二氧化碳减排产柴油微藻培养体系研究进展

污水资源化、二氧化碳减排及微藻生物柴油是当前能源与环境领域的前沿课题。以下围绕污水及烟道气资源化培养产油微藻的培养体系,就藻种、营养条件、培养方式、培养环境及微藻生物反应器等影响产油微藻培养的因素研究进展进行了综述。在综述的基础上提出：由于微藻具有特殊营养方式,通过藻种筛选、微藻营养条件和培养环境的优化以及高效光生物反应器和生产工艺等的创新,可利用污水进行产油微藻生产,以获得生物柴油等高附加值产品,实现微藻生物能源、污水资源化处理和CO2减排三者高度耦合的产油微藻生产体系,从而减少微藻培养费用及污水处理费用,因此,该体系具有重要的环境、社会、经济价值和商业化应用前景。     

以异养细胞作为种子的椭圆小球藻产油脂光自养培养优化

异养细胞种子/光自养培养方法是一种可异养培养的能源微藻培养的有效方法,但已有文献尚未从工艺优化角度考察其发展潜力。为了获得较高细胞密度的用于光自养培养的种子和提高光自养培养的细胞密度与油脂产率,对异养细胞种子/光自养培养的培养基和培养条件进行了优化。结果表明,采用优化后的培养基,椭圆小球藻在摇瓶中异养培养的最高藻细胞密度可达11.04 g/L,比在初始培养基条件下提高了28.0%,在5 L发酵罐中异养培养的藻细胞密度达到73.89 g/L;在2 L柱式光生物反应器中光自养培养的藻细胞密度、油脂含量和油脂产率分别达1.62 g/L、36.34%和6.1 mg/(L·h),油脂成分主要为含C16-C18碳链的脂肪酸,是制备生物柴油的理想原料。经过优化,异养细胞种子/光自养培养这一方法能够显著地提高椭圆小球藻产油脂的能力,这进一步表明异养细胞种子/光自养培养方法有望成为可异养的能源微藻的高效培养方式。     

微藻异养高密度培养研究进展与发展趋势*

微藻细胞可以积累大量油脂、蛋白质、多糖、色素、不饱和脂肪酸等物质,在能源、食品、饵料、保健品及药品等行业有巨大的应用价值。然而,微藻在传统光自养模式下很难实现高密度培养来大量生产这些重要的物质,进而限制了微藻的实际应用。相反,微藻在异养模式下生长速度快、生物质浓度高,可以短时间内获得大量微藻生物质。因此,异养高密度培养微藻具备大规模、高效率培养微藻生产目标产物的巨大潜力。阐述微藻异养培养的优缺点及相应技术难点的解决思路、影响微藻异养生长及目标产物积累的主要营养因子和环境因子、微藻异养高密度培养的方式及微藻异养高密度培养的当前发展水平。结合文献报道分析微藻异养高密度培养的四个具有极大发展潜力的发展方向,以期更好地利用异养模式来高效率、低成本培养微藻生产大量目标产物,满足上述多个行业对微藻原材料的巨大需求,从而加速微藻产业的发展。     

碳源、氮源对异养单针藻Monoraphidium sp. FXY-10油脂积累和脂肪酸组成的影响

研究了碳源与氮源对单针藻Monoraphidium sp. FXY-10异养培养的影响。以BG-11为基础培养基,通过添加不同类型、浓度梯度碳源和氮源,比较分析微藻生物量、油脂积累以及脂肪酸组成。结果表明,以葡萄糖作碳源,硝酸钠为氮源,微藻细胞积累的油脂是理想的生物柴油制备原料。硝酸钠浓度分别为1.00、3.00和5.00 g/L时,对油脂产量影响不显著(P>0.05)。葡萄糖浓度为10.00 g/L,硝酸钠为氮源油脂产量达到实验最高值0.84 g/L,其油脂脂肪酸组成主要由C16:0和C18:1等短链饱和脂肪酸和单不饱和脂肪酸组成,不饱和度值(DU)为61.98,相对偏低。     

微藻生物柴油研发态势分析

微藻是光合效率最高的原始植物之一,与农作物相比,单位面积的产率可高出数十倍。微藻生物柴油技术首先包括微藻的筛选和培育,获得性状优良的高含油量藻种,然后在光生物反应器中吸收阳光、CO2等,生成微藻生物质,最后经过采收、加工,转化为微藻生物柴油。完整的微藻生物柴油成套技术链涵盖多个技术环节,是一个复杂的系统工程,包括微藻生物工程技术、微藻高效规模化养殖技术,以及微藻生物质采收、加工与转化技术等。其中,降低生产成本是当前微藻生物柴油研究面临的主要挑战,各国的研究机构为此开展了多方面的研究。     

