Photoheterotrophic Conditions Research Articles

Metabolic responses of Euglena gracilis under photoheterotrophic and heterotrophic conditions

The protist Euglena gracilis has various trophic modes including heterotrophy and photoheterotrophy. To investigate how cultivation mode influences metabolic regulation, the chemical composition of cellular metabolites of Euglena gracilis grown under heterotrophic and photoheterotrophic conditions was monitored from the early exponential phase to the mid-stationary phase using two different techniques, i.e, nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS). The combined metabolomics approach allowed an in-depth understanding of the mechanism of photoheterotrophic and heterotrophic growth for biomolecule production. Heterotrophic conditions promoted the production of polar amino and oxygenated compounds such as proteins and polyphenol compounds, especially at the end of the exponential phase while photoheterotrophic cells enhanced the production of organoheterocyclic compounds, carbohydrates, and alkaloids.

Photoheterotroph improved the growth and nutrient levels of Chlorella vulgaris and the related molecular mechanism

Microalgae are rich in fatty acids, proteins, and other nutrients, which have gained the general attention of researchers all over the world. For the development of Chlorella vulgaris in food and feed industry, this study was conducted to investigate the differences in C. vulgaris’ growth and nutritional components under different culture conditions (autotrophic, heterotrophic, photoheterotrophic) and the internal factors through cell counting in combination with transcriptome and nutrient analyses. The results showed that, under the photoheterotrophic condition, Chlorella’s growth and the contents of lipid and protein were significantly higher than that under the heterotrophic condition, and the moisture content was lower than that under the heterotrophic condition. The saturated fatty acid content under the photoheterotrophic condition was the lowest, while the polyunsaturated fatty acid content was significantly higher than those under the other two conditions. There were 46,583 differentially expressed genes (DEGs), including 33,039 up-regulated DEGs (70.93%) and 13,544 down-regulated DEGs (29.07%), under the photoheterotrophic condition in comparison with the autotrophic condition. The fold change between the two conditions of samples of up-regulated genes was higher than that of the down-regulated genes. The KEGG enrichment showed that the up-regulated DEGs in the photoheterotrophic condition were significantly enriched in 5 pathways, including protein processing in endoplasmic reticulum pathway, photosynthesis pathway, photosynthesis-antenna protein pathway, endocytosis pathway, and phosphonate and phosphinate metabolism pathway. DEGs related to fatty acid metabolic pathways were significantly enriched in the fatty acid biosynthesis pathway and the biosynthesis of unsaturated fatty acid pathway. The qPCR analysis showed that the expression pattern of the selected genes was consistent with that of transcriptome analysis. The results of this study lay a theoretical foundation for the large-scale production of Chlorella and its application in food, feed, and biodiesel.Key points• Nutrient levels under photoheterotrophic condition were higher than other conditions.• Six important pathways were discovered that affect changes in nutritional composition.• Explored genes encode important enzymes in the differential metabolic pathways.

Enhancing carotenoid production and exploring the potential use of microalga Desmodesmus cf. pleiomorphus DSHM22 as a biodiesel feedstock through photoheterotrophic cultivation

Improving biomass and value-added compound production is crucial for the industrial application of microalgae, achieved through the utilization of low-cost nutrients and various cultivation modes. This study aimed to enhance the biomass and biochemical production of the indigenous microalgae by comparing different cultivation modes. Among six algal strains obtained from the East Sea in South Korea, Desmodesmus cf. pleiomorphus DSHM22 exhibited the highest biomass productivity (0.0118 g L−1 d−1) and was selected for optimization. Algal growth responses were assessed under four commercially available soluble fertilizer media to identify a cost-effective nutrient substrate. The most effective fertilizer demonstrated 2.2-fold higher growth compared to the BG-11 algal medium after 14 days. Glucose was supplemented in the culture medium to evaluate organic carbon availability under light and dark conditions. While no growth responses were observed under glucose-supplemented dark condition, the highest biomass production (approximately 1.22 g L−1) was achieved under glucose-supplemented light condition. The algal strain showed growth under both photoautotrophic (PA) and photoheterotrophic (PH) conditions, excluding the heterotrophic mode. Biochemical accumulations were compared by analyzing photosynthetic pigments and fatty acids under both PA and PH modes. The algal strain effectively produced lutein, neoxanthin, and omega-3, 6, and 9 fatty acids. Although PH condition yielded significantly higher biochemicals and lipid production compared to PA condition, both culture conditions exhibited similar fatty acid compositions and biodiesel properties. This study suggests that PH cultivation is an effective approach for producing value-added biochemicals and biodiesel using microalga D. cf. pleiomorphus DSHM22.

