Comparison of 2-chlorobenzoic acid biodegradation in a membrane bioreactor by B. cepacia and B. cepacia bearing the bacterial hemoglobin gene

The Discovery of Vitreoscilla Hemoglobin and Early Studies on Its Biochemical Functions, the Control of Its Expression, and Its Use in Practical Applications

In 1986, the surprising identification of a hemoglobin (VHb) in the bacterium Vitreoscilla greatly extended the range of taxa in which this oxygen binding protein functions. Elucidation of many of its biochemical properties and relation to overall cell physiology, as well as the sequence of the gene encoding it and aspects of control of its expression were determined in the following years. In addition, during the early years following its discovery, strategies were developed to use its expression in heterologous microbial hosts to enhance processes of practical usefulness. The VHb discovery also served as the foundation for what has become the fascinatingly rich field of bacterial hemoglobins. VHb's position as the first known bacterial hemoglobin and its extensive use in biotechnological applications, which continue today, make a review of the early studies of its properties and uses an appropriate and interesting topic thirty-five years after its discovery.

Recent Advances in the Physicochemical Properties and Biotechnological Application of Vitreoscilla Hemoglobin

Vitreoscilla hemoglobin (VHb), the first discovered bacterial hemoglobin, is a soluble heme-binding protein with a faster rate of oxygen dissociation. Since it can enhance cell growth, product synthesis and stress tolerance, VHb has been widely applied in the field of metabolic engineering for microorganisms, plants, and animals. Especially under oxygen-limited conditions, VHb can interact with terminal oxidase to deliver enough oxygen to achieve high-cell-density fermentation. In recent years, with the development of bioinformatics and synthetic biology, several novel physicochemical properties and metabolic regulatory effects of VHb have been discovered and numerous strategies have been utilized to enhance the expression level of VHb in various hosts, which greatly promotes its applications in biotechnology. Thus, in this review, the new information regarding structure, function and expressional tactics for VHb is summarized to understand its latest applications and pave a new way for the future improvement of biosynthesis for other products.

Next-Generation Ultrafiltration Membranes Enabled by Block Polymers

Reliable and equitable access to safe drinking water is a major and growing challenge worldwide. Membrane separations represent one of the most promising strategies for the energy-efficient purification of potential water sources. In particular, porous membranes are used for the ultrafiltration (UF) of water to remove contaminants with nanometric sizes. However, despite exhibiting excellent water permeability and solution processability, existing UF membranes contain a broad distribution of pore sizes that limit their size selectivity. To maximize the potential utility of UF membranes and allow for precise separations, improvements in the size selectivity of these systems must be achieved. Block polymers represent a potentially transformative solution, as these materials self-assemble into well-defined domains of uniform size. Several different strategies have been reported for integrating block polymers into UF membranes, and each strategy has its own set of materials and processing considerations to ensure that uniform and continuous pores are generated. This Review aims to summarize and critically analyze the chemistries, processing techniques, and properties required for the most common methods for producing porous membranes from block polymers, with a particular focus on the fundamental mechanisms underlying block polymer self-assembly and pore formation. Critical structure-property-performance metrics will be analyzed for block polymer UF membranes to understand how these membranes compare to commercial UF membranes and to identify key research areas for continued improvements. This Review is intended to inform readers of the capabilities and current challenges of block polymer UF membranes, while stimulating critical thought on strategies to advance these technologies.

Phytoremediation of waste dumping site soil and landfill leachate by using cattail (Typha latifolia)

In this study, cattail (Typha latifolia) was used to remove Na⁺ and Cl^- from polluted soil (PS) in a solid waste open dumping site. Hydroponic system was also evaluated to remove Na⁺ and Cl^- from landfill leachate. The results indicated that the cattail grown in PS had higher biomass yield of 44.4 ± 3.29 g compared to that of control (35.3 ± 4.28 g). Nitrogen and phosphorous contents of cattails grown in PS were also higher than that of control plants, and the electrical conductivity of PS significantly decreased from 245 ± 1.40 to 51.9 ± 9.30 ms/m over the 5-week experimental duration. Na⁺ and Cl^- contents from cattail grown on PS were 10.8 ± 1.85 and 64.7 ± 9.15 g/kg biomass, respectively. For cattails grown hydroponically in water containing leachate, nitrogen and phosphorous accumulation was 51.1 ± 5.94 and 9.32 ± 3.22 g/kg biomass, respectively. The corresponding biomass yield of these cattails was 13.5 ± 1.29 g at the end of 5 weeks. In addition, the Na⁺ and Cl^- accumulation of 55.5 ± 4.82 and 78.2 ± 28.3 g/Kg biomass, respectively, was higher in hydroponic cattails grown in this study. Overall, the results suggest the effectiveness of cattails for phytoremediation of contaminated soil and the high efficiency of hydroponic system for nutrient and salinity removal compared to the conventional soil test.