Ultrahigh-cell-density heterotrophic cultivation of the unicellular green microalga Scenedesmus acuminatus and application of the cells to photoautotrophic culture enhance biomass and lipid production

Although production of biodiesels from microalgae is proved to be technically feasible, a commercially viable system has yet to emerge. High-cell-density fermentation of microalgae can be coupled with photoautotrophic cultivation to produce oils. In this study, by optimizing culturing conditions and employing a sophisticated substrate feed control strategy, ultrahigh-cell-density of 286 and 283.5 g/L was achieved for the unicellular alga Scenedesmus acuminatus grown in 7.5-L bench-scale and 1,000-L pilot-scale fermenters, respectively. The outdoor scale-up experiments indicated that heterotrophically grown S. acuminatus cells are more productive in terms of both biomass and lipid accumulation when they are inoculated in photobioreactors for lipid production as compared to the cells originally grown under photoautotrophic conditions. Technoeconomic analysis based on the pilot-scale data indicated that the cost of heterotrophic cultivation of microalgae for biomass production is comparable with that of the open-pond system and much lower than that of tubular PBR, if the biomass yield was higher than 200 g/L. This study demonstrated the economic viability of heterotrophic cultivation on large-scale microalgal inocula production, but ultrahigh-productivity fermentation is a prerequisite. Moreover, the advantages of the combined heterotrophic and photoautotrophic cultivation of microalgae for biofuels production were also verified in the pilot-scale.     

Cultivation, photobioreactor design and harvesting of microalgae for biodiesel production: A critical review

Microalgae have the ability to mitigate CO₂ emission and produce oil with a high productivity, thereby having the potential for applications in producing the third-generation of biofuels. The key technologies for producing microalgal biofuels include identification of preferable culture conditions for high oil productivity, development of effective and economical microalgae cultivation systems, as well as separation and harvesting of microalgal biomass and oil. This review presents recent advances in microalgal cultivation, photobioreactor design, and harvesting technologies with a focus on microalgal oil (mainly triglycerides) production. The effects of different microalgal metabolisms (i.e., phototrophic, heterotrophic, mixotrophic, and photoheterotrophic growth), cultivation systems (emphasizing the effect of light sources), and biomass harvesting methods (chemical/physical methods) on microalgal biomass and oil production are compared and critically discussed. This review aims to provide useful information to help future development of efficient and commercially viable technology for microalgae-based biodiesel production.     

Biodiesel production by simultaneous extraction and conversion of total lipids from microalgae, cyanobacteria, and wild mixed-cultures

Microalgae have been identified as a potential biodiesel feedstock due to their high lipid productivity and potential for cultivation on marginal land. One of the challenges in utilizing microalgae to make biodiesel is the complexities of extracting the lipids using organic solvents followed by transesterification of the extracts to biodiesel. In the present work, reaction conditions were optimized that allow a single step extraction and conversion to biodiesel in high yield from microalgae. From the optimized conditions, it is demonstrated that quantitative conversion of triglycerides from several different microalgae and cyanobacteria could be achieved, including from mixed microbial biomass collected from a municipal wastewater lagoon. Evidence is presented that for some samples, significantly more biodiesel can be produced than would be expected from available triglycerides, indicating conversion of fatty acids contained in other molecules (e.g., phospholipids) using this approach. The effectiveness of the approach on wet algae is also reported.     

Isolation and evaluation of oil-producing microalgae from subtropical coastal and brackish waters

Microalgae have been widely reported as a promising source of biofuels, mainly based on their high areal productivity of biomass and lipids as triacylglycerides and the possibility for cultivation on non-arable land. The isolation and selection of suitable strains that are robust and display high growth and lipid accumulation rates is an important prerequisite for their successful cultivation as a bioenergy source, a process that can be compared to the initial selection and domestication of agricultural crops. We developed standard protocols for the isolation and cultivation for a range of marine and brackish microalgae. By comparing growth rates and lipid productivity, we assessed the potential of subtropical coastal and brackish microalgae for the production of biodiesel and other oil-based bioproducts. This study identified Nannochloropsis sp., Dunaniella salina and new isolates of Chlorella sp. and Tetraselmis sp. as suitable candidates for a multiple-product algae crop. We conclude that subtropical coastal microalgae display a variety of fatty acid profiles that offer a wide scope for several oil-based bioproducts, including biodiesel and omega-3 fatty acids. A biorefinery approach for microalgae would make economical production more feasible but challenges remain for efficient harvesting and extraction processes for some species.     

