Phototrophic Conditions Research Articles

Effect of aeration rate and light cycle on the growth characteristics of Chlorella sorokiniana in a photobioreactor

Microalgae is considered to be an important resource to address the global demand for sustainable energy and green technologies. Numerous applications of microalgae have already been identified in the past. They can be cultivated to produce food, animal feeds, nutraceuticals, and even biofuels. They can also be used for environmental applications such as carbon capture and storage, and wastewater treatment. There are different environmental factors that can affect the growth of microalgae such as light, nutrients, temperature, and aeration. Among different microalgae strains, Chlorella sorokiniana had been identified to be one of the most effective and commonly used strains across these different applications. In this study, the effect of aeration rate and light cycle on the growth characteristics of C. sorokiniana in a photobioreactor was investigated. Cultivation experiments were carried out at room temperature (24 – 26 °C) under phototrophic conditions in which the light intensity was set to 150 µmol/m2-s and the carbon source used was air enriched with carbon dioxide at 2.5% concentration. The aeration rates evaluated were 0.0125, 0.0250, 0.0500, 0.1000, and 0.2000 vvm while the light cycles evaluated were 24:0 (continuous illumination), and 12:12 (diurnal illumination). The results showed that in the 24:0 light cycle, increasing the aeration rate up to 0.1000 vvm led to an increase in the cumulative biomass production, specific growth rate, overall biomass productivity, and nitrate consumption of C. sorokiniana cultures. At 0.2000 vvm, no increase in any of these parameters were observed. Meanwhile, the aeration rate did not have any effect on the carbohydrate content of C. sorokiniana. On the other hand, cultivation under a 12:12 cycle resulted in a decrease in all of the parameters across all aeration rates evaluated. However, no significant interaction between the light cycle and the aeration rate was found in this study. Finally, among the conditions evaluated, the 24:0 light cycle and 0.1000 vvm aeration rate led to the best growth characteristics of C. sorokiniana. The results from this study indicate that aeration rate and light cycle have significant effects on cultivating microalgae such as C. sorokiniana. The results also showed that appropriate levels of these factors should be considered depending on the application of the microalgae cultivation. For future work, the growth of C. sorokiniana can be modeled to dynamically optimize these factors to improve its growth and reduce its cultivation costs.

Resolving the roles of the Rhodobacter sphaeroides HemA and HemT 5-aminolevulinic acid synthases.

Strains of the phototrophic alpha-proteobacterium Rhodobacter sphaeroides vary in the number of enzymes catalyzing the formation of 5-aminolevulinic acid (ALA synthases) that are encoded in their genomes. All have hemA, but not all have hemT. This study compared transcription of these genes, and also properties of their products among three wild-type strains; 2.4.3 has hemA alone, 2.4.1 and 2.4.9 have both hemA and hemT. Using lacZ reporter plasmids all hemA genes were found to be upregulated under anaerobic conditions, but induction amplitudes differ. hemT is transcriptionally silent in 2.4.1 but actively transcribed in 2.4.9, and strongly upregulated under anaerobic-dark growth conditions when cells are respiring dimethyl sulfoxide, vs. aerobic-dark or phototrophic (anaerobic-light) conditions. Two extracytoplasmic function (ECF)-type sigma factors present in 2.4.9, but absent from 2.4.1 are directly involved in hemT transcription. Kinetic properties of the ALA synthases of all three strains were similar, but HemT enzymes are far less sensitive to feedback inhibition by hemin than HemA enzymes, and HemT is less active under oxidizing conditions. A model is presented that compares and contrast events in strains 2.4.1 and 2.4.9.

Engineering linear, branched-chain triterpene metabolism in monocots.