Phytodesalination of landfill leachate using Puccinellia nuttalliana and Typha latifolia

Landfilling has been widely used for solid waste disposal; however, the generation of leachate can pose a major threat to the surrounding environment in the form of soil salinity. Two native plants of North America Puccinellia nuttalliana (alkaligrass) and Typha latifolia (cattail) were selected in this study to investigate bioaccumulation of sodium (Na⁺) and chloride (Cl^-) under controlled greenhouse conditions. The treatments include irrigation of the plants using fertilizer (F), landfill leachate (LL), and tap water (control, C). Plants cultivated after one season (12 weeks) were harvested by separating aboveground tissues and roots, and soil from each treatment was collected for analysis. The results show that alkaligrass irrigated with LL had 2.13% more biomass yield than control, but 17.63% less than that with F. However, cattail yielded 19.70% more biomass with the irrigation of LL than C and 3.04% less compared to F. Alkaligrass and cattail accumulated 6.85 and 7.00 g Na⁺/Kg biomass with the irrigation of LL, respectively. Alkaligrass and cattail irrigated with LL accumulated 120.14% and 94.47% more Cl^- than C. When alkaligrass and cattail were irrigated with LL, the electrical conductivity of soil was reduced by 71.70% and 45.36%, respectively. This study demonstrated that using North American native halophytes could be a cost-effective and promising approach for phytoremediation of landfill leachate.

Microbial community structure and dynamics in a membrane bioreactor supplemented with the flame retardant dibromoneopentyl glycol

Brominated flame retardants (BFRs) are a group of widely used compounds that, due to their limited biodegradability, exhibit excessive persistence in the environment. The persistence and high toxicity of these compounds to the natural biota causes great environmental concern. We investigated the biodegradation of the BFR dibromoneopentyl glycol (DBNPG) under continuous culture conditions using a miniature membrane bioreactor (mMBR) to assess its feasibility as a bioremediation approach. This system demonstrated long-term, stable biodegradation of DBNPG (>90 days), with an average removal rate of about 50%. Pyrosequencing of the 16S rRNA gene of the microorganisms involved in this process revealed the dominance of reads affiliated with the genus Brevundimonas of the Alphaproteobacteria class during the different mMBR operational stages. The bacterial community was also dominated by reads affiliated with the Sinorhizobium and Sphingopyxis genera within the Alphaproteobacteria class and the Sediminibacterium genus of the Sphingobacteria class. Real-time PCR used to analyze possible changes in the population dynamics of these four dominant groups revealed their consistent presence throughout the long-term mMBR biodegradation activity. Two genera, Brevundimonas and Sphingopyxis, were found to increase in abundance during the acclimation period and then remained relatively stable, forming the main parts of the consortium over the prolonged active stage.

Recent applications of Vitreoscilla hemoglobin technology in bioproduct synthesis and bioremediation

Since its first use in 1990 to enhance production of α-amylase in E. coli, engineering of heterologous hosts to express the hemoglobin from the bacterium Vitreoscilla (VHb) has become a widely used strategy to enhance production of a variety of bioproducts, stimulate bioremediation, and increase growth and survival of engineered organisms. The hosts have included a variety of bacteria, yeast, fungi, higher plants, and even animals. The beneficial effects of VHb expression are presumably the result of one or more of its activities. The available evidence indicates that these include oxygen binding and delivery to the respiratory chain and oxygenases, protection against reactive oxygen species, and control of gene expression. In the past 4 to 5 years, the use of this "VHb technology" has continued in a variety of biotechnological applications in a wide range of organisms. These include enhancement of production of an ever wider array of bioproducts, new applications in bioremediation, a possible role in enhancing aerobic waste water treatment, and the potential to enhance growth and survival of both plants and animals of economic importance.

