Cell fragility — The key problem of microalgae mass production in closed photobioreactors

The gap between the theoretical biological potential of microalgae and the biomass productivity obtained with algal culture in tubular biophotoreactors is due to a reduced growth rate related to hydrodynamic stress of pumping. High levels of mixing are necessary to reach a turbulent flow of the culture, in order to optimize the light regime. The optimal conditions of pumping to produce this significant liquid mixing may produce some cell damage. Factors affecting this hydrodynamic stress (geometry of the bioreactor involved, type of pump utilized, morphology of algal cells, physiological conditions of microalgae, etc.) are discussed.     

Algal swimming velocities signal fatty acid accumulation

The use of microalgae for biofuel production will be beneficial to society if we can produce biofuels at large scales with minimal mechanical energy input in the production process. Understanding micro‐algal physiological responses under variable environmental conditions in bioreactors is essential for the optimization of biofuel production. We demonstrate that measuring micro‐algal swimming speed provides information on culture health and total fatty acid accumulation. Three strains of Chlamydomonas reinhardtii were grown heterotrophically on acetate and subjected to various levels of nitrogen starvation. Other nutrient levels were explored to determine their effect on micro‐algal kinetics. Swimming velocities were measured with two‐dimensional micro‐particle tracking velocimetry. The results show an inverse linear relationship between normalized total fatty acid mass versus swimming speed of micro‐algal cells. Analysis of RNA sequencing data confirms these results by demonstrating that the biological processes of cell motion and the generation of energy precursors are significantly down‐regulated. Experiments demonstrate that changes in nutrient concentration in the surrounding media also affect swimming speed. The findings have the potential for the in situ and indirect assessment of lipid content by measuring micro‐algal swimming kinetics. Biotechnol. Bioeng. 2013; 110: 143–152. © 2012 Wiley Periodicals, Inc.     

Outdoor cultivation of microalgae for carotenoid production: current state and perspectives

Microalgae are a major natural source for a vast array of valuable compounds, including a diversity of pigments, for which these photosynthetic microorganisms represent an almost exclusive biological resource. Yellow, orange, and red carotenoids have an industrial use in food products and cosmetics as vitamin supplements and health food products and as feed additives for poultry, livestock, fish, and crustaceans. The growing worldwide market value of carotenoids is projected to reach over US$1,000 million by the end of the decade. The nutraceutical boom has also integrated carotenoids mainly on the claim of their proven antioxidant properties. Recently established benefits in human health open new uses for some carotenoids, especially lutein, an effective agent for the prevention and treatment of a variety of degenerative diseases. Consumers’ demand for natural products favors development of pigments from biological sources, thus increasing opportunities for microalgae. The biotechnology of microalgae has gained considerable progress and relevance in recent decades, with carotenoid production representing one of its most successful domains. In this paper, we review the most relevant features of microalgal biotechnology related to the production of different carotenoids outdoors, with a main focus on β-carotene from Dunaliella, astaxanthin from Haematococcus, and lutein from chlorophycean strains. We compare the current state of the corresponding production technologies, based on either open-pond systems or closed photobioreactors. The potential of scientific and technological advances for improvements in yield and reduction in production costs for carotenoids from microalgae is also discussed.     

Efficient utilization of high irradiance for production of photoautotropic cell mass: a survey

For mass production of microalgae outdoors to be justified as a significant commercial entity, solar energy should be utilized at a much higher efficiency, yielding greatly increased photosynthetic productivity than presently obtained. Development of photobioreactors to provide an answer for this challenge rests at the root and the very future of this biotechnology. Most available Photobioreactors yield increased volumetric outputs of cell mass, but the areal yield which relates to the photosynthetic efficiency is rather similar to that obtained in the basically inefficient open raceway, the most prevalent commercial reactor today. The key for efficient utilization of the super saturating solar irradiance existing outdoors rests in distributing it, in effect, to as large a number of cells per given volume in as high a frequency as possible. This unfolds the design principles underlying efficient utilization of high irradiance for photoautotrophic production of cell mass: Reactors should be maximally exposed to sun light, have a narrow light-path coupled with a safe mixing system designed to create fast, turbulent streaming for moving the algal cells in and out of the photic volume at maximal frequency. Reactors designed along these principles may support ultrahigh cell densities resulting in high volumetric as well as areal yields, hopefully expanding thereby the economic basis of microalgal biotechnology.     

