Optimization of the yield of dark microaerobic production of hydrogen from lactate by Rhodopseudomonas palustris

The genome-scale metabolic model for the purple non-sulfur bacterium Rhodopseudomonas palustris Bis A53 accurately predicts phenotypes under chemoheterotrophic, chemoautotrophic, photoheterotrophic, and photoautotrophic growth conditions

The purple non-sulfur bacterium Rhodopseudomonas palustris is recognized as a critical microorganism in the nitrogen and carbon cycle and one of the most common members in wastewater treatment communities. This bacterium is metabolically extremely versatile. It is capable of heterotrophic growth under aerobic and anaerobic conditions, but also able to grow photoautotrophically as well as mixotrophically. Therefore R. palustris can adapt to multiple environments and establish commensal relationships with other organisms, expressing various enzymes supporting degradation of amino acids, carbohydrates, nucleotides, and complex polymers. Moreover, R. palustris can degrade a wide range of pollutants under anaerobic conditions, e.g., aromatic compounds such as benzoate and caffeate, enabling it to thrive in chemically contaminated environments. However, many metabolic mechanisms employed by R. palustris to breakdown and assimilate different carbon and nitrogen sources under chemoheterotrophic or photoheterotrophic conditions remain unknown. Systems biology approaches, such as metabolic modeling, have been employed extensively to unravel complex mechanisms of metabolism. Previously, metabolic models have been reconstructed to study selected capabilities of R. palustris under limited experimental conditions. Here, we developed a comprehensive metabolic model (M-model) for R. palustris Bis A53 (iDT1294) consisting of 2,721 reactions, 2,123 metabolites, and comprising 1,294 genes. We validated the model using high-throughput phenotypic, physiological, and kinetic data, testing over 350 growth conditions. iDT1294 achieved a prediction accuracy of 90% for growth with various carbon and nitrogen sources and close to 80% for assimilation of aromatic compounds. Moreover, the M-model accurately predicts dynamic changes of growth and substrate consumption rates over time under nine chemoheterotrophic conditions and demonstrated high precision in predicting metabolic changes between photoheterotrophic and photoautotrophic conditions. This comprehensive M-model will help to elucidate metabolic processes associated with the assimilation of multiple carbon and nitrogen sources, anoxygenic photosynthesis, aromatic compound degradation, as well as production of molecular hydrogen and polyhydroxybutyrate.

Characteristics and Application of Rhodopseudomonas palustris as a Microbial Cell Factory

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

Enhancing Hydrogen Productivity of Photosynthetic Bacteria from the Formulated Carbon Source by Mixing Xylose with Glucose

To develop an efficient photofermentative process capable of higher rate biohydrogen production using carbon components of lignocellulosic hydrolysate, a desired carbon substrate by mixing xylose with glucose was formulated. Effects of crucial process parameters affecting cellular biochemical reaction of hydrogen by photosynthetic bacteria (PSB), i.e., variation in initial concentration of total carbon, glucose content in initial carbon substrate, and light intensity, were experimentally investigated using response surface methodology (RSM) with a Box-Behnken design (BBD). Hydrogen production rate (HPR) in the maximum value of 30.6 mL h^-1 L^-1 was attained under conditions of 39 mM initial concentration of total carbon, 59% (mol/mol) glucose content in initial carbon substrate, and 12.6 W m^-2 light intensity at light wavelength of 590 nm. Synergic effects of metabolizing such a well-formulated carbon substrate for sustaining the active microbial synthesis to sufficiently accumulate biomass in bioreactor, as well as stimulating enzyme activity of nitrogenase for the higher rate biohydrogen production, were attributed to this carbon substrate that can enable PSB to maintain the relatively consistent microenvironment in suitable culture pH condition during the optimized photofermentative process.

Improving squalene production by blocking the competitive branched pathways and expressing rate-limiting enzymes in Rhodopseudomonas palustris

Squalene is a medically valuable bioactive compound that can be used as a raw material for fuels. Microbial fermentation is the preferred method for the squalene production. In this study, we employed several metabolic engineering strategies to increase squalene yield in Rhodopseudomonas palustris. A 57% increase in squalene titer was achieved by blocking the carotenoid pathway, thus directing more FPP into the squalene biosynthetic pathway. In order to cut down the conversion of squalene to haponoids, a recombinant strain R. palustris [Δshc, ΔcrtB] in which both carotenoid and haponoid pathways were blocked was then constructed, resulting in a 50-fold increase in squalene titer. Based on the expression of rate-limiting enzymes involved in the squalene pathway, the final squalene content reached 23.3 mg/g DCW, which was 178-times higher than that of the wild-type strain. In this study, several methods effective in improving squalene yield have been described and the potential of R. palustris for producing squalene has been demonstrated.

Photosynthetic bacteria-based technology is a potential alternative to meet sustainable wastewater treatment requirement?

