Effects of light intensity and photoperiod on pigments production and corresponding key gene expression of Rhodopseudomonas palustris in a photobioreactor system

Light intensity effects on bioproduct recovery from fuel synthesis wastewater using purple phototrophic bacteria in a hybrid biofilm-suspended growth system

This research looked at how three different light intensities (1600, 4300, and 7200 lx) affect the biomass development, treatment of fuel synthesis wastewater and the recovery of valuable bioproducts between biofilm and suspended growth in a purple-bacteria enriched photobioreactor. Each condition was run in duplicate using an agricultural shade cloth as the biofilm support media in a continuously mixed batch reactor. The results showed that the highest chemical oxygen demand (COD) removal rate (56.8 ± 0.9 %) was found under the highest light intensity (7200 lx), which also led to the most biofilm formation and highest biofilm biomass production (1225 ± 95.7 mg). The maximum carotenoids (Crts) and bacteriochlorophylls (BChls) content occurred in the suspended growth of the 7200 lx reactor. BChls decreased with light intensity in suspended growth, while in biofilm both Crts and BChls were relatively stable between light conditions, likely due to an averaging effect as biofilm thickened at higher light intensity. Light intensity did not affect protein content of the biomass, however, biofilm showed a lower average (41.2 % to 43.7 %) than suspended biomass (45.4 % to 47.7 %). For polyhydroxybutyrate (PHB) the highest cell concentration in biofilm occurred at 1600 lx (11.4 ± 2.4 %), while for suspended growth it occurred at 7200 lx (22.7 ± 0.3 %), though total PHB productivity remained similar between reactors. Shading effects from the externally located biofilm could explain most variations in bioproduct distribution. Overall, these findings suggest that controlling light intensity can effectively influence the treatment of fuel synthesis wastewater and the recovery of valuable bioproducts in a biofilm photobioreactor.

Feasibility and mechanism of adsorption and bioreduction of hexavalent chromium using Rhodopseudomonas palustris immobilized on multiple materials

Rhodopseudomonas palustris immobilized on multiple materials was used to invistigate Cr(VI) adsorption and bioreduction. The highest Cr(VI) removal (97.5%) was achieved at 276h under the opitimed conditions of 2.5% SA, 8% PVA, and 50% filling degree. The highest adsorption capacity was obtained at 11.75 mg g-1 under 300 mg L-1 Cr(VI). Results from adsorption kinetics and isotherms indicated that Cr(VI) adsorption of immobilized photosynthetic bacteria (IPSB) was consistent with the Freundich model and the pseudo-second-order kinetic model (qe = 14.00 mg g-1). SEM and FTIR analyses verified that the porous multilayer network structure of IPSB provided more adsorption sites and functional groups for the removal of Cr(VI). Furthermore, the maximum Cr(VI) reduction efficiency of IPSB was achieved at 10.80 mg g-1, which correlated with the up-regulation of chrR gene expressions at 100 mg L-1 Cr(VI). This study demonstrated the dual mechanisms of Cr(VI) removal in IPSB-treated Cr wastewater, involving both chemisorption and bioreduction working synergistically.

In-situ construction of biomineralized cadmium sulfide-Rhodopseudomonas palustris hybrid system: Mechanism of synergistic light utilization

Sulfide biomineralization is a microorganism-induced process for transforming the environmentally hazardous cadmium into useful resource utilization. This study successfully constructed cadmium sulfide nanoparticles-Rhodopseudomonas palustris (Bio-CdS NPs-R. palustris) hybrids. For the self-assembling hybrids, Bio-CdS NPs were treated as new artificial-antennas to enhance photosynthesis, especially under low light (LL). Bacterial physiological results of hybrids were significantly increased, particularly for cells under LL, with higher enhancement photon harvesting ability. The enhancement included the pigment contents, and the ratio of the peripheral light-harvesting complex Ⅱ (LH2) to light-harvesting Ⅰ (1.33 ± 0.01 under LL), leading to the improvements of light-harvesting, transfer, and antenna conversion efficiencies. Finally, the stimulated electron chain of hybrids improved bacterial metabolism with increased nicotinamide adenine dinucleotide (NADH, 174.5% under LL) and adenosine triphosphate (ATP, 41.1% under LL). Furthermore, the modified photosynthetic units were induced by the up-regulated expression of fixK, which was activated by reduced oxygen tension of the medium for hybrids. fixK up-regulated genes encoding pigments (crt, and bch) and complexes (puf, pucAB, and pucC), leading to improved light-harvesting and transfer, and transform ability. This study provides a comprehensive understanding of the solar energy utilization mechanism of in-situ semiconductor-phototrophic microbe hybrids, contributing to further theoretical insight into their practical application.