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.

EFFECT OF VARIOUS CARBON SOURCES ON THE GROWTH PROPERTIES AND MORPHOLOGY OF $SPIRULINA~PLATENSIS$

Cyanobacteria generate biomass under photoautotrophic conditions during photosynthesis. Cultivation of cyanobacteria under photoheterotrophic conditions using various organic carbon sources can increase yield of biomass. In the current study, the effect of organic carbon sources on the growth properties and morphology of cyanobacteria Spirulina platensis, as well as the production of the photosynthetic pigments: chlorophyll a, carotenoids, and phycocyanin were investigated. Carbon sources, such as glucose, fructose and succinate, caused not only an increase in the cyanobacterial growth rate, but also morphological changes in S. platensis trichomes. An increase in the amounts of photosynthetic pigments was also observed. Thus, glucose, fructose, and succinate are more efficient substrates for photoheterotrophic cultivation of these cyanobacteria.

Demonstration of the Antioxidant Potential of Three Newly Isolated Carotenoid-Producing Microscopic Algae from KwaZulu-Natal (South Africa)

Natural carotenoids from green microalgae exhibit beneficial effects in treating health-related diseases, primarily due to their antioxidant capacity. Therefore, carotenoid-producing microalgae were isolated and characterized from aqueous samples in KwaZulu-Natal (South Africa) under photoautotrophic conditions. Three isolates were characterized comprehensively using light and electron microscopy. In addition, the growth of the three selected microalgae was evaluated under photoautotrophic and photoheterotrophic conditions. Based on the cytological characteristics, the three strains matched the genera Haematococcus, Parachlorella, and Scenedesmus, the latter two additionally confirmed via analysis of the 18S rRNA gene sequence at the genus level. Light and electron microscopy and UV-Vis spectroscopy confirmed pigment production by all three microalgal strains. Both isolate Parachlorella sp. AA1 and Scenedesmus sp. AA2 showed the highest growth rate when cultured with acetate (25 mM) photoheterotrophically, while the isolate Haematococcus sp. AA3 grew best with glucose (50 mM). In addition, all three isolates utilized salicylate as a carbon source under photoheterotrophic conditions, evidently resulting in increased carotenoid production in strains AA1 and AA3. The 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) antioxidant assay confirmed that methanol extracts of all three isolates contained carotenoids that can scavenge radicals, and thin layer chromatography (TLC) analysis showed that β-carotene and astaxanthin were formed by strain AA3 as main carotenoids.

Attachment of Ferredoxin: NADP+ Oxidoreductase to Phycobilisomes Is Required for Photoheterotrophic Growth of the Cyanobacterium Synechococcus sp. PCC 7002.