SummaryTriterpenes are thirty‐carbon compounds derived from the universal five‐carbon prenyl precursors isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). Normally, triterpenes are synthesized via the mevalonate (MVA) pathway operating in the cytoplasm of eukaryotes where DMAPP is condensed with two IPPs to yield farnesyl diphosphate (FPP), catalyzed by FPP synthase (FPS). Squalene synthase (SQS) condenses two molecules of FPP to generate the symmetrical product squalene, the first committed precursor to sterols and most other triterpenes. In the green algae Botryococcus braunii, two FPP molecules can also be condensed in an asymmetric manner yielding the more highly branched triterpene, botryococcene. Botryococcene is an attractive molecule because of its potential as a biofuel and petrochemical feedstock. Because B. braunii, the only native host for botryococcene biosynthesis, is difficult to grow, there have been efforts to move botryococcene biosynthesis into organisms more amenable to large‐scale production. Here, we report the genetic engineering of the model monocot, Brachypodium distachyon, for botryococcene biosynthesis and accumulation. A subcellular targeting strategy was used, directing the enzymes (botryococcene synthase [BS] and FPS) to either the cytosol or the plastid. High titres of botryococcene (>1 mg/g FW in T0 mature plants) were obtained using the cytosolic‐targeting strategy. Plastid‐targeted BS + FPS lines accumulated botryococcene (albeit in lesser amounts than the cytosolic BS + FPS lines), but they showed a detrimental phenotype dependent on plastid‐targeted FPS, and could not proliferate and survive to set seed under phototrophic conditions. These results highlight intriguing differences in isoprenoid metabolism between dicots and monocots.

In situ biological CO2 fixation and wastewater nutrient removal with Neochloris oleoabundans in batch photobioreactor.

Microalgae cultivation in wastewater media in phototrophic condition is a promising approach for integrated CO2 biofixation and wastewater treatment. For this, Neochloris oleoabundans was used to investigate the tertiary treatment of wastewater along with CO2 biofixation. In this investigation, biomass productivity, CO2 biofixation rate and percentage of total nitrogen (TN) and total phosphorus (TP) removal from synthetic wastewater are considered under three different operating conditions: temperature, CO2 feed concentration and nitrogen to phosphorus (NP) ratio in the media. Cultivation of N. oleoabundans was found to be highly temperature sensitive. With the increase of cultivation temperature from 25 to 45°C, declining trends of biomass concentration, productivity and percentage of TN and TP removal were observed. Cultivation temperature of 25°C was found to be most favorable in terms of biomass productivity, CO2 biofixation rate, percentage of TN and TP removal of 92 (mg L-1day-1), 145 (mg L-1day-1), 100% and 32%, respectively. Arrhenius-type kinetic model was used and the model showed good agreement with the experimental findings. Activation energy for the active stage and decay stage was found to be [Formula: see text] = 88.8 kJmol-1 and [Formula: see text] = 8.4 kJmol-1, respectively. With the increase of CO2 feed concentration, biomass productivity increased and the maximum biomass concentration and productivity was achieved at 6%. After that with the increase in CO2, a declining trend was observed. With the increase of NP ratio from 1:1 to 2:1, both the biomass productivity and CO2 biofixation were increased, but later were subsequently decreased with increase of NP ratio from 4:1 to 8:1. It is interesting that TP removal was increased with NP ratio and 100 percent of TP removal was achieved at 4:1 and 8:1 conditions.

Euglena gracilis growth and cell composition under different temperature, light and trophic conditions.

BackgroundEuglena gracilis, a photosynthetic protist, produces protein, unsaturated fatty acids, wax esters, and a unique β-1,3-glucan called paramylon, along with other valuable compounds. The cell composition of E. gracilis was investigated in this study to understand how light and organic carbon (photo-, mixo- and heterotrophic conditions) affected growth and cell composition (especially lipids). Comparisons were primarily carried out in cultures grown at 23 °C, but the effect of growth at higher temperatures (27 or 30 °C) was also considered.Cell growthSpecific growth rates were slightly lower when E. gracilis was grown on glucose in either heterotrophic or mixotrophic conditions than when grown photoautotrophically, although the duration of exponential growth was longer. Temperature determined the rate of exponential growth in all cultures, but not the linear growth rate during light-limited growth in phototrophic conditions. Temperature had less effect on cell composition.Cell compositionAlthough E. gracilis was not expected to store large amounts of paramylon when grown phototrophically, we observed that phototrophic cells could contain up to 50% paramylon. These cells contained up to 33% protein and less than 20% lipophilic compounds, as expected. The biomass contained about 8% fatty acids (measured as fatty acid methyl esters), most of which were unsaturated. The fatty acid content of cells grown in mixotrophic conditions was similar to that observed in phototrophic cells, but was lower in cells grown heterotrophically. Heterotrophic cells contained less unsaturated fatty acids than phototrophic or mixotrophic cells. α-Linolenic acid was present at 5 to 18 mg g-1 dry biomass in cells grown in the presence of light, but at < 0.5 mg g-1 biomass in cells grown in the dark. Eicosapentaenoic and docosahexaenoic acids were detected at 1 to 5 mg g-1 biomass. Light was also important for the production of vitamin E and phytol.