Luxury uptake of phosphorus by microalgae in waste stabilisation ponds: current understanding and future direction

There is a need to improve phosphorus removal for the tens of thousands of small communities around the world that currently rely on waste stabilisation ponds for their wastewater treatment. We now know that microalgae in pond systems are capable of accumulating phosphorus within their cells as polyphosphate in a process known as luxury uptake, but this knowledge has not yet been applied to engineer a new process to improve phosphorus removal. This paper summarises the current understanding of this mechanism, discusses the key factors influencing polyphosphate accumulation and provides future direction for research in this field. There have only been a limited number of studies that have focussed on luxury uptake of polyphosphate by microalgae in high phosphorus concentration environments such as those found in waste stabilisation pond systems. However, from this review it has been shown that the environmental factors which influence polyphosphate storage are the phosphate concentration, light intensity and temperature. Currently we are limited to a black box understanding of the bulk population and as a result future research needs to focus on a systematic evaluation of different microalgal genera under a wide range of environmental, biological and process variables in order to reveal the conditions needed to reliably trigger this phenomenon. This will then provide the basis for developing a new algal biotechnology optimised for luxury uptake of polyphosphate. While there are still several key questions that need to be answered there is potential for this to lead to a process which could be as significant to pond systems as the development of enhanced biological phosphorus removal was for the activated sludge process.     

In vitro culture and conservation of microalgae: Applications for aquaculture, biotechnology and environmental research

Summary Microalgae are a highly diverse group of unicellular organisms comprising the eukaryotic protists and the prokaryotic cyanobacteria or blue-green algae. The microalgae have a unique environmental status; being virtually ubiquitous in euphotic aquatic niches, they can occupy extreme habitats ranging from tropical coral reefs to the polar regions, and they contribute to half of the globe’s photosynthetic activity. Furthermore, they form the basis of the food chain for more than 70% of the world’s biomass. Microalgae are a valuable environmental and biotechnological resource, and the aim of this review is to explore the use of in vitro technologies in the conservation and sustainable exploitation of this remarkable group of organisms. The first part of the review evaluates the importance of in vitro methods in the maintenance and conservation of microalgae and describes the central role of culture collections in applied algal research. The second part explores the application of microalgal in vitro technologies, particularly in the context of the aquaculture and biotechnology industries. Emphasis is placed upon the exploitation of economically important algal products including aquaculture feed, biomass production for the health care sector, green fertilizers, pigments, vitamins, antioxidants, and antimicrobial agents. The contribution that microalgae can make to environmental research is also appraised; for example, they have an important role as indicator organisms in environmental impact assessments. Similarly, designated culture collection strains of microalgae are used for ecotoxicity testing. Throughout the review, emphasis is placed on the application of in vitro techniques for the continued advancement of microalgal research. The paper concludes by assessing future perspectives for the novel application of microalgae and their products.     

Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances

Microalgae represent an exceptionally diverse but highly specialized group of micro-organisms adapted to various ecological habitats. Many microalgae have the ability to produce substantial amounts (e.g. 20–50% dry cell weight) of triacylglycerols (TAG) as a storage lipid under photo-oxidative stress or other adverse environmental conditions. Fatty acids, the building blocks for TAGs and all other cellular lipids, are synthesized in the chloroplast using a single set of enzymes, of which acetyl CoA carboxylase (ACCase) is key in regulating fatty acid synthesis rates. However, the expression of genes involved in fatty acid synthesis is poorly understood in microalgae. Synthesis and sequestration of TAG into cytosolic lipid bodies appear to be a protective mechanism by which algal cells cope with stress conditions, but little is known about regulation of TAG formation at the molecular and cellular level. While the concept of using microalgae as an alternative and renewable source of lipid-rich biomass feedstock for biofuels has been explored over the past few decades, a scalable, commercially viable system has yet to emerge. Today, the production of algal oil is primarily confined to high-value specialty oils with nutritional value, rather than commodity oils for biofuel. This review provides a brief summary of the current knowledge on oleaginous algae and their fatty acid and TAG biosynthesis, algal model systems and genomic approaches to a better understanding of TAG production, and a historical perspective and path forward for microalgae-based biofuel research and commercialization.     

Microalgae immobilization: current techniques and uses

Information about advances in immobilization techniques and biotechnology use of freshwater and marine microalgae is scattered. This work aims to bring together the main recent research about the topic. Passive and active immobilization techniques used on microalgae are listed and described in the text. Effect of immobilization on growth and metabolism of the cells is also reviewed. Current uses of immobilized microalgae include metabolite production, culture collection handling, obtaining of energy and removing of undesired or valuable substances from media (nutrients, metals and different pollutant agents). Applications of immobilized microalgae in environmental aquatic research have been recently increased: novel immobilization techniques as well as the use of living microalgae as biosensors in electronic devices designed to measure toxicity of substances and effluents demonstrated to be a very promising topic in biotechnology research. Recent research pointed out the advantages of mixed bacterial-algal co-immobilized systems in water treatment plants. Application of immobilized systems to the production of non-contaminant energy (as H(2) obtained from algal cultures) is also an important topic to be explored in the next years.     