Two types of cyanobacterial phycobilisomes (PBS) are present: the hemidiscoidal PBS (CpcG-PBS) and the membrane-bound PBS (CpcL-PBS). Both types of PBS have ferredoxin:NADP+ oxidoreductase (FNR) attached to the termini of their rods through a CpcD domain. To date, the physiological significance of the attachment remains unknown. We constructed a mutant (dF338) which contains an FNR lacking the N-terminal CpcD domain in Synechococcus sp. PCC 7002. Isolated CpcG-PBS from dF338 did not contain FNR and the cell extracts of the mutant had a 35 kDa protein cross-reacting to anti-FNR antibodies. dF338 grows normally under photoautotrophic conditions, but little growth was observed under photoheterotrophic conditions. A cpcL (cpcG2) mutant grows extremely slowly under photoheterotrophic conditions while a cpcG (cpcG1) mutant, in which PBS rods could not attach to the cores of the CpcG-PBS, can grow photoheterotrophically, strongly suggesting that the attachment of FNR to CpcL-PBS is critical to photoheterotrophic growth. We show that electron transfer to the plastoquinone pool in dF338 and the cpcL mutant was impaired. We also provide evidence that trimeric photosystem I (PSI) and intact CpcL-PBS with a full-length FNR is critical to plastoquinone reduction. The presence of a NADPH-dehydrogenase (NDH)-CpcL-PBS-PSI trimer supercomplex and its roles are discussed.

Enhanced biomass production of Synechocystis sp. PCC 6803 by two associated bacteria Paenibacillus camelliae and Curtobacterium ammoniigenes.

Synechocystis sp. PCC 6803 grown continuously in a 5-L photo-bioreactor for 20 months was found to have associated consortia with heterotrophic microorganisms. Two strains of bacteria were isolated from the long-term cultures of cyanobacteria with the aim to test whether their presence affects cyanobacterial growth and metabolism. The two strains were phylogenetically identified as Paenibacillus camelliae and Curtobacterium ammoniigenes, respectively. Co-culturing the Synechocystis sp. with either of the isolates under photoautotrophic and photoheterotrophic conditions exerted a statistically significant growth enhancement effect on cyanobacteria. Under co-culture experiments, the addition of P. camelliae resulted in a four-fold higher biomass yield with a considerable decrease in the stationary period. The growth was more pronounced on the addition of acetate to the culture media. Growth-enhancing factors like indole acetic acid (IAA) and siderophores were detected in the co-culture conditions which proved to be the main driving force in boosting cyanobacterial growth. Thus, the cyanobacteria-bacteria consortia can be very useful for augmenting biomass production by circumventing the time factor which can be further exploited for various biotechnological applications.

Slaughterhouse wastewater treatment using purple phototrophic bacteria: A comparison between photoheterotrophic and chemoheterotrophic conditions

This study aimed to evaluate the effectiveness of purple phototrophic bacteria (PPB) biomass, dominated by Rhodopseudomonas, for treating slaughterhouse wastewater (SWW) under photoheterotrophic and chemoheterotrophic conditions in batch bioreactors. Under both the photoheterotrophic and chemoheterotrophic conditions, PPB hydrolyzed SWW quickly, with both carbon (soluble COD: sCOD) and nutrients (soluble total nitrogen: sTN and phosphate: PO43−) reaching maximum concentrations within the first day. Active assimilation caused simultaneous decrease in carbon and nutrient concentrations within the first 8 days under the photoheterotrophic condition, resulting in high removal of sCOD (91.9%), sTN (70.1%), and PO43− (90.9%). Although removal rate constants (k) were almost five-fold lower than those under the photoheterotrophic condition, we observed substantial removal of sCOD (88.4%) and PO43− (78.2%) under the chemoheterotrophic condition, indicating the activation of versatile metabolic pathways (e.g. respiration and phosphorus accumulation). Biomass carbohydrate and protein content on the last day of the photoheterotrophic condition increased by 50% and 76%, respectively, compared with inoculated PPB biomass; the chemoheterotrophic condition saw 13% and 36% increases. Our results confirmed the effectiveness of the photoheterotrophic pathway in SWW treatment. The bacteria were also able to remove carbon and phosphorus from SWW during the prolonged dark period.

Tandem gene amplification restores photosystem II accumulation in cytochrome b559 mutants of cyanobacteria.