Enhancement of aerobic granulation and nutrient removal by an algal–bacterial consortium in a lab-scale photobioreactor

This study aimed to evaluate the properties, nutrients removal and microbial community dynamics of algal–bacterial granules in a lab-scale photobioreactor treating synthetic domestic wastewater. Two identical bioreactors were operated under non-phototrophic (Rc, the control) and phototrophic (Rp) conditions (12 h/12 h dark–light cycles). The results showed that aggregation of algal and bacterial biomass occurred rapidly in Rp, possibly as a granule nucleus, which favored aerobic granulation. Mature algal–bacterial granules had a compact structure and large size as well as exhibited excellent settling properties. In addition, increased biomass production was facilitated in Rp compared to Rc. Notably, increased production of extracellular polymeric substances (EPS) was induced in Rp, especially aromatic and tryptophan-like substances, implying that proteins significantly contributed to the formation and stability of the algal–bacterial granules. Further investigations revealed that algal–bacterial granules provided resistance to temperature fluctuations and enhanced the removal efficiencies of organics and nutrients (nitrogen and phosphorus), probably attributable to the enrichment of the Betaproteobacteria, Gammaproteobacteria and Flavobacterium bacterial classes, and Chlorophyta, Trebouxiophyceae and Bacillariophyceae algae classes, whereas Cyanobacteria only occupied a small proportion. These findings demonstrated that the algal–bacterial consortium was conducive to accelerating aerobic granulation, enhancing system stability and improving nutrient removal efficiency.

Phototrophic cultivation of NaCl-tolerant mutant of Spirulina platensis for enhanced C-phycocyanin production under optimized culture conditions and its dynamic modeling.

Commercial cultivation of Spirulina sp. is highly popular due to the presence of high amount of C-phycocyanin (C-PC) and other valuable chemicals like carotenoids and γ-linolenic acid. In this study, the pH and the concentrations of nitrogen and carbon source were manipulated to achieve improved cell growth and C-PC production in NaCl-tolerant mutant of Spirulina platensis. In this study,highest C-PC (147mg·L-1 ) and biomass (2.83g·L-1 ) production was achieved when a NaCl-tolerant mutant of S.platensis was cultivated in a nitrate and bicarbonate sufficient medium (40 and 60mM, respectively) at pH 9.0 under phototrophic conditions. Kinetic study of wildtype S.platensis and its NaCl-tolerant mutant was also done to determine optimum nitrate concentrations for maximum growth and C-PC production. Kinetic parameter of inhibition (Haldane model) was fitted to the relationship between specific growth rate and substrate concentration obtained from the growth curves. Results showed that the maximum specific growth rate (μmax ) for NaCl-tolerant mutant increased by 17.94% as compared to its wildtype counterpart, with a slight increase in half-saturation constant (Ks ), indicating that this strain could grow well at high concentration of NaNO3 . C-PC production rate (Cmax ) in mutant cells increased by 12.2% at almost half the value of Ks as compared to its wildtype counterpart. Moreover, the inhibition constant (Ki ) value was 207.85% higher in NaCl-tolerant mutant as compared to its wildtype strain, suggesting its ability to produce C-PC even at high concentrations of NaNO3 .

Investigating mixotrophic metabolism in the model diatom Phaeodactylum tricornutum.