Microalgae as bioreactors

Microalgae already serve as a major natural source of valuable macromolecules including carotenoids, long-chain polyunsaturated fatty acids and phycocolloids. As photoautotrophs, their simple growth requirements make these primitive plants potentially attractive bioreactor systems for the production of high-value heterologous proteins. The difficulty of producing stable transformants has meant that the field of transgenic microalgae is still in its infancy. Nonetheless, several species can now be routinely transformed and algal biotechnology companies have begun to explore the possibilities of synthesizing recombinant therapeutic proteins in microalgae and the engineering of metabolic pathways to produce increased levels of desirable compounds. In this review, we compare the current commercially viable bioreactor systems, outline recent progress in microalgal biotechnology and transformation, and discuss the potential of microalgae as bioreactors for the production of heterologous proteins.     

Strategies to enhance production of microalgal biomass and lipids for biofuel feedstock

ABSTRACT

Microalgae have enormous potential as feedstock for biofuel production compared with other sources, due to their high areal productivity, relatively low environmental impact, and low impact on food security. However, high production costs are the major limitation for commercialization of algal biofuels. Strategies to maximize biomass and lipid production are crucial for improving the economics of using microalgae for biofuels. Selection of suitable algal strains, preferably from indigenous habitats, and further improvement of those ‘platform strains’ using mutagenesis and genetic engineering approaches are desirable. Conventional approaches to improve biomass and lipid productivity of microalgae mainly involve manipulation of nutritional (e.g. nitrogen and phosphorus) and environmental (e.g. temperature, light and salinity) factors. Approaches such as the addition of phytohormones, genetic and metabolic engineering, and co-cultivation of microalgae with yeasts and bacteria are more recent strategies to enhance biomass and lipid productivity of microalgae. Improvement in culture systems and the use of a hybrid system (i.e. a combination of open ponds and photobioreactors) is another strategy to optimize algal biomass and lipid production. In addition, the use of low-cost substrates such as agri-industrial wastewater for the cultivation of microalgae will be a smart strategy to reduce production costs. Such systems not only generate high algal biomass and lipid productivity, but are also useful for bioremediation of wastewater and bioremoval of waste CO₂. The aim of this review is to highlight the advances in the use of various strategies to enhance production of algal biomass and lipids for biofuel feedstock.     

The 96-well twin-layer system: a novel approach in the cultivation of microalgae

A novel system for the growth and maintenance of microalgae has been developed that allows the cultivation of a large number of strains with little manual effort. The system is based on a 96-well microtiter plate in which a membrane filter constitutes the bottom of each well. Algal strains are immobilised on the membranes and provided with culture medium through contact with layers of glass fibre located beneath the membranes in a special cultivation chamber. The configuration effectively separates culture medium from algal cells which allows the simultaneous exchange of the culture medium for 96 strains within a few minutes without the need to transfer the algae. If necessary, algal strains can be transferred using multi-channel pipettes. We demonstrate that a large variety of microalgal strains including delicate flagellates can be reliably grown in the system under axenic conditions and without cross-contamination. As an array system, the 96-well twin-layer system using immobilised algae is also amenable to high-throughput and massively parallel applications increasingly sought after in algal bio- and environmental technology.     

Mass production of the blue-green alga Spirulina: An overview

Progress has been made in the past decade in developing appropriate technology for microalgal mass cultivation. This review details basic requirements required in order to achieve high productivity and low cost of production. There is a need for a wide variety of algal species and strains that will favorably respond to the varying environmental conditions existing outdoors. Another essential requirement is for better bioreactors, either by improving existing open raceway types or developing tubular closed systems. The latter solution seems more promising. These developments must overcome the main limitation confronting the industry today which is the overall low areal yields which fall too short of the theoretical maximum and which are associated with scaling up microalgal culture to commercial size.     

The development of microalgal biotechnology in the Czech Republic

Microscopic algae and cyanobacteria are excellent sources of numerous compounds, from raw biomass rich in proteins, oils, and antioxidants to valuable secondary metabolites with potential medical use. In the former Czechoslovakia, microalgal biotechnology developed rapidly in the 1960s with the main aim of providing industrial, high-yield sources of algal biomass. Unique cultivation techniques that are still in use were successfully developed and tested. Gradually, the focus changed from bulk production to more sophisticated use of microalgae, including production of bioactive compounds. Along the way, better understanding of the physiology and cell biology of productive microalgal strains was achieved. Currently, microalgae are in the focus again, mostly as possible sources of bioactive compounds and next-generation biofuels for the 21st century.     

