Diatom artificial medium (DAM): a new artificial medium for the diatom <Emphasis Type="Italic">Haslea ostrearia</Emphasis> and other marine microalgae

Artificial media are used in physiological studies of microalgae to maintain consistent conditions from one experiment to another and these media must be adapted to the needs of the organism studied. The artificial medium, in this case named diatom artificial medium (DAM), was designed to maintain long-term cultures of Haslea ostrearia and 19 other planktonic microalgae, and to allow physiological studies related to metal metabolism. The biomass and biochemical composition of H. ostrearia grown in the DAM and in a modified Provasoli medium were compared to assess the suitability of this new artificial medium for the culture of this diatom. The DAM provided sufficient nutrients to allow H. ostrearia to grow as efficiently as in the enriched seawater medium, without negative impact on metabolism. The DAM was tested with 19 other microalgae in order to widen its potential use, and 18 of the 19 showed a good adaptation to this medium. The chemical speciation of metals (Cd, Cu, Pb, Zn) was assessed using a speciation mathematical model. The presence of EDTA resulted in the total complexation of the trace metals implying that they were present in a sole chemical species in the DAM.     

Engineering a powerful green cell factory for robust photoautotrophic diterpenoid production

Diterpenoids display a large and structurally diverse class of natural compounds mainly found as specialized plant metabolites. Due to their diverse biological functions they represent an essential source for various industrially relevant applications as biopharmaceuticals, nutraceuticals, and fragrances. However, commercial production utilizing their native hosts is inhibited by low abundances, limited cultivability, and challenging extraction, while the precise stereochemistry displays a particular challenge for chemical synthesis. Due to a high carbon flux through their native 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway towards photosynthetically active pigments, green microalgae hold great potential as efficient and sustainable heterologous chassis for sustainable biosynthesis of plant-derived diterpenoids. In this study, innovative synthetic biology and efficient metabolic engineering strategies were systematically combined to re-direct the metabolic flux through the MEP pathway for efficient heterologous diterpenoid synthesis in C. reinhardtii. Engineering of the 1-Deoxy-D-xylulose 5-phosphate synthase (DXS) as the main rate-limiting enzyme of the MEP pathway and overexpression of diterpene synthase fusion proteins increased the production of high-value diterpenoids. Applying fully photoautotrophic high cell density cultivations demonstrate potent and sustainable production of the high-value diterpenoid sclareol up to 656 mg L⁻¹ with a maximal productivity of 78 mg L⁻¹ day⁻¹ in a 2.5 L scale photobioreactor, which is comparable to sclareol titers reached by highly engineered yeast. Consequently, this work represents a breakthrough in establishing a powerful phototrophic green cell factory for the competetive use in industrial biotechnology.     

Optimization of a raceway pond system for wastewater treatment: a review

Microalgae are photosynthetic microorganisms with potential for biofuel production, CO₂ mitigation and wastewater treatment; indeed they have the capacity to assimilate pollutants in wastewaters. Light supply and distribution among the microalgae culture is one of the major challenges of photo-bioreactor design, with many studies focusing on microalgae culture systems such as raceway ponds (RWP), widely used and cost-effective systems for algal biomass production. This review focuses on possible improvements of the RWP design in order to achieve optimal microalgal growth conditions and high biomass productivities, to minimize energy consumption and to lower the capital costs of the pond. The improvement strategy is based on three aspects: (1) hydrodynamic characteristics of the raceway pond, (2) evaluation of hydrodynamic and mass transfer capacities of the pond and (3) design of the RWP. Finally, a possible optimal design for the RWP is discussed in the context of wastewater treatment.     

Effects of green LED light and three stresses on biomass and lipid accumulation with two-phase culture of microalgae

The effects of wavelengths of light-emitting diode (LED), nitrate concentration, and salt concentration were evaluated for the two-phase culture of the microalgal species Phaeodactylum tricornutum, Dunaliella tertiolecta, and Isochrysis galbana on cell growth and lipid production. Blue LEDs produced the highest biomass of P. tricornutum at a nitrate concentration of 8 mg/L, reaching 0.97 g dcw/L with a specific growth rate (μ) of 0.047 h⁻¹, followed by I. galbana with 0.79 g dcw/L and μ = 0.040 h⁻¹ and D. tertiolecta with 0.55 g dcw/L and μ = 0.028 h⁻¹. Of the three microalgae, P. tricornutum had the highest specific growth rate of μ_max = 0.070 h⁻¹ and lowest saturation constant of K_s = 4.18 mg/L, resulting in fast cell growth. The highest lipid production was obtained under green LED wavelength stress on day 14, reaching 60.6% (w/w) of the dry cell weight among the three microalgae. The main fatty acids produced by the three microalgae were myristic acid (C14:0), palmitic acid (C16:0), oleic acid (C18:1), and arachidic acid (C20:0), which comprised 72.68%–84.16% (w/w) of the total fatty acids content under three stresses.     

