately halophilic bacteria. The potential use of bacteriorhodopsin, the retinal protein proton pump of Halobacterium, in optoelectronic devices and photochemical processes is being explored, and may well lead to commercial applications in the near future. Demand for salt-tolerant enzymes in current manufacturing or related processes is limited. Other possible uses of halophilic microorganisms such as treatment of saline and hypersaline wastewaters, and the production of exopolysaccharides, poly-beta-hydroxyalkanoate bioplastics and biofuel are being investigated, but no large-scale applications have yet been reported.

Poly(3-hydroxybutyrate) production from glycerol by Zobellella denitrificans MW1 via high-cell-density fed-batch fermentation and simplified solvent extraction

Industrial production of biodegradable polyesters such as polyhydroxyalkanoates is hampered by high production costs, among which the costs for substrates and for downstream processing represent the main obstacles. Inexpensive fermentable raw materials such as crude glycerol, an abundant by-product of the biodiesel industry, have emerged to be promising carbon sources for industrial fermentations. In this study, Zobellella denitrificans MW1, a recently isolated bacterium, was used for the production of poly(3-hydroxybutyrate) (PHB) from glycerol as the sole carbon source. Pilot-scale fermentations (42-liter scale) were conducted to scale up the high PHB accumulation capability of this strain. By fed-batch cultivation, at first a relatively high cell density (29.9 +/- 1.3 g/liter) was obtained during only a short fermentation period (24 h). However, the PHB content was relatively low (31.0% +/- 4.2% [wt/wt]). Afterwards, much higher concentrations of PHB (up to 54.3 +/- 7.9 g/liter) and higher cell densities (up to 81.2 +/- 2.5 g/liter) were obtained by further fed-batch optimization in the presence of 20 g/liter NaCl, with optimized feeding of glycerol and ammonia to support both cell growth and polymer accumulation over a period of 50 h. A high specific growth rate (0.422/h) and a short doubling time (1.64 h) were attained. The maximum PHB content obtained was 66.9% +/- 7.6% of cell dry weight, and the maximum polymer productivity and substrate yield coefficient were 1.09 +/- 0.16 g/liter/h and 0.25 +/- 0.04 g PHB/g glycerol, respectively. Furthermore, a simple organic solvent extraction process was employed for PHB recovery during downstream processing: self-flotation of cell debris after extraction of PHB with chloroform allowed a convenient separation of a clear PHB-solvent solution from the cells. Maximum PHB recovery (85.0% +/- 0.10% [wt/wt]) was reached after 72 h of extraction with chloroform at 30 degrees C, with a polymer purity of 98.3% +/- 1.3%.

Production of poly-3-hydroxyalkanoic acids by a moderately halophilic bacterium, Halomonas marina HMA 103 isolated from solar saltern of Orissa, India

Halomonas marina HMA 103 (MTCC 8968), the moderately halophilic bacterium isolated and characterized from the solar saltern of Orissa, India, grows optimally at 10% (w/v) NaCl in culture medium and is able to synthesize poly(3-hydroxybutyrate) [P(3HB)] during growth. This study is an attempt to optimize the cultural conditions for efficient production of P(3HB) by H. marina in batch cultivation. Growth of the organism under shake-flask culture using 2% (w/v) glucose resulted in P(3HB) accumulation accounting for more than 59% of cell dry weight after 50 h of incubation. The optimum P(3HB) production was attained with a combined supply of NH4Cl and yeast extract as N-source, 0.01% (w/v) phosphate, 1.5% (w/v) sulphate and 10% (w/v) NaCl. Qualitative and quantitative 1HNMR and FT-IR analysis of cells grown in alkanoic acids (C3-C6) as sole source of carbon and co-substrates revealed synthesis of PHA co-polymers composed of 3-hydroxybutyric acid and 3-hydroxyvaleric acid [P(3HB-co-3HV)]. In two-step cultivation, accumulation of the co-polymer was significantly improved (80% CDW) in glucose medium supplemented with valerate (0.1%, w/v) as co-substrate and the polymer contained 88.1 and 12.8 mol% 3HB and 3HV monomers, respectively.