Modelling photoperiod in enhancing hydrogen production from Chlorella vulgaris sp. while bioremediating ammonium and organic pollutants in municipal wastewater

Municipal wastewater is ubiquitously laden with myriad pollutants discharged primarily from a combination of domestic and industrial activities. These heterogeneous pollutants are threating the natural environments when the traditional activated sludge system fails sporadically to reduce the pollutants' toxicities. Besides, the activated sludge system is very energy intensive, bringing conundrums for decarbonization. This research endeavoured to employ Chlorella vulgaris sp. In converting pollutants from municipal wastewater into hydrogen via alternate light and dark fermentative process. The microalgae in attached form onto 1 cm3 of polyurethane foam cubes were adopted in optimizing light intensity and photoperiod during the light exposure duration. The highest hydrogen production was recorded at 52 mL amidst the synergistic light intensity and photoperiod of 200 μmolm-2s-1 and 12:12 h (light:dark h), respectively. At this lighting condition, the removals of chemical oxygen demand (COD) and ammoniacal nitrogen were both achieved at about 80%. The sustainability of microalgal fermentative performances was verified in recyclability study using similar immobilization support material. There were negligible diminishments of hydrogen production as well as both COD and ammoniacal nitrogen removals after five cycles, heralding inconsequential microalgal cells' washout from the polyurethane support when replacing the municipal wastewater medium at each cycle. The collected dataset was finally modelled into enhanced Monod equation aided by Python software tool of machine learning. The derived model was capable to predict the performances of microalgae to execute the fermentative process in producing hydrogen while subsisting municipal wastewater at arbitrary photoperiod. The enhanced model had a best fitting of R2 of 0.9857 as validated using an independent dataset. Concisely, the outcomes had contributed towards the advancement of municipal wastewater treatment via microalgal fermentative process in producing green hydrogen as a clean energy source to decarbonize the wastewater treatment facilities.

Removal of pollutants and accumulation of high-value cell inclusions in a batch reactor containing Rhodopseudomonas for treating real heavy oil refinery wastewater

Treating wastewater using purple non-sulfur bacteria (PNSB) is an environmentally friendly technique that can simultaneously remove pollutants and lead to the accumulation of high-value cell inclusions. However, no PNSB system for treating heavy oil refinery wastewater (HORW) and recovering high-value cell inclusions has yet been developed. In this study, five batch PNSB systems dominated by Rhodopseudomonas were used to treat real HORW for 186 d. The effects of using different hydraulic retention times (HRT), sludge retention times (SRT), trace element solutions, phosphate loads, and influent loads were investigated, and the bacteriochlorophyll, carotenoid, and coenzyme Q10 concentrations were determined. The community structure and quantity of Rhodopseudomonas in the systems were determined using a high-sequencing technique and quantitative polymerase chain reaction technique. The long-term results indicated that phosphate was the limiting factor for treating HORW in the PNSB reactor. The soluble chemical oxygen demand (SCOD) removal rates were 67.03% and 85.26% without and with phosphate added, respectively, and the NH4+-N removal rates were 32.18% and 89.22%, respectively. The NO3--N concentration in the effluent was stable at 0-3 mg/L with or without phosphate added. Adding phosphate increased the Rhodopseudomonas relative abundance and number by 13.21% and 41.61%, respectively, to 57.35% and 8.52 × 106 gene copies/μL, respectively. The SRT was the limiting factor for SCOD removal, and the bacteria concentration was the limiting factor for nitrogen removal. Once the inflow load had been increased, the total nitrogen (TN) removal rate increased as the HRT increased. Maximum TN removal rates of 64.46%, 68.06%, 73.89%, 82.15%, and 89.73% were found at HRT of 7, 10, 13, 16, and 19 d, respectively. The highest bacteriochlorophyll, carotenoid, and coenzyme Q10 concentrations were 2.92, 4.99, and 4.53 mg/L, respectively. This study provided a simple and efficient method for treating HORW and reutilizing resources, providing theoretical support and parameter guidance for the application of Rhodopseudomonas in treating HORW.