Bioprocess engineering of microalgae to optimize lipid production through nutrient management

Microalgae have been used commercially as a feedstock for the production of high-value compounds, pigments, cosmetics, and nutritional supplements. In addition, because of their rapid growth rates, high photosynthetic efficiency, and high lipid and protein content, commodity products including biodiesel, feed supplements, and polyunsaturated fatty acids derived from algal biomass are of current interest. Since microalgae lack non-photosynthetic structures and float in water, they do not need massive amounts of structural cellulose found in land plants. Thus, under optimal culture conditions, some oleaginous species can allocate up to 70 % of their biomass to lipids. Lipid production and its regulation in microalgae are species-specific and influenced by environmental conditions. Various strategies have been developed to improve lipid productivity and fatty acid composition to meet specific production goals. Manipulation of physiochemical parameters, trophic modes, and nutrient levels, known as process engineering, is a simple approach that leads to desired alterations in the biochemical composition of algal biomass, including lipid quantity and quality. In this paper, we review the effects of manipulating biochemical parameters such as necessary nutrients (C, N, P, S, Fe, and Si), NaCl concentration, and pH of culture medium to optimize lipid content and profile in some algae strains with commercial potential.     

Microalgal biotechnology: Carotenoid production by the green algae<Emphasis Type="Italic">Dunaliella salina</Emphasis>

Unicellular green algae of the genusDunaliella thrive in extreme environmental conditions such as high salinity, low pH, high irradiance and subzero temperatures. Species ofDunaliella are well known in the alga biotechnological industry and are employed widely for the production of valuable biochemicals, such as carotenoids. Some strains ofDunaliella are cultivated commercially in large outdoor ponds and are harvested to produce dry algal meals, such as polyunsaturated fatty acids and oils for the health food industry, and coloring agents for the food and cosmetic industries. During the past decade, the advances in molecular biology and biochemistry of microalgae, along with the advances in biotechnology of microalgal mass cultivation, enabled this microalga to become a staple of commercial exploitation. In particular, the advent of molecular biology and mutagenesis inDunaliella has permitted enhancements in the carotenoids content of this green alga, making it more attractive for biotechnological applications. Accordingly, the present review summarizes the recent developments and advances in biotechnology of carotenoid production inDunaliella.     

Perspectives of phototrophic microorganisms in environment protection and ecology

The ecophysiological properties – light effectuated neosynthesis of biomass from inorganics – make microalgae become interesting organisms of preventive and sanitating environmental protection with regard to the inorganic impact. They are filling an underestimated gap. Medium-term solutions of the heavy metal accumulation in the tertiary sewage disposal, the trophy sanitation of surface and ground waters and of algal induced recultivation of waste-land seem to be attainable. The investigation of the role of microalgae in world-wide material cycles will be of long-term importance to biotechnology and global ecology.     

风力胁迫对具鞘微鞘藻结皮光合活性的影响

抵抗风力胁迫是荒漠藻类适应干旱区环境的重要生物学机制,也是藻结皮能够拓殖流沙的必要条件之一,但有关藻类对风力胁迫的响应机理国内外尚无研究报道。以具鞘微鞘藻(Microcoleus vaginatus Gom.)人工结皮为实验对象,研究了不同强度风力吹蚀对结皮含水量、藻类活力、生物量、及其光合活性的影响。结果表明:不论低于当地起沙风(3m/s)还是高于起沙风(5m/s和7m/s)的风力吹蚀,结皮中藻类生物量均明显下降,而且结皮生物量的变化与风速大小和吹蚀时间呈线性关系(y=14.78+0.035a-1.48b,a风速,b时间,r2=0.79)。进一步分析发现,风力吹蚀后结皮中藻类活力并没有降低但主要光合色素和天线色素的含量普遍降低,叶绿素荧光(Fv/Fm)、表观电子传递速率(ETR)和净光合速率(Pn)明显下降,并且风速越大,降幅越大。这些结果说明风力胁迫对藻结皮生长和光合活性的影响主要是通过影响光合色素代谢合成和电子传递速率引起的,对其生命力没有明显影响。       

The use of natural infochemicals for sustainable and efficient harvesting of the microalgae <Emphasis Type="Italic">Scenedesmus</Emphasis> spp. for biotechnology: insights from a meta-analysis

Open raceway ponds are regarded as the most economically viable option for large-scale cultivation of microalgae for low to mid-value bio-products, such as biodiesel. However, improvements are required including reducing the costs associated with harvesting biomass. There is now a growing interest in exploiting natural ecological processes within biotechnology. We review how chemical cues produced by algal grazers induce colony formation in algal cells, which subsequently leads to their sedimentation. A statistical meta-analysis of more than 80 studies reveals that Daphnia grazers can induce high levels of colony formation and sedimentation in Scenedesmus obliquus and that these natural, infochemical induced sedimentation rates are comparable to using commercial chemical equivalents. These data suggest that natural ecological interactions can be co-opted in biotechnology as part of a promising, low energy and clean harvesting method for use in large raceway systems.