利用废水培养微藻的研究进展

微藻被认为是一种有潜力的、可被开发为再生能源的重要生物材料。一些微藻种类具有较强的异养和混养能力,能直接利用有机物作为碳源。工农业生产和城市生活中所排放的废水中通常含有大量的有机碳、氮、磷等营养物质。利用废水培养微藻,一方面可以将废水中的碳、氮、磷等营养物质转化为具有更高价值的微藻生物质,另一方面又可实现废水的净化和营养物质的再利用。本综述了不同种类废水的特点,讨论了两类微藻培养模式的优劣,同时还探讨了微藻对营养元素的利用,并总结了微藻培养需突破的瓶颈。     

Improvement in Oil Production by Increasing Malonyl-CoA and Glycerol-3-Phosphate Pools in Scenedesmus quadricauda

In recent years, microalgae have attracted considerable interest as a biofuel resource owing to their rapid growth, tolerance to harsh conditions, and ability to accumulate a large amount of triacylglycerols (TAGs). However, the economic effectiveness of algal biofuel is still low. In this study, we attempted to increase oil production of the microalga Scenedesmus quadricauda by elevating intracellular malonyl-CoA and glycerol-3-phosphate (G3P) pools. To increase intracellular oil content, yeast-derived genes encoding acetyl-CoA carboxylase (ACC1), glycerol kinase (GPD1), and glycerol-3-phosphate dehydrogenase (GUT1) were overexpressed under the control of CaMV 35S and NOS promoters with SV40 large T antigen components. Fatty acid profiling, G3P content, and the number of cells with high oil content were analyzed by gas chromatography-mass spectrometry, G3P assay kit, and flow cytometry, respectively. Overexpression of ACC1 increased the total fatty acid content by 1.6-fold. Overexpression of GPD1 and GUT1 increased intracellular G3P content by 1.6- and 1.9-fold, respectively. Multi-gene expression of ACC1, GPD1, and GUT1 increased the number of cells with high oil content by 1.45-fold compared with that observed with the wild-type. This study is the first to report increased oil production by overexpression of the key genes (ACC1, GPD1, and GUT1) for TAG biosynthesis in microalgae.

Electronic supplementary material

The online version of this article (doi:10.1007/s12088-015-0546-4) contains supplementary material, which is available to authorized users.     

Perspectives on engineering strategies for improving biofuel production from microalgae — A critical review

Although the potential for biofuel production from microalgae via photosynthesis has been intensively investigated, information on the selection of a suitable operation strategy for microalgae-based biofuel production is lacking. Many published reports describe competitive strains and optimal culture conditions for use in biofuel production; however, the major impediment to further improvements is the absence of effective engineering strategies for microalgae cultivation and biofuel production. This comprehensive review discusses recent advances in understanding the effects of major environmental stresses and the characteristics of various engineering operation strategies on the production of biofuels (mainly biodiesel and bioethanol) using microalgae. The performances of microalgae-based biofuel-producing systems under various environmental stresses (i.e., irradiance, temperature, pH, nitrogen depletion, and salinity) and cultivation strategies (i.e., fed-batch, semi-continuous, continuous, two-stage, and salinity-gradient) are compared. The reasons for variations in performance and the underlying theories of the various production strategies are also critically discussed. The aim of this review is to provide useful information to facilitate development of innovative and feasible operation technologies for effectively increasing the commercial viability of microalgae-based biofuel production.     

细胞固定化方法制备微藻光电极的研究

通过将微藻细胞固定在平面多孔碳纸上,制备微藻光电极,并在三电极体系电解液中加入电子介体进行测试,可产生与光照同步的光电流响应。考察了不同固定化方法、不同微藻及不同电子介体的光电流响应,结果表明硅溶胶-凝胶法制备的光电极光电流响应最佳,且对于亚心形四爿藻、金藻、莱茵衣藻、蛋白核小球藻、聚球藻等 5 种微藻都适用,表明该制备方法对不同微藻具有较好的通用性。电子介体的研究表明苯醌及其衍生物由于氧还电位较高,具有较好的阳极光电流响应特性,而甲基紫精氧还电位较低,具有较好的阴极光电流响应。     

Effect of mixed organic substrate on α-tocopherol production by <Emphasis Type="Italic">Euglena gracilis</Emphasis> in photoheterotrophic culture

Effects of organic carbon sources on cell growth and alpha-tocopherol productivity in wild and chloroplast-deficient W14ZUL strains of Euglena gracilis under photoheterotrophic culture were investigated. In both strains, the increase in cell growth was particularly high when glucose was added as the sole organic carbon source. On the other hand, alpha-tocopherol production per dry cell weight was enhanced by adding ethanol. Ethanol addition also increased the chlorophyll concentration in wild strain and mitochondria activity in W14ZUL strain. For effective alpha-tocopherol production, the effects of mixture of glucose and ethanol were investigated. The results showed that, when a mixture of glucose (6 g/l) and ethanol (4 g/l) was used, alpha-tocopherol productivity per culture broth was 3.89 x 10(-2) mg l(-1) h(-1), which was higher than the value obtained without addition of organic carbon source (0.92 x 10(-2) mg l(-1) h(-1)). In addition, under fed-batch cultivation using an internally illuminated photobioreactor, the alpha-tocopherol production per culture broth was 23.43 mg/l, giving a productivity of 16.27 x 10(-2) mg l(-1) h(-1).     

微藻生物质制备燃料乙醇关键技术研究进展

燃料乙醇作为一种优良的可再生液体燃料,其开发利用受到了人们的广泛关注。微藻是一种高光合、高产生物量的生物质资源,很多的藻体细胞中含有大量的淀粉、纤维素（Iα型）等多糖物质,是制备燃料乙醇的优良原料。发展利用微藻制备燃料乙醇技术工艺,对于缓解我国目前日益短缺的能源问题,减少温室气体排放和环境污染等具有很好的应用前景。综述了国内外利用微藻生物质制备燃料乙醇中所用到的关键技术、存在的问题以及今后的发展前景等。