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.

Stability and biomineralization of cadmium sulfide nanoparticles biosynthesized by the bacterium Rhodopseudomonas palustris under light

Cadmium (Cd) pollution is regarded as a potent problem due to its hazard risks to the environment, making it crucial to be removed. Compared to the physicochemical techniques (e.g., adsorption, ion exchange, etc.), bioremediation is a promising alternative technology for Cd removal, due to its cost-effectiveness, and eco-friendliness. Among them, microbial-induced cadmium sulfide mineralization (Bio-CdS NPs) is a process of great significance for environmental protection. In this study, microbial cysteine desulfhydrase coupled with cysteine acted as a strategy for Bio-CdS NPs by Rhodopseudomonas palustris. The synthesis, activity, and stability of Bio-CdS NPs-R. palustris hybrid was explored under different light conditions. Results show that low light (LL) intensity could promote cysteine desulfhydrase activities to accelerate hybrid synthesis, and facilitated bacterial growth by the photo-induced electrons of Bio-CdS NPs. Additionally, the enhanced cysteine desulfhydrase activity effectively alleviated high Cd-stress. However, the hybrid rapidly dissolved under changed environmental factors, including light intensity and oxygen. The factors affecting the dissolution were ranked as follows: darkness/microaerobic ≈ darkness/aerobic < LL/microaerobic < high light (HL)/microaerobic < LL/aerobic < HL/aerobic. The research provides a deeper understanding of Bio-CdS NPs-bacteria hybird synthesis and its stability in Cd-polluted water, allowing advanced bioremediation treatment of heavy metal pollution in water.

Light intensity defines growth and photopigment content of a mixed culture of purple phototrophic bacteria

Purple bacteria (PPB), anoxygenic photoorganoheterotrophic organisms with a hyper-versatile metabolism and high biomass yields over substrate, are promising candidates for the recovery of nutrient resources from wastewater. Infrared light is a pivotal parameter to control and design PPB-based resource recovery. However, the effects of light intensities on the physiology and selection of PPB in mixed cultures have not been studied to date. Here, we examined the effect of infrared irradiance on PPB physiology, enrichment, and growth over a large range of irradiance (0 to 350 W m−2) in an anaerobic mixed-culture sequencing batch photobioreactor. We developed an empirical mathematical model that suggests higher PPB growth rates as response to higher irradiance. Moreover, PPB adapted to light intensity by modulating the abundances of their phototrophic complexes. The obtained results provide an in-depth phylogenetic and metabolic insight the impact of irradiance on PPB. Our findings deliver the fundamental information for guiding the design of light-driven, anaerobic mixed-culture PPB processes for wastewater treatment and bioproduct valorization.

Spectral bands of incandescent lamp leading to variable productivity of purple bacteria biomass and microbial protein: Full is better than segmented

Purple non‑sulfur bacteria (PNSB) are competent microorganisms capable of producing value-added products from waste streams. Light source is one of the most influential factors determining the efficiency of this process. Previous studies mostly focused on optimizing light intensity, while the impact of spectral bands on PNSB growth is still unknown. To fill the knowledge gap, this study investigated the responses of PNSB (i.e., Rhodobacter sphaeroides) growth, protein content and enzyme activity to various spectral bands of an incandescent lamp for the first time. It was found that the full spectrum of the incandescent lamp was propitious to cultivate PNSB than segmented spectral bands, as demonstrated by the maximum biomass yield of 1.05 g biomass g^-1 COD_removed, specific growth rate of 0.53 d^-1 and protein concentration of 0.48 g L^-1. The production of biomass and protein under infrared (IR) spectral band were slightly lower than those under full spectrum, but 3.2 and 1.7 times higher than the average values (0.14 g L^-1 and 0.07 g L^-1) under visible spectral bands, respectively. The variation trends of enzymatic activities, such as fructose-1,6-bisphosphatase (FBP) and photopigments were consistent with that of PNSB biomass upon varying spectral bands, suggesting that the spectral bands might induce a variable PNSB biomass via affecting the Calvin cycle and photophosphorylation process. These results provide a new perspective that spectrum bands of light sources should be considered in the process optimization.