Diatoms are prominent marine microalgae, interesting not only from an ecological point of view, but also for their possible use in biotechnology applications. They can be cultivated in phototrophic conditions, using sunlight as the sole energy source. Some diatoms, however, can also grow in a mixotrophic mode, wherein both light and external reduced carbon contribute to biomass accumulation. In this study, we investigated the consequences of mixotrophy on the growth and metabolism of the pennate diatom Phaeodactylum tricornutum, using glycerol as the source of reduced carbon. Transcriptomics, metabolomics, metabolic modelling and physiological data combine to indicate that glycerol affects the central-carbon, carbon-storage and lipid metabolism of the diatom. In particular, provision of glycerol mimics typical responses of nitrogen limitation on lipid metabolism at the level of triacylglycerol accumulation and fatty acid composition. The presence of glycerol, despite provoking features reminiscent of nutrient limitation, neither diminishes photosynthetic activity nor cell growth, revealing essential aspects of the metabolic flexibility of these microalgae and suggesting possible biotechnological applications of mixotrophy.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Long alkyl-chain imidazolium ionic liquids: Antibiofilm activity against phototrophic biofilms

Biofilm formation is problematic and hence undesirable in medical and industrial settings. In addition to bacteria, phototrophic organisms are an integral component of biofilms that develop on surfaces immersed in natural waters. 1-Alkyl-3-methyl imidazolium ionic liquids (IL) with varying alkyl chain length were evaluated for their influence on the formation of monospecies (Navicula sp.) and multispecies biofilms under phototrophic conditions. An IL with a long alkyl side chain, 1-hexadecyl-3-methylimidaazolium chloride ([C16(MIM)][Cl]) retarded growth, adhesion and biofilm formation of Navicula sp. at concentrations as low as 5μM. Interestingly, [C16(MIM)][Cl] was very effective in preventing multispecies phototrophic biofilms on fibre reinforced plastic surfaces immersed in natural waters (fresh and seawater). SYTOX® Green staining and chlorophyll leakage assay confirmed that the biocidal activity of the IL was exerted through cell membrane disruption. The data show that [C16(MIM)][Cl] is a potent inhibitor of phototrophic biofilms at micromolar concentrations and a promising agent for biofilm control in re-circulating cooling water systems. This is the first report that ionic liquids inhibit biofilm formation by phototrophic organisms which are important members of biofilms in streams and cooling towers.

Synthesis of Biodegradable Polymer Polyhydroxyalkanoate (PHA) in CyanobacteriaSynechococcus elongatesUnder Mixotrophic Nitrogen- and Phosphate-Mediated Stress Conditions

Abstract Biopolymers like polyhydroxyalkanoates (PHA) are a class of metabolites with promising applications in the fields of environmental, agricultural, and biomedical sciences. Commercial production of polyhydroxyalkanoates is a high-cost process generally carried out using heterotrophic species of bacteria. Using algae for intracellular PHA accumulation is an alternative method for bioplastic production. In the present study, cyanobacterium Synechococcus elongates was identified as a potential PHA-producing organism. A range of experimental conditions that include different growth parameters of nitrogen and phosphate starvation with the addition of external carbon sources under photoautotrophic and chemoheterotrophic growth conditions were examined. Our observations revealed that the algal cells cultured in nitrogen-starved medium under phototrophic conditions yield the highest quantity of PHA (17.15%) when 1% sucrose is used as a carbon source. Similarly, under chemotrophic conditions, the highest PH...

Screening of fatty acid composition in Nitzschia sp.

Abstract Introduction The main target of this study was to compare the effects of nitrogen limitation and light intensities on cell growth, lipid content and fatty acid profile of Nitzschia sp. Methods F/2 medium and N-free F/2 medium were both tested at two different light intensities of 11 and 56 μEm−2s−1 in the internal loop air-lift photobioreactor for Nitzschiasp. Results The presence of nitrogen led to more cells grown efficiently. Furthermore, the increase in chlorophyll content went parallel to the increase in dry weight. The most abundant saturated and monounsaturated fatty acids were pentadecanoic acid (C15:0) and palmitoleic acid (C16:1) which constituted 17%–42% and 15%–48% of total fatty acids for all growth conditions, respectively. It was recorded that palmitoleic acid was present at higher concentrations than palmitic acid. Discussion and conclusion The noteworthy finding was that the lipid content increased with increasing the growth rate of Nitzschia sp. under phototrophic conditions.