Summary Cytochrome (Cyt) b 559 is a key component of the photosystem II complex (PSII) that is essential for its proper functioning and assembly. Site‐directed mutants of the model cyanobacterium Synechocystis sp. PCC6803 with mutated heme axial ligands of Cyt b 559 have little PSII and are therefore unable to grow photoautotrophically.Here we describe two types of Synechocystis autotrophic transformants that retained the same mutations in Cyt b 559 but are able to accumulate PSII and grow photoautotrophically. Whole‐genome sequencing revealed that all of these autotrophic transformants carried a variable number of tandem repeats (from 5 to 15) of chromosomal segments containing the psbEFLJ operon.RNA‐seq analysis showed greatly increased transcript levels of the psbEFLJ operon in these autotrophic transformants. Multiple copies of the psbEFLJ operon in these transformants were only maintained during autotrophic growth, while its copy numbers gradually decreased under photoheterotrophic conditions. Two‐dimensional PAGE analysis of membrane proteins revealed a strong deficiency in PSII complexes in the Cyt b 559 mutants that was reversed in the autotrophic transformants.These results illustrate how tandem gene amplification restores PSII accumulation and photoautotrophic growth in Cyt b 559 mutants of cyanobacteria, and may serve as an important adaptive mechanism for cyanobacterial survival.

Analysis of the Involvement of the Isoleucine Biosynthesis Pathway in Photoheterotrophic Metabolism of Rhodospirillum rubrum.

Purple non-sulfur bacteria (PNSB) are recognized as a highly versatile group of bacteria that assimilate a broad range of carbon sources. Growing heterotrophically, PNSB such as Rhodospirillum rubrum (Rs. rubrum) generate reduced equivalents that are used for biomass production. However, under photoheterotrophic conditions, more reduced electron carriers than required to produce biomass are generated. The excess of reduced equivalents still needs to be oxidized for the metabolism to optimally operate. These metabolic reactions are known as electron sinks. Most PNSB rely on the CO2-fixing Calvin cycle and H2 production to oxidize these reduced equivalents. In addition to these well-described electron sinks, the involvement of some pathways, such as polyhydroxyalkanoate (PHA) biosynthesis, in redox poise is still controversial and requires further studies. Among them, isoleucine biosynthesis has been recently highlighted as one of these potential pathways. Here, we explore the role of isoleucine biosynthesis in Rs. rubrum. Our results demonstrate that the isoleucine content is higher under illuminated conditions and that submitting Rs. rubrum to light stress further increases this phenomenon. Moreover, we explore the production of (p)ppGpp in Rs. rubrum and its potential link with light stress. We further demonstrate that a fully functional isoleucine biosynthesis pathway could be an important feature for the onset of Rs. rubrum growth under photoheterotrophic conditions even in the presence of an exogenous isoleucine source. Altogether, our data suggest that isoleucine biosynthesis could play a key role in redox homeostasis.

Analysis of microalgal growth kinetic model and carbohydrate biosynthesis cultivated using agro-industrial waste residuals as carbon source

Microalgal carbohydrate is considered one of the potential feedstock for biofuel produced via the bioconversion process. However, the current cultivation process using commercial medium exhibited low biomass production and its carbohydrate productivity which become a bottleneck for sustainable microalgal-carbohydrate-based biofuel production. Thus, the objective of this study is to assess the utilization of industrial waste including molasses and glycerol on the Halochlorella rubescens and Tetraselmis suecica growth as well as its carbohydrate content under different cultivation modes such as autotrophic, heterotrophic and photoheterotrophic conditions. From this study, the highest maximum biomass of H. rubenscens and T. suecica of 0.653 ± 0.009 and 0.669 ± 0.01gL−1 were obtained when the cultivation was performed under photoheterotrophic using molasses. High carbohydrate content of H. rubescens and T. seucica of 56.81 ± 0.39% and 71.52 ± 0.03% with glucose represent the dominant sugar was observed under this condition. The growth kinetic model of the analysis indicated that Huang and Gompertz Models described well the growth of H. rubescens and T. suecica under photoheteroptroph condition with a high significant R 2 of 0.99. The information generated could be beneficial for the future development of low-cost microalgal cultivation media formulation for future microalgal carbohydrate-based products such as bioethanol.