Enhanced heme protein expression by ammonia-oxidizing communities acclimated to low dissolved oxygen conditions

This study has investigated the acclimation of ammonia-oxidizing communities (AOC) to low dissolved oxygen (DO) concentrations. Under controlled laboratory conditions, two sequencing batch reactors seeded with activated sludge from the same source were operated at high DO (near saturation) and low DO (0.1 mg O₂/L) concentrations for a period of 220 days. The results demonstrated stable and complete nitrification at low DO conditions after an acclimation period of approximately 140 days. Acclimation brought about increased specific oxygen uptake rates and enhanced expression of a particular heme protein in the soluble fraction of the cells in the low DO reactor as compared to the high DO reactor. The induced protein was determined not to be any of the enzymes or electron carriers present in the conventional account of ammonia oxidation in ammonia-oxidizing bacteria (AOB). Further research is required to determine the specific nature of the heme protein detected; a preliminary assessment suggests either a type of hemoglobin protein or a lesser-known component of the energy-transducing pathways of AOB. The effect of DO on AOC dynamics was evaluated using the 16S rRNA gene as the basis for phylogenetic comparisons and organism quantification. Ammonium consumption by ammonia-oxidizing archaea and anaerobic ammonia-oxidizing bacteria was ruled out by fluorescent in situ hybridization in both reactors. Even though Nitrosomonas europaea was the dominant AOB lineage in both high and low DO sequencing batch reactors at the end of operation, this enrichment could not be linked in the low DO reactor to acclimation to oxygen-limited conditions.

Characterization of multiple chlorobenzoic acid-degrading organisms from pristine and contaminated systems: mineralization of 2,4-dichlorobenzoic acid

Multiple bacterial strains with CBA metabolic properties were isolated using a simple selective strategy. Phylogenetic analysis of the 16S rRNA gene sequences grouped them into two main clusters consisting of four bacterial phyla and belonging to 17 genera. Whereas growth was more frequent with 2-CBA (∼68%), 50% grew on 4-CBA and ∼7% utilized 3-CBA. One third of the strains exhibited 2,4-dichlorobenzoic acid (2,4-diCBA) catabolic function and were mainly representatives of α-, β- and γ-Proteobacteria. In batch experiments, growth was concomitant with substrate disappearance and near-stoichiometric release of chloride. Doubling times for 2,4-diCBA degradation doubled those determined for mono-substituted CBAs. Out of the six 2,4-diCBA degraders submitted for enzyme assays, significant induction of catechol 1,2-dioxygenase types I and II activities in cell-free extracts were found in four while protocatechuate 3,4-dioxygenase activity was detected in the remaining two. Activities in CBA-grown cells were 20 orders-of-magnitude higher than those grown on benzoic acid.

The single-domain globin of Vitreoscilla: augmentation of aerobic metabolism for biotechnological applications

Extensive studies have revealed that large-scale, high-cell density bioreactor cultivations have significant impact on metabolic networks of oxygen-requiring production organisms. Oxygen transfer problems associated with fluid dynamics and inefficient mixing efficiencies result in oxygen gradients, which lead to reduced performance of the bioprocess, decreased product yields, and increased production costs. These problems can be partially alleviated by improving bioreactor configuration and setting, but significant improvements have been achieved by metabolic engineering methods, especially by heterologously expressing Vitreoscilla hemoglobin (VHb). Vast numbers of studies have been accumulating during the past 20 years showing the applicability of VHb to improve growth and product yields in a variety of industrially significant prokaryotic and eukaryotic hosts. The global view on the metabolism of globin-expressing Escherichia coli cells depicts increased energy generation, higher oxygen uptake rates, and a decrease in fermentative by-product excretion. Transcriptome and metabolic flux analysis clearly demonstrate the multidimensional influence of heterologous VHb on the expression of stationary phase-specific genes and on the regulation of cellular metabolic networks. The exact biochemical mechanisms by which VHb is able to improve the oxygen-limited growth remain poorly understood. The suggested mechanisms propose either the delivery of oxygen to the respiratory chain or the detoxification of reactive nitrogen species for the protection of cytochrome activity. The expression of VHb in E. coli bioreactor cultures is likely to assist bacterial growth through providing an increase in available intracellular oxygen, although to fully understand the exact role of VHb in vivo, further analysis will be required.