Beech wood Fagus sylvatica dilute-acid hydrolysate as a feedstock to support Chlorella sorokiniana biomass, fatty acid and pigment production

This work evaluates the possibility of using beech wood (Fagus sylvatica) dilute-acid (H2SO4) hydrolysate as a feedstock for Chlorella sorokiniana growth, fatty acid and pigment production. Neutralized wood acid hydrolysate, containing organic and mineral compounds, was tested on Chlorella growth at different concentrations and compared to growth under phototrophic conditions. Chlorella growth was improved at lower loadings and inhibited at higher loadings. Based on these results, a 12% neutralized wood acid hydrolysate (Hyd12%) loading was selected to investigate its impact on Chlorella growth, fatty acid and pigment production. Hyd12% improved microalgal biomass, fatty acid and pigment productivities both in light and in dark, when compared to photoautotrophic control. Light intensity had substantial influence on fatty acid and pigment composition in Chlorella culture during Hyd12%-based growth. Moreover, heterotrophic Chlorella cultivation with Hyd12% also showed that wood hydrolysate can constitute an attractive feedstock for microalgae cultivation in case of lack of light.

Development of a thin-film solar photobioreactor with high biomass volumetric productivity (AlgoFilm©) based on process intensification principles

This work presents the rational development up to its final characterization and validation of an intensified solar photobioreactor (PBR) for microalgal production, AlgoFilm©. Our aim was to achieve very high volumetric performance for phototrophic conditions, in the range of that found in fermentation processes.The overall design procedure was underpinned by robust engineering rules derived from knowledge models developed for PBR in-depth modeling. This approach was used to pinpoint the main engineering parameters governing PBR kinetic performance. It introduces generic principles of PBR performance enhancement for the setting-up of culture systems combining high volumetric and areal productivities. These principles were then applied to the design of a solar culture system, integrating the attendant constraints. The result was the AlgoFilm© PBR, based on the falling-film principle, which enables very thin culture depth (around 1.5–2mm) and provides a high specific illuminated surface area (around 500m2·m−3, corresponding to 2.1L/m2 illuminated surface).A complete experimental characterization was then conducted to (i) determine operating conditions for the setting of optimal parameters governing AlgoFilm© PBR performance, such as depth of the falling film, and (ii) ensure that no limitation other than light occurred, a mandatory condition to control the system and ensure high biomass productivities.AlgoFilm© PBR performance was characterized by Chlorella vulgaris culture. Batch, continuous and semi-continuous cultures were run, and typical irradiation conditions of a year's operation in Nantes (France) were applied using a LED panel to simulate constant, and then fully-controlled day/night cycles. Our results demonstrate the high performance of the AlgoFilm© PBR, with volumetric productivities very close to those expected from the prior theoretical design. The best measured productivity was 5.7kg·m−3·day−1 (7.07kg·m−3·day−1 in constant light). This value was significantly higher than those reported in the literature, and similar to those generally found for microalgal production in heterotrophic processes.

Heterotrophic cultivation of microalgae: production of metabolites of commercial interest

Several microalgal species are capable of growing heterotrophically, exhibiting considerable metabolic versatility and flexibility. As demonstrated in this review, heterotrophic conditions can enhance the biomass concentration by as much as 25-fold compared with phototrophic conditions. Currently, these types of cultivation are economically feasible only for high-value products, including polyunsaturated fatty acids (PUFAs), pigments, antioxidants, polysaccharides, food and aquaculture feed from carbon sources, such as glucose, acetate or glycerol. To make heterotrophic cultivation economically viable for high-volume, low-value commodities, such as biofuels, the use of unconventional carbon sources, such as food and agricultural wastes and wastewater, is recommended. Since microalgae are capable of modifying their metabolism according to varying culture conditions, it is possible to modify, control and therefore maximize the production of target compounds. This manuscript not only offers a review of the most relevant and recent findings in the use of heterotrophic microalgal cultivation for enhanced metabolite production but also provides recommendations for future research on this promising subject. © 2016 Society of Chemical Industry

Morphological and ultrastructural characterization of the acidophilic and lipid-producer strain Chlamydomonas acidophila LAFIC-004 (Chlorophyta) under different culture conditions.