Enhanced lipid accumulation of Chlorella vulgaris with agricultural waste under optimized photoheterotrophic conditions

Enhanced lipid accumulation of Chlorella vulgaris with agricultural waste under optimized photoheterotrophic conditions

Assessment of Voltage Influence in Carbon Dioxide Fixation Process by a Photo-Bioelectrochemical System under Photoheterotrophy

Bioelectrochemical systems are a promising technology capable of reducing CO2 emissions, a renewable carbon source, using electroactive microorganisms for this purpose. Purple Phototrophic Bacteria (PPB) use their versatile metabolism to uptake external electrons from an electrode to fix CO2. In this work, the effect of the voltage (from −0.2 to −0.8 V vs. Ag/AgCl) on the metabolic CO2 fixation of a mixed culture of PPB under photoheterotrophic conditions during the oxidation of a biodegradable carbon source is demonstrated. The minimum voltage to fix CO2 was between −0.2 and −0.4 V. The Calvin–Benson–Bassham (CBB) cycle is the main electron sink at these voltages. However, lower voltages caused the decrease in the current intensity, reaching a minimum at −0.8 V (−4.75 mA). There was also a significant relationship between the soluble carbon uptake in terms of chemical oxygen demand and the electron consumption for the experiments performed at −0.6 and −0.8 V. These results indicate that the CBB cycle is not the only electron sink and some photoheterotrophic metabolic pathways are also being affected under electrochemical conditions. This behavior has not been tested before in photoheterotrophic conditions and paves the way for the future development of photobioelectrochemical systems under heterotrophic conditions.

The periplasmic component of the DctPQM TRAP-transporter is part of the DctS/DctR sensory pathway in Rhodobacter sphaeroides.

Rhodobacter sphaeroides can use C4-dicarboxylic acids to grow heterotrophically or photoheterotropically, and it was previously demonstrated in Rhodobacter capsulatus that the DctPQM transporter system is essential to support growth using these organic acids under heterotrophic but not under photoheterotrophic conditions. In this work we show that in R. sphaeroides this transporter system is essential for photoheterotrophic and heterotrophic growth, when C4-dicarboxylic acids are used as a carbon source. We also found that over-expression of dctPQM is detrimental for photoheterotrophic growth in the presence of succinic acid in the culture medium. In agreement with this, we observed a reduction of the dctPQM promoter activity in cells growing under these conditions, indicating that the amount of DctPQM needs to be reduced under photoheterotrophic growth. It has been reported that the two-component system DctS and DctR activates the expression of dctPQM. Our results demonstrate that in the absence of DctR, dctPQM is still expressed albeit at a low level. In this work, we have found that the periplasmic component of the transporter system, DctP, has a role in both transport and in signalling the DctS/DctR two-component system.

Insights into discrepancy in power generation among glucose and malate grown Rubrivivax benzoatilyticus JA2 microbial fuel cells

In this study, Rubrivivax benzoatilyticus JA2T (=ATCC BAA-35T = JCM 13220T = MTCC 7087T), an anoxygenic photosynthetic bacterium, was subjected to altered conditions and observed for changes in power outcome in the two chambered microbial fuel cells (MFCs), the basis of which was established using metabolomic studies. This is an extension to our previous studies, which showed that, under photo heterotrophic conditions, glucose in the form of a solitary carbon resource in minimal media, caused the strain JA2 to exhibit altered growth rates, progressive loss of pigmentation and reduced cell size (3–4 μm), compared to malate grown cells (6–7 μm). When R. benzoatilyticus JA2 cells were grown in malate bio-anodes, they presented higher potentials (289.22 ± 4.6 mV or 436.22 OCV per mg dry weight) compared to glucose bioanodes (163 ± 5.5 mv or 188.98 OCV per mg dry weight). Insights from the metabolomic footprints and fingerprints have revealed differential regulation of key components in the central metabolic pathway such as fumarate, citrate and succinate, which are significantly increased in malate grown bio anodes. Strain JA2 cells when grown with malate as substrate are densely grown on the electrodes and exhibited reduced size, when observed under SEM, which contrasts with control cells grown on malate broth. The artificial selection pressure of the MFC and the different metabolic pathways followed by these bacteria are the reasons for such discrepancy in the power production by the strain JA2. These adaptations may indicate survival advantage during the electron transfer and growth in bio anodes. The study throws light on what types of effluents would be more suitable as substrates for R. benzoatilyticus JA2 microbial fuel cells.