Poly(3-hydroxybutyrate) production by Halomonas boliviensis in fed-batch culture

High poly(3-hydroxybutyrate) (PHB) content and volumetric productivity were achieved by fed-batch culture of Halomonas boliviensis using a defined medium. Initial shake flask cultivations in a minimal medium revealed that the growth of H. boliviensis was supported only when the medium was supplemented with aspartic acid, glycine, or glutamine. Addition of 0.1% (w/v) glutamine in the medium resulted in the highest cell dry weight (CDW; 3.9 g l(-1)). Glutamine was replaced by the less expensive monosodium glutamate (MSG) in the medium without any notable change in the final cell density. Effect of initial concentrations of NH(4)Cl and K(2)HPO(4) on cell growth and PHB accumulation by H. boliviensis was then analyzed using a fed-batch fermentation system. The best conditions for PHB production by H. boliviensis were attained using 0.4% (w/v) NH(4)Cl and 0.22% (w/v) K(2)HPO(4) and adding MSG intermittently to the fermentor. Poly(3-hydroxybutyrate) content and CDW reached 90 wt.% and 23 g l(-1), respectively, after 18 h of cultivation. In order to increase CDW and PHB content, MSG, NH(4)Cl, and K(2)HPO(4) were initially fed to the fermentor to maintain their concentrations at 2%, 0.4%, and 0.22% (w/v), respectively, and subsequently their feed was suppressed. This resulted in a CDW of 44 g l(-1), PHB content of 81 wt.%, and PHB volumetric productivity of 1.1 g l(-1) h(-1).

Optimizing conditions for poly(β-hydroxybutyrate) production by Halomonas boliviensis LC1 in batch culture with sucrose as carbon source

Halomonas boliviensis LC1 is able to accumulate poly(beta-hydroxybutyrate) (PHB) under conditions of excess carbon source and depletion of essential nutrients. This study was aimed at an efficient production of PHB by growing H. boliviensis to high cell concentrations in batch cultures. The effect of ammonium, phosphate, and yeast extract concentrations on cell concentration [cell dry weight (CDW)] and PHB content of H. boliviensis cultured in shake flasks was assayed using a factorial design. High concentrations of these nutrients led to increments in cell growth but reduced the PHB content to some extent. Cultivations of H. boliviensis under controlled conditions in a fermentor using 1.5% (w/v) yeast extract as N source, and intermittent addition of sucrose to provide excess C source, resulted in a polymer accumulation of 44 wt.% and 12 g l(-1) CDW after 24 h of cultivation. Batch cultures in a fermentor with initial concentrations of 2.5% (w/v) sucrose and 1.5% (w/v) yeast extract, and with induced oxygen limitation, resulted in an optimum PHB accumulation, PHB concentration and CDW of 54 wt.%, 7.7 g l(-1) and 14 g l(-1), respectively, after 19 h of cultivation. The addition of casaminoacids in the medium increased the CDW to 14.4 g l(-1) in 17 h but reduced the PHB content in the cells to 52 wt.%.

Preparation and characterization of poly(hydroxyalkanoate) from the fermentation of Haloferax mediterranei

In this study, fed-batch fermentation of Haloferax mediterranei using glucose and yeast extract as carbon and nitrogen source, respectively, was carried out to produce poly(hydroxyalkanoate) (PHA). After fermentation for 117 h, the concentration of H. mediterranei and PHA content reached 85.8 g/l and 48.6%, respectively. 1H- and 13C-NMR spectra proved that the produced PHA was poly(3-hydroxybutyrate-co-3-hydroxyvalerate) P(3HB-co-3HV) co-polymer. However, further fractionation using chloroform/acetone revealed that the produced PHA consisted of at least two compositionally different co-polymers (P1 and P2). One P(3HB-co-3HV) co-polymer (P1, 93.4 wt%) contains 10.7 mol% of 3-HV unit in the chain structure and has a high molecular weight of 569.5 kg/mol. The other one (P2, 6.6 wt%) has a slightly higher 3-HV content, ca. 12.3 mol%, but its molecular weight is relatively low, 78.2 kg/mol. Both fractions exhibit two overlapped melting peaks measured by differential scanning calorimetry when the heating rate is at and below 20 degrees C/min. For example, at a heating rate of 10 degrees C/min, the two melting peaks occur at 134.8 degrees C and 144.3 degrees C for P1, and 131.1 degrees C and 140.6 degrees C for P2. Through observing the variation of relative intensity of these two melting peaks by changing the heating rate, it was proven that the phenomenon is caused by a melt/recrystallization process. Glass-transition temperature, crystallization temperature and thermal degradation behavior of these co-polymers were also discussed.