Heterotrophic cultivation of microalgae for pigment production: A review

Pigments (mainly carotenoids) are important nutraceuticals known for their potent anti-oxidant activities and have been used extensively as high end health supplements. Microalgae are the most promising sources of natural carotenoids and are devoid of the toxic effects associated with synthetic derivatives. Compared to photoautotrophic cultivation, heterotrophic cultivation of microalgae in well-controlled bioreactors for pigments production has attracted much attention for commercial applications due to overcoming the difficulties associated with the supply of CO₂ and light, as well as avoiding the contamination problems and land requirements in open autotrophic culture systems. In this review, the heterotrophic metabolic potential of microalgae and their uses in pigment production are comprehensively described. Strategies to enhance pigment production under heterotrophic conditions are critically discussed and the challenges faced in heterotrophic pigment production with possible alternative solutions are presented.     

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。     

Best practices in heterotrophic high-cell-density microalgal processes: achievements,potential and possible limitations

Microalgae of numerous heterotrophic genera (obligate or facultative) exhibit considerable metabolic versatility and flexibility but are currently underexploited in the biotechnological manufacturing of known plant-derived compounds, novel high-value biomolecules or enriched biomass. Highly efficient production of microalgal biomass without the need for light is now feasible in inexpensive, well-defined mineral medium, typically supplemented with glucose. Cell densities of more than 100 g l⁻¹ cell dry weight have been achieved with Chlorella, Crypthecodinium and Galdieria species while controlling the addition of organic sources of carbon and energy in fedbatch mode. The ability of microalgae to adapt their metabolism to varying culture conditions provides opportunities to modify, control and thereby maximise the formation of targeted compounds with non-recombinant microalgae. This review outlines the critical aspects of cultivation technology and current best practices in the heterotrophic high-cell-density cultivation of microalgae. The primary topics include (1) the characteristics of microalgae that make them suitable for heterotrophic cultivation, (2) the appropriate chemical composition of mineral growth media, (3) the different strategies for fedbatch cultivations and (4) the principles behind the customisation of biomass composition. The review confirms that, although fundamental knowledge is now available, the development of efficient, economically feasible large-scale bioprocesses remains an obstacle to the commercialisation of this promising technology.     

The effect of physicochemical conditions and nutrient sources on maximizing the growth and lipid productivity of green microalgae

Biodiesel is a renewable fuel produced mostly from edible and non‐edible vegetables, by transesterification of neutral lipids (triacylglycerols). However, vegetable oil‐based biodiesel production competes with food crops for arable land, increasing food prices and leading to biodiversity loss. The production of biodiesel from oleaginous microorganisms – particularly microalgae – has attracted attention due to the higher lipid productivity of these organisms, when compared with vegetables. Several environmental factors – including light, temperature, pH and the presence of nutrients (particularly nitrogen, phosphorus and iron) – influence directly the ability of microalgae to produce and store triacylglycerols and other lipids, and also modulate microalgal growth. Although some environmental factors affect several species in a similar manner, differential responses between species are frequent, highlighting the importance of identifying optimal cultivation conditions for each species, to balance growth and lipid productivity for biodiesel production. Here, we reviewed the particular influence of the physicochemical and nutritional factors on the growth and lipid productivity of different green oleaginous microalgae species.     

A state of the art of metabolic networks of unicellular microalgae and cyanobacteria for biofuel production

The most promising and yet challenging application of microalgae and cyanobacteria is the production of renewable energy: biodiesel from microalgae triacylglycerols and bioethanol from cyanobacteria carbohydrates. A thorough understanding of microalgal and cyanobacterial metabolism is necessary to master and optimize biofuel production yields. To this end, systems biology and metabolic modeling have proven to be very efficient tools if supported by an accurate knowledge of the metabolic network. However, unlike heterotrophic microorganisms that utilize the same substrate for energy and as carbon source, microalgae and cyanobacteria require light for energy and inorganic carbon (CO₂ or bicarbonate) as carbon source. This double specificity, together with the complex mechanisms of light capture, makes the representation of metabolic network nonstandard. Here, we review the existing metabolic networks of photoautotrophic microalgae and cyanobacteria. We highlight how these networks have been useful for gaining insight on photoautotrophic metabolism.     

微藻生物柴油技术的研究现状及展望

微藻生物柴油是一种优良的可再生新能源,对于解决人类面临的能源短缺和全球变暖两大危机具有潜在的重大战略意义。综述了微藻生物柴油的技术流程、油脂含量较高的微藻藻种、微藻生物柴油的最大技术瓶颈、提高微藻油脂总产量的方法、微藻的大规模培养、微藻的采收和微藻生物柴油的制取等方面的研究现状,并对微藻生物柴油未来的核心研究方向提出了初步见解。     

Cyanobacteria cultivation in industrial wastewaters and biodiesel production from their biomass: a review

As an alternative fuel biodiesel has become increasingly important due to diminishing petroleum reserves and adverse environmental consequences of exhaust gases from petroleum-fueled engines. Recently, research interest has focused on the production of biofuel from microalgae. Cyanobacteria appeared to be suitable candidates for cultivation in wastes and wastewaters because they produce biomass in satisfactory quantity and can be harvested relatively easily due to their size and structure. In addition, their biomass composition can be manipulated by several environmental and operational factors to produce biomass with concrete characteristics. Herein, we review the culture of cyanobacteria in wastewaters and also the potential resources that can be transformed into biodiesel successfully for meeting the ever-increasing demand for biodiesel production.     