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.

Material biosynthesis, mechanism regulation and resource recycling of biomass and high-value substances from wastewater treatment by photosynthetic bacteria: A review

The environmental-friendly and economic benefits generated from photosynthetic bacteria (PSB) wastewater treatment have attracted significant attention. This process of resource recovery can produce PSB biomass and high-value substances including single cell protein, Coenzyme Q₁₀, polyhydroxyalkanoates (PHA), 5-aminolevulinic acid, carotenoids, bacteriocin, and polyhydroxy chain alkyl esters, etc. for application in various fields, such as agriculture, medical treatment, chemical, animal husbandry and food industry while treating wastewaters. The main contents of this review are summarized as follows: physiological characteristics, mechanism and application of PSB and potential of single cell protein (SCP) production are described; PSB wastewater treatment technology, including procedures and characteristics, typical cases, influencing factors and bioresource recovery by membrane bioreactor are detailed systematically. The future development of PSB-based resource recovery and wastewater treatment are also provided, particularly concerning PSB-membrane reactor (MBR) process, regulation of biosynthesis mechanism of high-value substances and downstream separation and purification technology. This will provide a promising and new alternative for wastewater treatment recycling.

The Role of Photo-Cycles in the Modulation of Growth and Biochemical Profile of Microalgae: Part I—Food Interest Compounds

The objective of this work was to evaluate the effect of different photo-cycles on the growth and biochemical profile of Scenedesmus obliquus CPCC05, focusing on food interest compounds. The photo-cycle conditions were separated into three groups: long-term photo-cycles (24:0, 22:2, 20:4, 18:6, 12:12, and 10:14 (h:h)), frequency photo-cycles (2, 4, 8, 12, 24, and 48 times per day (t/d)), and short photo-cycles (0.91:0.09, 0.83:0.17, 0.75:0.25, and 0.50:0.50 (s:s)) of light:dark, respectively. The results showed these microalgae can store enough energy to support cell growth for continuous periods of up to 2 h in the dark, without affecting the productivity of the process. This 2 h, when divided into 2 cycles per day (2 t/d), showed the best growth condition (3700 mg L⁻¹), generation time (14.40 h), and maximum biomass productivity (21.43 mg L h⁻¹). This photo-cycle of 2 t/d was also the best condition for the production of total sterols. However, the values of polyunsaturated fatty acids, lipid content, and amino acids obtained higher yields in the short photo-cycle of 0.75:0.25. Thus, the modulation of light cycles becomes an important tool for boosting and directing the production of target molecules in phototrophic cultures of microalgae.

Photosynthetic bacteria with iron oxide nanoparticles as catalyst for cooking oil removal and valuable products recovery with heavy metal co-contamination

Waste cooking oil discharge causes environmental pollution in receiving waters, particularly when associated with heavy metals that can lead to formation of hazardous organometallic compounds. This study combined iron oxide nanomaterial and the anoxygenic photosynthetic bacterium Rhodopseudomonas faecalis PA2 for removal of cooking oil in the presence of heavy metals. R. faecalis PA2, with known capability to generate beneficial substances from several wastes, was capable of cooking oil removal with production of valuable products. Oil removal, biomass, protein, and carotenoid production were 82.38%, 1.48 g/L, 1,600.19 mg/L, and 1,046.33 mg/L, respectively, under optimal conditions (cooking oil as carbon source and 30% inoculum density). Iron (Fe) stimulates growth of R. faecalis; in this study, Fe₃O₄ nanoparticles were synthesized and used as a catalyst to facilitate interaction and high reactivity between Fe and R. faecalis PA2. Size measurement by transmission electron microscopy (17.44 nm), X-ray diffraction peaks, and magnetic susceptibility confirmed that the synthesized nanoparticles were magnetite Fe₃O₄. Biomass, protein, and carotenoid production of the Fe₃O₄ supplemented experiment increased by 61.56%, 70.78%, and 57.2%, respectively, when compared with the control. When different concentrations of heavy metals (Pb, Ni, Co, and Zn) were supplemented in the media containing cooking oil, Fe₃O₄ addition increased heavy metal tolerance, improved bacterial growth, and enhanced valuable products when compared with the non-supplemented group. This study reports the positive impact of nanoparticle application as a catalyst for valorization of cooking oil waste with heavy metal co-contamination by the photosynthetic bacterium R. faecalis PA2.