Potential of Various Archae- and Eubacterial Strains as Industrial Polyhydroxyalkanoate Producers from Whey

Three different microbial wild-type strains are compared with respect to their potential as industrial scale polyhydroxyalkanoate (PHA) producers from the feed stock whey lactose. The halophilic archaeon Haloferax mediterranei as well as two eubacterial strains (Pseudomonas hydrogenovora and Hydrogenophaga pseudoflava) are investigated. H. mediterranei accumulated 50 wt.-% of poly-3-(hydroxybutyrate-co-8%-hydroxyvalerate) from hydrolyzed whey without addition of 3-hydroxyvalerate (3HV) precursors (specific productivity q(p): 9.1 mg x g(-1) x h(-1)). Using P. hydrogenovora, the final percentage of poly-3-hydroxybutyrate (PHB) amounted to 12 wt.-% (q(p): 2.9 mg x g(-1) x h(-1)). With H. pseudoflava, it was possible to reach 40 wt.-% P-3(HB-co-5%-HV) on non-hydrolyzed whey lactose plus addition of valeric acid as 3HV precursor (q(p): 12.5 mg x g(-1) x h(-1)). A detailed characterization of the isolated biopolyesters and an evaluation with regard to the economic feasibility completes the study.

Interrelationships between Dunaliella and halophilic prokaryotes in saltern crystallizer ponds

Thanks to their often very high population densities and their simple community structure, saltern crystallizer ponds form ideal sites to study the behavior of halophilic microorganisms in their natural environment at saturating salt concentrations. The microbial community is dominated by square red halophilic Archaea, recently isolated and described as Haloquadratum walsbyi, extremely halophilic red rod-shaped Bacteria of the genus Salinibacter, and the unicellular green alga Dunaliella as the primary producer. We review here, the information available on the microbial community structure of the saltern crystallizer brines and the interrelationships between the main components of their biota. As Dunaliella produces massive amounts of glycerol to provide osmotic stabilization, glycerol is often postulated to be the most important source of organic carbon for the heterotrophic prokaryotes in hypersaline ecosystems. We assess here, the current evidence for the possible importance of glycerol and other carbon sources in the nutrition of the Archaea and the Bacteria, the relative contribution of halophilic Bacteria and Archaea to the heterotrophic activity in the brines, and other factors that determine the nature of the microbial communities that thrive in the salt-saturated brines of saltern crystallizer ponds.

Production of polyhydroxyalkanoates from inexpensive extruded rice bran and starch by Haloferax mediterranei

Low-cost raw materials can be used to reduce significantly the production cost of polyhydroxyalkanoates (PHA). In this study, extruded rice bran (ERB) and extruded cornstarch (ECS) were used as carbon sources to produce PHA by an archaea, Haloferax mediterranei, which cannot use native rice bran or cornstarch as a carbon source. By employing pH-stat control strategy to maintain pH at 6.9-7.1 in a 5-liter jar fermentor using ERB:ECS (1:8 g/g) as the major carbon source, we obtained a cell concentration of 140 g/L, PHA concentration of 77.8 g/L and PHA content of 55.6 wt.% in a repeated fed-batch fermentation. In contrast, when ECS was used as the major carbon source, we obtained 62.6 g/L cell concentration, 24.2 g/L PHA concentration and 38.7 wt.% PHA content. Under a hyper-saline condition and with no nitrogen-limitation restriction, the repeated fed-batch process can be sustained a long time for the mass production of PHA.