Chlamydomonas acidophila LAFIC-004 is an acidophilic strain of green microalgae isolated from coal mining drainage. In the present work, this strain was cultivated in acidic medium (pH3.6) under phototrophic, mixotrophic, and heterotrophic regimes to determine the best condition for growth and lipid production, simultaneously assessing possible morphological and ultrastructural alterations in the cells. For heterotrophic and mixotrophic treatments, two organic carbon sources were tested: 1% glucose and 1% sodium acetate. Lipid content and fatty acid profiles were only determined in phototrophic condition. The higher growth rates were achieved in phototrophic conditions, varying from 0.18 to 0.82day-1. Glucose did not result in significant growth increase in either mixotrophic or heterotrophic conditions, and acetate proved to be toxic to the strain in both conditions. Oil content under phototrophic condition was 15.9% at exponential growth phase and increased to 54.63% at stationary phase. Based on cell morphology (flow cytometry and light microscopy) and ultrastructure (transmission electron microscopy), similar characteristics were observed between phototrophic and mixotrophic conditions with glucose evidencing many lipid bodies, starch granules, and intense fluorescence. Under the tested conditions, mixotrophic and heterotrophic modes did not result in increased neutral lipid fluorescence. It can be concluded that the strain is a promising lipid producer when grown until stationary phase in acidic medium and under a phototrophic regime, presenting a fatty acid profile suitable for biodiesel production. The ability to grow this strain in acidic mining residues suggests a potential for bioremediation with production of useful biomass.

Uptake of inorganic and organic nutrient species during cultivation of a Chlorella isolate in anaerobically digested dairy waste.

A natural assemblage of microalgae from a facultative lagoon system treating municipal wastewater was enriched for growth in the effluents of an anaerobic digester processing dairy waste. A green microalga with close resemblance to Chlorella sp. was found to be dominant after multiple cycles of sub-culturing. Subsequently, the strain (designated as LLAI) was isolated and cultivated in 20× diluted digester effluents under various incident light intensities (255-1,100 µmoles m-2 s-1 ) to systematically assess growth and nutrient utilization. Our results showed that LLAI production increased with increasing incident light and a maximum productivity of 0.34 g L-1 d-1 was attained when the incident irradiance was 1,100 µmoles m-2 s-1 . Lack of growth in the absence of light indicated that the cultures did not grow heterotrophically on the organic compounds present in the medium. However, the cultures were able to uptake organic N and P under phototrophic conditions and our calculations suggest that the carbon associated with these organic nutrients contributed significantly to the production of biomass. Overall, under high light conditions, LLAI cultures utilized half of the soluble organic nitrogen and >90% of the ammonium, orthophosphate, and dissolved organic phosphorus present in the diluted waste. Strain LLAI was also found to accumulate triacylglycerides (TAG) even before the onset of nutrient limitation and a lipid productivity of 37 mg-TAG L-1 d-1 was measured in cultures incubated at an incident irradiance of 1,100 µmoles m-2 s-1 . The results of this study suggest that microalgae isolates from natural environments are well-suited for nutrient remediation and biomass production from wastewater containing diverse inorganic and organic nutrient species. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1336-1342, 2016.

Pilot-scale cultivation of wall-deficient transgenic Chlamydomonas reinhardtii strains expressing recombinant proteins in the chloroplast.

Microalgae have emerged as potentially powerful platforms for the production of recombinant proteins and high-value products. Chlamydomonas reinhardtii is a potentially important host species due to the range of genetic tools that have been developed for this unicellular green alga. Transformation of the chloroplast genome offers important advantages over nuclear transformation, and a wide range of recombinant proteins have now been expressed in the chloroplasts of C. reinhardtii strains. This is often done in cell wall-deficient mutants that are easier to transform. However, only a single study has reported growth data for C. reinhardtii grown at pilot scale, and the growth of cell wall-deficient strains has not been reported at all. Here, we report the first pilot-scale growth study for transgenic, cell wall-deficient C. reinhardtii strains. Strains expressing a cytochrome P450 (CYP79A1) or bifunctional diterpene synthase (cis-abienol synthase, TPS4) were grown for 7days under mixotrophic conditions in a Tris-acetate-phosphate medium. The strains reached dry cell weights of 0.3g/L within 3-4days with stable expression levels of the recombinant proteins during the whole upscaling process. The strains proved to be generally robust, despite the cell wall-deficient phenotype, but grew poorly under phototrophic conditions. The data indicate that cell wall-deficient strains may be highly amenable for transformation and suitable for commercial-scale operations under mixotrophic growth regimes.

Heterotrophic Production of Omega-3 Long-Chain Polyunsaturated Fatty Acids by Trophically Converted Marine Diatom Phaeodactylum tricornutum.