A paradigm shift is underway in wastewater treatment from pollution removal to resource or energy recovery. However, conventional activated sludge (CAS) as the core technology of wastewater treatment is confronted with severe challenges on high energy consumption, sludge disposal and inevitable greenhouse gas emission, which are posing a serious impact on the current wastewater industry. It is urgent to find new alternative methods to remedy these defects. Photosynthetic bacteria (PSB) have flexible metabolic modes and high tolerance, which enhance the removal of nutrients, heavy metals and organic contaminants efficiency in different wastewater. The unique phototrophic growth of PSB breaks the restriction of nutrient metabolism in the CAS system. Recent studies have shown that PSB-based technologies can not only achieve the recovery of nutrient and energy, but also improve the degradation efficiency of refractory substances. If the application parameters can be determined, there will be great prospects and economic effects. This review summarizes the research breakthroughs and application promotion of PSB-based wastewater treatment technology in recent years. Comparing discussed the superiority and inferiority from the perspective of application range, performance differences and recovery possibility. Pathways involved in the nutrient substance and the corresponding influencing parameters are also described in detail. The mode of PSB biodegradation processes presented a promising alternative for new wastewater treatment scheme. In the future, more mechanical and model studies, deterministic operating parameters, revolutionary process design is need for large-scale industrial promotion of PSB-based wastewater treatment.

Grid columnar flat panel photobioreactor with immobilized photosynthetic bacteria for continuous photofermentative hydrogen production

A biophotoreactor with a transparent glass flat panel with polymethyl methacrylate (PMMA) grid columnar for enhanced biofilm growth with Rhodopseudomonas palustris GCA009 was developed and tested at 590 nm incident light. Continuous photofermentative hydrogen production from glucose was tested using this novel reactor. At light intensity of 210 W/m², feed substrate concentration of 56.0 mmol/L, and crossflow velocity of 1.68 × 10^-6 m/s, a maximum hydrogen production rate of 32.6 mmol/L-d (3.56 mmol/m²-h), hydrogen yield of 1.15 mol H₂/mol glucose and light conversion efficiency of 5.34% can be achieved. Since the revised grid columnar effectively enlarged the surface area of reactor and enhanced cell attachment, the present reactor design led to higher hydrogen production rates than literature works.

A comparative study of the effects of microbial agents and anaerobic sludge on microalgal biotransformation into organic fertilizer

The effects of Lactobacillus bulgaricus, Rhodopseudomonas palustris, Issatchenkia orientalis and anaerobic sludge on anaerobic digestion of microalgae to organic fertilizer were studied. High-throughput sequencing was used to analyze characteristics of microbial community structure during anaerobic digestion of microalgae using different inocula. Lactobacillales and Saccharomycetales were more likely to become dominant bacteria and eukaryotes. The relative abundance of Lactobacillales was 98.15%, 88.61% and 81.73% of total bacteria at the beginning, middle and end of the experiment, respectively. Meanwhile, the relative abundance of Saccharomycetales was 90.91%, 98.41% and 98.8% of eukaryotes at the beginning, middle and end of the experiment, respectively. At the end of digestion, the microcystin content in the reactor inoculated with Issatchenkia orientalis decreased to 0.71 μg/kg, which met drinking water standards. Rhodopseudomonas palustris did not become a dominant microorganism and had the most negative impact on the atmosphere. Volatile organic compounds were 11.92 mg/kg while the odor concentration reached 97,724 ou/m³. The organic matter content in reactors inoculated with specific groups of microbial agents, which was higher than the standard required for bio-organic fertilizer, occupying over 96% dry weight. In addition, the effective microorganism counts of Issatchenkia orientalis and Lactobacillus bulgaricus in fermentation products reached 1.8E+09 colony-forming units (cfu)/g and 1.6E+09 cfu/g, respectively, which are suitable values for microbial fertilizer.

Photosynthetic bacteria improved hydrogen yield of combined dark- and photo-fermentation

Integration of dark- and photo-fermentation is a promising strategy to enhance saline wastewater treatment efficiency and biohydrogen production. In this study, dark- and photo-fermentative bacterial consortium was respectively enriched and their communities were analyzed using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). Both consortia were mainly composed of hydrogen-producing strains. After the first-stage dark-fermentation, the following conditions were applied prior to the second-stage fermentation: fermentative broth pH regulation (the pH group), glucose addition (the glucose group), glucose addition and pH regulation (the glucose + pH group), photosynthetic bacteria addition (the photo group), and photosynthetic bacteria addition and pH regulation (the photo + pH group), respectively. Dark fermentative broth with no pretreatment was used as control (the control group). Then the second stage began. The results showed that pH restriction had more influence than substrate or products restriction on dark-fermentative hydrogen production. Addition of photo-fermentative bacteria after dark-fermentation increased the hydrogen yield (134%) and substrate utilization (67%). These findings indicated syntrophic interactions between dark- and photo-fermentative bacteria during the hydrogen production process.

Demonstration and optimization of sequential microaerobic dark- and photo-fermentation biohydrogen production by immobilized Rhodobacter capsulatus JP91

Hydrogen generation from complex substrates composed of simple sugars has the potential to mitigate future worldwide energy demand. The biohydrogen potential of a sequential microaerobic dark- and photo-fermentative system was investigated using immobilized Rhodobacter capsulatus JP91. Biological hydrogen production from glucose was carried out using a batch process and a bench-scale bioreactor. Response surface methodology with a Box-Behnken design was employed to optimize key parameters such as inoculum concentration, oxygen concentration, and glucose concentration. The maximum hydrogen production (21 ± 0.25 mmol H₂/L) and yield (7.8 ± 0.1 mol H₂/mol glucose) were obtained at 6 mM glucose, 4.5% oxygen and 62.5 v/v% inoculum concentration, demonstrating the feasibility of enhanced hydrogen production by immobilized R. capsulatus JP91 in a sequential system. This is the first time that a sequential process using an immobilized system has been described. This system also achieved the highest hydrogen yield obtained by an immobilized system so far.