Poly(β-hydroxybutyrate) production by a moderate halophile,Halomonas boliviensisLC1 using starch hydrolysate as substrate

AIM

The objective of the present work was to enable the use of starch hydrolysate, generated by the action of a recombinant maltooligosaccharide forming amylase from Bacillus halodurans LBK 34, as the carbon source for the production of poly-beta-hydroxybutyrate (PHB) by Halomonas boliviensis LC1.

METHODS AND RESULTS

In this work, different amounts of the alpha-amylase (Amy 34) were utilized for starch hydrolysis, resulting in the production of mixtures of maltooligosaccharides (G1-G6) at varying ratios. The highest PHB accumulation (56 wt%) by H. boliviensis cultivated in shake flasks (with agitation at 160 rev min(-1)) was obtained when 6.4 U ml(-1) of the amylase was used for starch hydrolysis. When H. boliviensis cells were grown in a fermentor with no oxygen limitation the accumulation of PHB was decreased to 35 wt%. Although some improvements in PHB accumulation and cell mass concentration were reached by the addition of peptone and phosphate, respectively, major enhancements were attained when oxygen limitation was induced in the fermentor.

CONCLUSIONS

Halomonas boliviensis uses preferentially maltose for PHB formation from starch hydrolysate. It is also able to hydrolyse higher sugars if no other simpler carbon source is available but with a significantly lower polymer yield. Furthermore, H. boliviensis is able to adjust its metabolism to oxygen limitation, most probably by directing the excess NAD(P)H to PHB accumulation.

SIGNIFICANCE AND IMPACT OF THE STUDY

There have been no reports related to PHB production amongst the members of the genus Halomonas. The use of a maltooligosaccharide forming alpha-amylase, which is active at a temperature and pH close to that required for growth of H. boliviensis, and the versatility of this bacterium in the selection of the carbon source may provide an attractive alternative for the utilization of starch-derived raw materials.

A novel heat-stable lipolytic enzyme from Sulfolobus acidocaldarius DSM 639 displaying similarity to polyhydroxyalkanoate depolymerases

A fragment of genomic DNA from Sulfolobus acidocaldarius DSM 639 encoding a lipolytic enzyme was cloned and sequenced. The 314-amino acid polypeptide displays a maximum sequence similarity (43%) to a putative polyhydroxyalkanoate depolymerase from Pseudomonas oleovorans and contains the pentapeptide G-X1-S-X2-G which is typical of serine hydrolases. The protein is highly thermostable and is able to hydrolyse a variety of lipid substrates thus providing a promising tool for potential biotechnological applications.

Metabolic pathway for poly(3-hydroxybutyrate-co-3-hydroxyvalerate) formation in Nocardia corallina: inactivation of mutB by chromosomal integration of a kanamycin resistance gene

The gene encoding the large subunit of the methylmalonyl-coenzyme A (CoA) mutase in Nocardia corallina (mutBNc) was cloned. A 4.3-kbp BamHI fragment containing almost the entire mutBNc was identified by Southern hybridization experiments employing a digoxigenin-labeled probe deduced from mutB of Streptomyces cinnamonensis, mutBNc was interrupted by insertion of a kanamycin resistance gene block (mutB::kan or mutB::neo) and introduced into N. corallina to obtain mutB-negative strains by homologous recombination. Four of sixteen kanamycin-resistant clones occurred via double-crossover events and harbored only the interrupted mutBNc. These exhibited no growth on odd-chain fatty acids in the presence of kanamycin but exhibited wild-type growth on even-chain fatty acids, glucose, and succinate. Whereas the wild type of N. corallina accumulates a copolyester of 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) containing more than 60 mol% 3HV from most carbon sources, mutB-negative strains accumulated poly(3HB-co-3HV) containing only 2 to 6 mol% 3HV. Methylmalonyl-CoA mutase activity was not found in these clones. Therefore, this study provides strong evidence that the majority of 3HV units in poly(3HB-co-3HV) accumulated by N. corallina are synthesized via the methylmalonyl-CoA pathway.