Electrochemistry of R. palustris Azul during phototrophic growth

Facultative phototrophs are considered within the most versatile bacteria with respect to energy metabolism, since their genomes code for the machinery for living under any of the four modes of metabolism. In photoautotrophic conditions, Rhodopseudomonas palustris' growth requires electrons provided by inorganic donors to produce enough reducing equivalents to complement energy generation by light. When growing photo heterotrophically, these electrons are provided by organic donors, and the excess of reducing power is balanced through carbon dioxide (CO2) fixation or through the reduction of other electron acceptors. Here we report the electrochemical characterization of an autochthonous R. palustris strain identified as AZUL, which is capable of not only accepting electrons from an electrode (cathodic conditions) during photoautotrophic growth but also using the electrode as an electron acceptor (anodic conditions) under a photoheterotrophic conditions. In the first condition, cells were able to grow and accept electrons from an electrode polarized at negative potentials (i.e. replacing inorganic electron donors). We propose that R. palustris AZUL presents mechanisms for both direct and indirect electron uptake, as evidenced by growth curves and cyclic voltammetry experiments. When grown in anodic conditions, cells formed biofilms with a particular architecture on poised graphite electrodes. We detected a specific voltammetric profile characterized by two reduction processes at 0.2 and 0.4 V and an oxidation process at 0.57 V (vs Ag/AgCl). These redox signals progressively changed with the time of growth towards an oxidation catalysis signal with a gate potential of 0.5 V. Biofilm charge accumulation experiments indicated that this bacteria accumulates electrons that can be subsequently discharged upon polarization. The results presented herein suggest that R. palustris AZUL is capable of exchanging electrons with an electrode in a bidirectional fashion. In addition, our experiments indicate that this strain has a mechanism of redox balance that includes charge storage in redox molecules that can transfer charge to the extracellular space when a high potential acceptor is available.

Exploring the inhibition boundaries of mixed cultures of purple phototrophic bacteria for wastewater treatment in anaerobic conditions

The development of novel wastewater platforms should include the analysis of the most critical functional factors including the effects of toxic or inhibitory substances. Due to the novelty of purple phototrophic bacteria (PPB)-based wastewater treatment systems, this analysis has not been done yet in mixed cultures. In this work, various relevant chemical compounds, including aromatic (phenol, 2,4,6-trichlorophenol or 246TCP, 4-nitrophenol or 4CP, sulfathiazole) and aliphatic organics (methanol, trichlorethylene or TCE, oleic acid, ethanol, propionic acid), inorganic salts (ammonium, ClO3−, Na+), and metals (Fe3+, Fe2+, Cu2+, Zn2+, Ni2+, Al3+), as well as pH, are analyzed for their effect on mixed PPB cultures in anaerobic photoheterotrophic conditions using acetate as the model organic substrate. The most toxic substances detected were 246TCP, 4NP, Cu2+, Fe2+ and Ni2+, (Ki for activity: 23 ± 2, 97 ± 12, 3.1 ± 0.4, 13 ± 3, 13 ± 1 mg/L, and Ki (or toxicity threshold) for growth: 17 ± 2, (119), 3.5 ± 0.4, (4.8), (22.9) mg/L, respectively). Some substances inhibited the activity more than the growth (sulfathiazole, Ni2+ and Fe3+), or the growth more than the activity (TCE, 4NP and Fe2+). In addition, some organic substrates, such as phenol, ethanol and propionate, specifically inhibited the acetate uptake, being noncompetitive in the case of phenol and ethanol, and most likely competitive in the case of propionate. These findings are relevant for the wastewater treatment and resource recovery applications of the PPB technology, as well as for the upgrading of current models (Photo-Anaerobic Model). In addition, the data will open possibilities to promote the production of specific compounds (as PHA or single-cell proteins) by selectively inhibiting some parts of the PPB metabolism.