Biodiesel fuel from microalgae-promising alternative fuel for the future: a review

The use of organic matter such as vegetable oil to produce biodiesel fuel has been a practical technology for a number of years. However, the search for new technologies and raw materials for biodiesel fuel production has gained increased attention recently because of financial and environmental concerns. Of particular interest are raw materials that are not food-related. Microalgae have gained a great deal of attention as a potential biodiesel raw material because of their high growth rates and ability to accumulate oil, bind carbon dioxide, and remove contaminants from wastewater. This article is a literature review of technologies for biodiesel production from microalgae. The technologies relate to microalgal cultivation, microalgal growth enhancement to simultaneously increase biomass and reduce pollution, the preparation of microalgal biomass for biodiesel production, and biodiesel production itself.     

Comparative life cycle assessment of autotrophic cultivation of Scenedesmus dimorphus in raceway pond coupled to biodiesel and biogas production

Life cycle assessment (LCA) of indigenous freshwater microalgae, Scenedesmus dimorphus, cultivation in open raceway pond and its conversion to biodiesel and biogas were carried out. The LCA inventory inputs for the biogas scenario was entirely based on primary data obtained from algal cultivation (in pilot scale raceway pond), harvesting, and biogas production; while only the downstream processing involved in biodiesel production namely drying, reaction and purification were based on secondary data. Overall, eight scenarios were modeled for the integrated process involving: algae-based CO₂ capture and downstream processing scenarios for biodiesel and biogas along with impact assessment of nutrient addition and extent of recycling in a life cycle perspective. The LCA results indicated a huge energy deficit and net CO₂ negative in terms of CO₂ capture for both the biodiesel and biogas scenarios, majorly due to lower algal biomass productivity and higher energy requirements for culture mixing. The sensitivity analysis indicated that variability in the biomass productivity has predominant effect on the primary energy demand and global warming potential (GWP, kg CO₂ eq.) followed by specific energy consumption for mixing algal culture. Furthermore, the LCA results indicated that biogas conversion route from microalgae was more energy efficient and sustainable than the biodiesel route. The overall findings of the study suggested that microalgae-mediated CO₂ capture and conversion to biodiesel and biogas production can be energy efficient at higher biomass productivity (> 10 g m⁻² day⁻¹) and via employing energy-efficient systems for culture mixing (< 2 W m⁻³).

     

我国微藻生物柴油的研究背景与发展战略

生物柴油是可再生能源开发利用的重要发展方向。藻类制备生物柴油具有产油量高、生长速度快、环境适应能力强、不与农作物争夺农田和淡水资源等优势。从微藻制备生物柴油着手,简要介绍了生物柴油的生产原料与发展历程、微藻油脂的组成与生物合成途径、微藻制备柴油的工艺与瓶颈及解决策略,最后对微藻制备生物柴油技术提出了近中远期发展目标及展望。     

Microalgal mass culture systems and methods: Their limitation and potential

Cultivation of microalgae using natural and man-made open-ponds istechnologically simple, but not necessary cheap due to the high downstream processing cost. Products of microalgae cultured in open-pondscould only be marketed as value-added health food supplements, specialityfeed and reagents for research. The need to achieve higher productivityand to maintain monoculture of algae led to the development of enclosedtubular and flat plate photobioreactors. Despite higher biomassconcentration and better control of culture parameters, data accumulatedin the past 25 years have shown that the illuminated areal, volumetricproductivity and cost of production in these enclosed photobioreactors arenot better than those achievable in open-pond cultures. The technicaldifficulty in sterilizing these photobioreactors has hindered their applicationfor the production of high value pharmaceutical products. The alternativeof cultivating microalgae in heterotrophic mode in sterilizable fermentorshas achieved some commercial success. The maximum specific growth ratesof heterotrophic algal cultures are in general slower than those measured inphotosynthetic cultures. The biomass productivity of heterotrophic algalcultures has yet to achieve a level that is comparable to industrialproduction of yeast and other heterotrophic microrganisms. Mixotrophiccultivation of microalage takes advantage of their ability to utilise organicenergy and carbon substrates and perform photosynthesis concurrently. Moreover, production of some algal metabolites is light regulated. Futuredesign of sterilizable bioreactors for mixotrophic cultivation of microalgaemay have to consider the organic substrate the main source of energy andlight the supplemental source of energy, a change in mindset.