Cost-effective process for the production of Monascus pigments using potato pomace as carbon source by fed-batch submerged fermentation

Potato pomace, generated from starch-processing industry, was applied as a cost-effective resource for producing Monascus pigments via submerged fermentation. First, the pigment-production capacity of potato pomace and its hydrolysate was compared. The results indicated that potato pomace was superior to its hydrolysate when used for producing Monascus pigments. The red and yellow pigments produced in potato pomace medium reached 27.8 and 19.7 OD units/ml in 7 days, with the yield of total pigments at 1,187.5 OD units/g, respectively, increased by 127.9%, 19.4%, and 46.3% compared with the data obtained from hydrolysate. Meanwhile, the citrinin produced in potato pomace medium decreased by 22.6%. Afterward, potato pomace, without hydrolysis, was used as carbon source to obtain the optimal pigment production conditions. In the batch fermentation process, it was found that high amount of pomace inhibited the growth rate of mycelia and the productivity of pigments, and the fed-batch fermentation process could enhance the yield and productivity of pigments. With the same final amount of pomace (80 g/L), the maximal levels of total pigments and productivity obtained from fed-batch process reached 118.8 OD units/ml and 13.2 OD units/(ml·day), which presented an increase of 35.2% and 67.1% compared with the not fed-batch group, respectively. The results demonstrated that potato pomace was a cost-effective substrate for producing Monascus pigments in terms of pigment production capacity and productivity when fed-batch submerged fermentation was applied.

Single-phase and two-phase cultivations using different light regimes to improve production of valuable substances in the anoxygenic photosynthetic bacterium Rhodopseudomonas faecalis PA2

This study aimed to improve biomass, carotenoid, bacteriochlorophyll, protein, lipid, and carbohydrate contents of Rhodopseudomonas faecalis PA2 using different light regimes. Light intensity (4000, 6000, 8000, and 10,000 lx), together with photoperiod (24:0, 16:8, 12:12, and 8:16 h light/dark), was assigned as single-phase (SP) cultivation while two-phase (TP) cultivation used two light intensities (using 4000 lx as the first phase), together with the control of phase shift (3, 6, and 9 days) and photoperiod. Biomass, carotenoid, and bacteriochlorophyll contents were maximized by SP cultivation; light at 8000 lx with light-dark cycle of 24:0 was optimal for pigments synthesis. In contrast, TP was useful to enhance storage compounds; protein, lipid, and carbohydrate productivities were significantly increased by 121.69%, 101.69%, and 92.44%, respectively, in TP when compared with SP. This indicates that the novel light strategy proposed in this study was able to manipulate the production of valuable substances in this strain.

Polyhydroxyalkanoates from organic waste streams using purple non-sulfur bacteria

Many microorganisms can produce intracellular and extracellular biopolymers, such as polyhydroxyalkanoates (PHA). Despite PHA's benefits, their widespread at the industrial level has not occurred due mainly to high production costs. PHA production under a biorefinery scheme is proposed to improve its economic viability. In this context, purple non-sulfur bacteria (PNSB) are ideal candidates to produce PHA and other substances of economic interest. This review describes the PHA production by PNSB under different metabolic pathways, by using a wide range of wastes and under diverse operational conditions such as aerobic and anaerobic metabolism, irradiance level, light or dark conditions. Some strategies, such as controlling the feed regime, biofilm reactors, and open photobioreactors in outdoor conditions, were identified from the literature review as the approach needed to improve the process's economic viability when using mixed cultures of PNSB and wastes as substrates.

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.