We have created via metabolic engineering a heterotrophic strain of Phaeodactylum tricornutum that accumulates enhanced levels of the high value omega-3 long chain polyunsaturated fatty acid (LC-PUFAs) docosahexaenoic acid (DHA). This was achieved by generation of transgenic strains in which the Δ5-elongase from Ostreococcus tauri was co-expressed with a glucose transporter from the moss Physcomitrella patens. This double transformant has the capacity to grow in the dark in liquid medium supplemented with glucose and accumulate substantial levels of omega-3 LC-PUFAs. The effects of glucose concentrations on growth and LC-PUFA production of wild type and transformed strains cultivated in the light and dark were studied. The highest omega-3 LC-PUFAs accumulation was observed in cultures grown under mixotrophic conditions in the presence of 1% glucose (up to 32.2% of total fatty acids, TFA). Both DHA and EPA are detected at high levels in the neutral lipids of transgenic cells grown under phototrophic conditions, averaging 36.5% and 23.6% of TFA, respectively. This study demonstrates the potential for P. tricornutum to be developed as a viable commercial strain for both EPA and DHA production under mixo- and heterotrophic conditions.

A genome-scale metabolic network reconstruction of tomato (Solanum lycopersicum L.) and its application to photorespiratory metabolism.

Tomato (Solanum lycopersicum L.) has been studied extensively due to its high economic value in the market, and high content in health-promoting antioxidant compounds. Tomato is also considered as an excellent model organism for studying the development and metabolism of fleshy fruits. However, the growth, yield and fruit quality of tomatoes can be affected by drought stress, a common abiotic stress for tomato. To investigate the potential metabolic response of tomato plants to drought, we reconstructed iHY3410, a genome-scale metabolic model of tomato leaf, and used this metabolic network to simulate tomato leaf metabolism. The resulting model includes 3410 genes and 2143 biochemical and transport reactions distributed across five intracellular organelles including cytosol, plastid, mitochondrion, peroxisome and vacuole. The model successfully described the known metabolic behaviour of tomato leaf under heterotrophic and phototrophic conditions. The in silico investigation of the metabolic characteristics for photorespiration and other relevant metabolic processes under drought stress suggested that: (i) the flux distributions through the mevalonate (MVA) pathway under drought were distinct from that under normal conditions; and (ii) the changes in fluxes through core metabolic pathways with varying flux ratio of RubisCO carboxylase to oxygenase may contribute to the adaptive stress response of plants. In addition, we improved on previous studies of reaction essentiality analysis for leaf metabolism by including potential alternative routes for compensating reaction knockouts. Altogether, the genome-scale model provides a sound framework for investigating tomato metabolism and gives valuable insights into the functional consequences of abiotic stresses.

Phylogenetically Diverse Aerobic Anoxygenic Phototrophic Bacteria Isolated from Epilithic Biofilms in Tama River, Japan.

The diversity of aerobic anoxygenic phototrophic (AAP) bacteria in freshwater environments, particularly in rivers, has not been examined in as much detail as in ocean environments. In the present study, we investigated the phylogenetic and physiological diversities of AAP bacteria in biofilms that developed on submerged stones in a freshwater river using culture methods. The biofilms collected were homogenized and inoculated on solid media and incubated aerobically in the dark. Sixty-eight red-, pink-, yellow-, orange-, or brown-colored colonies were isolated, and, of these, 28 isolates contained the photosynthetic pigment, bacteriochlorophyll (BChl) a. Phylogenetic analyses based on 16S rRNA gene sequences showed that the isolates were classified into 14 groups in 8 operational taxonomic units (OTUs) and distributed in the orders Rhodospirillales, Rhodobacterales, and Sphingomonadales of Alphaproteobacteria and in Betaproteobacteria. Physiological analyses confirmed that none of the representative isolates from any of the groups grew under anaerobic phototrophic conditions. Seven isolates in 4 OTUs showed a 16S rRNA gene sequence identity of 98.0% or less with any established species, suggesting the presence of previously undescribed species of AAP bacteria. Six isolates in 2 other OTUs had the closest relatives, which have not been reported to be AAP bacteria. Physiological comparisons among the isolates revealed differences in preferences for nutrient concentrations, BChl contents, and light-harvesting proteins. These results suggest that diverse and previously unknown AAP bacteria inhabit river biofilms.