New perspectives on butyrate assimilation in Rhodospirillum rubrum S1H under photoheterotrophic conditions

BackgroundThe great metabolic versatility of the purple non-sulfur bacteria is of particular interest in green technology. Rhodospirillum rubrum S1H is an α-proteobacterium that is capable of photoheterotrophic assimilation of volatile fatty acids (VFAs). Butyrate is one of the most abundant VFAs produced during fermentative biodegradation of crude organic wastes in various applications. While there is a growing understanding of the photoassimilation of acetate, another abundantly produced VFA, the mechanisms involved in the photoheterotrophic metabolism of butyrate remain poorly studied.ResultsIn this work, we used proteomic and functional genomic analyses to determine potential metabolic pathways involved in the photoassimilation of butyrate. We propose that a fraction of butyrate is converted to acetyl-CoA, a reaction shared with polyhydroxybutyrate metabolism, while the other fraction supplies the ethylmalonyl-CoA (EMC) pathway used as an anaplerotic pathway to replenish the TCA cycle. Surprisingly, we also highlighted a potential assimilation pathway, through isoleucine synthesis and degradation, allowing the conversion of acetyl-CoA to propionyl-CoA. We tentatively named this pathway the methylbutanoyl-CoA pathway (MBC). An increase in isoleucine abundance was observed during the early growth phase under butyrate condition. Nevertheless, while the EMC and MBC pathways appeared to be concomitantly used, a genome-wide mutant fitness assay highlighted the EMC pathway as the only pathway strictly required for the assimilation of butyrate.ConclusionPhotoheterotrophic growth of Rs. rubrum with butyrate as sole carbon source requires a functional EMC pathway. In addition, a new assimilation pathway involving isoleucine synthesis and degradation, named the methylbutanoyl-CoA (MBC) pathway, could also be involved in the assimilation of this volatile fatty acid by Rs. rubrum.

Carbonic Anhydrase in Anoxygenic Phototrophic Bacteria

The genes encoding carbonic anhydrase (CA) were found in all anoxygenic purple bacteria. The genes of α- and β-CA were found in purple nonsulfur bacteria of the class Alphaproteobacteria:Rhodospirillum rubrum, Rhodospirillum fulvum, Rhodoblastus acidophilus, and Rhodopseudomonas palustris. The alphaproteobacteria Rhodomicrobium vannielii, Blastochlorisviridis, Rhodobacter sphaeroides, Rhodobacter capsulatus, Rhodobacter veldkampii, Rhodovulumeuryhalinum, and Rhodovulum sulfidophilum possessed only the β-CA genes. Both nonsulfur purple bacteria of the class Betaproteobacteria (Rubrivivax gelatinosus) and purple sulfur bacteria (class Gammaproteobacteria) contained the α- and β-CA. No CA genes were found in green sulfur bacteria Chlorobaculum limnaeum and Chlorobaculum parvum, as well as in filamentous green nonsulfur bacteria Chloroflexus aurantiacus. However, the β-CA gene was revealed in Oscillochloris trichoides, which belonged to the latter taxonomic group. No γ-CA genes were detected in the genomes of the phototrophic bacteria studied in the present work. Although CA genes were present in all purple bacteria, the α- and β-CA activity was observed only in four species of purple nonsulfur Alphaproteobacteria: Rhodospirillum rubrum, Rhodopseudomonas palustris, Rhodoblastusacidophilus, and Rhodospirillum fulvum. These bacteria are able to synthesize CA under both photoautotrophic and photoheterotrophic conditions in the media with acetate, malate, or fructose. These bacteria (except for Rhodospirillum fulvum, which is unable to grow under aerobic conditions), also exhibit CA activity when grown under aerobic conditions in the dark.