Life-cycle assessment of microalgae culture coupled to biogas production

Due to resource depletion and climate change, lipid-based algal biofuel has been pointed out as an interesting alternative because of the high productivity of algae per hectare and per year and its ability to recycle CO₂ from flue gas. Another option for taking advantage of the energy content of the microalgae is to directly carry out anaerobic digestion of raw algae in order to produce methane and recycle nutrients (N, P and K). In this study, a life-cycle assessment (LCA) of biogas production from the microalgae Chlorella vulgaris is performed and the results are compared to algal biodiesel and to first generation biodiesels. These results suggest that the impacts generated by the production of methane from microalgae are strongly correlated with the electric consumption. Progresses can be achieved by decreasing the mixing costs and circulation between different production steps, or by improving the efficiency of the anaerobic process under controlled conditions. This new bioenergy generating process strongly competes with others biofuel productions.     

Microalgae aquaculture feeds

Microalgae feeds are currently used in relatively small amounts in aquaculture, mainly for the production of larvae and juvenile shell- and finfish, as well as for raising the zooplankton required for feeding of juvenile animals. The blue-green algaSpirulina is used in substantial amounts (over 100 t y^–1) as a fish and shrimp feed, and even larger markets can be projected if production costs could be reduced. Another potential large-scale application of microalgae is the cultivation ofHaematococcus for the production of the carotenoid astaxanthin, which gives salmon flesh its reddish color. In the long-term microalgae biomass high in lipids (omega-3 fatty acids) may be developed as substitutes for fish oil-based aquaculture feeds. In shrimp ponds the indigenous algal blooms supply a part of the dietary requirements of the animals, but it is difficult to maximize algal productivities. A separate algal production system could feed the shrimps and minimize the need for added feed. Bivalves feed essentially exclusively on marine microalgae throughout their life cycle. The development of cultivation technologies for such microalgae would allow the onshore production of these animals, with greatly improved product quality and safety.This paper was presented at the Symposium on Applied Phycology at the Fourth International Phycological Congress, Duke University.     

Evaluation of extraction methods for recovery of fatty acids from Botryococcus braunii LB 572 and Synechocystis sp. PCC 6803

Microalgae are a very diverse group of organisms that consist in both prokaryotic and eukaryotic forms. Some species of microalgae can be induced to overproduce particular fatty acids through simple manipulations of the physical and chemical properties of the culture medium. In this paper, the effect of different extraction techniques on the recovery of fatty acids from the freeze-dried biomass from two lipid-producing microalgal strains: Botryococcus braunii LB 572 (green algae) and Synechocystis sp. PCC 6803 (cyanobacteria) was examined. Five procedures were used: after conversion of the lipid material into the corresponding fatty acid methyl esters (FAMEs) via suitable derivatization reactions (extraction-transesterification) and direct transesterification of biomass to produce FAMEs (without the initial extraction step) that used differential types of catalysts and processing conditions. This study has shown that procedure 3, a one step practical procedure for lipid extraction and in situ methyl ester derivation could be used successfully for the determination of the fatty acid compositions of microalgae and cyanobacteria.     

Indole-3-acetic acid in the culture medium of two axenic green microalgae

In the view of the facts that algal extracts have been used in agriculture asa source of plant growth stimulating agents and IAA has been shown to bepresent in the extracts, a study was planned to establish whether or notaxenic algae can produce IAA. Evidence is provided for extracellular IAAproduction during culture of two axenic green microalgae. IAAidentification was based on co-chromatography with the standard, analysisof UV and fluorescent spectra, and gas chromatography – selectedion-monitoring mass spectrometry. HPLC analyses showed that underthe experimental conditions the amounts of IAA released to the mediumby Scenedesmus armatus and Chlorella pyrenoidosa weregenerally low. IAA tended to occur in Scenedesmus armatus culturemedium at higher concentrations than in that of Chlorellapyrenoidosa. In fast-growing cultures of Scenedesmus armatus,constantly aerated with CO₂/air mixture, the concentration of IAAcalculated per cell was less than in the slow-growing cultures.     

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.     

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.     

Use of response surface methodology to optimise carotenogenesis in the microalga,Haematococcus pluvialis

The factors controlling biomass production and the synthesis of astaxanthin esters in the microalga Haematococcus pluvialis (CCAP 34/7) have been investigated using a statistical approach employing response surface methodology (RSM). The culture conditions required for optimal growth and carotenogenesis in this alga are very different. Of particular importance is the photon flux density: for growth the optimum is 50–60 μmol m⁻² s⁻¹ whereas the optimum for astaxanthin synthesis is much higher at ∼-1600 μmol m⁻² s⁻¹. The addition of low levels of NaCl to the medium also stimulates to a small extent synthesis of astaxanthin, but photon flux density remains the overriding factor. The optimal temperature for this strain is quite low at 14–15 °C. RSM has been shown to be a rapid and effective technique leading to the optimisation of algal culture conditions. This statistical approach can be applied readily to the majority of microalgae and their products.     

小球藻的甲基磺酸乙酯诱变及产EPA的条件研究

小球藻是海水养殖系统中常用的单细胞微藻,繁殖能力强,易于规模化培养,且可合成不饱和脂肪酸EPA、DHA等多种活性物质,在医疗和保健品开发中具有很高的应用价值。目前商业化培养的藻种多从自然界中直接获得,活性物质的产量较低且藻种易退化。为了获得EPA产量更高的藻种,该研究利用0.6%EMS对小球藻进行诱变,利用尼罗红染色法进行初筛并通过单细胞分离技术得到1株突变株EC1,通过气象色谱测定其EPA产量。结果表明:与出发藻株相比,突变株EPA(二十碳五烯酸)产量提高了8.97%。根据单因素试验确定突变株生长及产EPA的合适培养条件,再通过正交试验筛选出培养条件的优化组合,表明突变藻EC1株产EPA的较适条件为NaNO_375 mg·L~-1,p H7.5,昼夜温度17~15℃,接种量为12%,在此条件下培养7d其EPA的产量可达25.38 mg·g~-1,传代实验表明突变藻株具有较好的遗传稳定性。该研究结果为进一步利用小球藻规模化生产EPA奠定了基础。     

Massive cultivation of microalgae: Results and prospects

An account is given of the development of the utilization of microalgae for food and feed with special emphasis on the advantages of algal technologies for tropical and subtropical countries. The present status of microalgae mass production is characterized with respect to technology, product properties, yields, nutrition, toxicology and economics. As a multipurpose operation, the treatment of liquid wastes with algae-bacteria systems is the most promising microalgal technology. It yields proteinaceous microbial biomass as a comparatively inexpensive by-product of the operation of high-rate algal ponds, either at the simplified rural level or at the technically more elaborate industrial level. The aspect of hard-currency saving by employing algae-bacteria systems in sewage treatment for animal feed production is stressed.     

Biological constraints in algal biotechnology

In the past decade, considerable progress has been made in developing the appropriate biotechnology for microalgal mass cultivation aimed at establishing a new agro-industry. This review points out the main biological constraints affecting algal biotechnology outdoors and the requirements for making this biotechnology economically viable. One of them is the availability of a wide variety of algal species and improved strains that favorably respond to varying environmental conditions existing outdoors. It is thus just a matter of time and effort before a new methodology like genetic engineering can and will be applied in this field as well. The study of stress physiology and adaptation of microalgae has also an important application in further development of the biotechnology for mass culturing of microalgae. In outdoor cultures, cells are exposed to severe changes in light and temperature much faster than the time scale required for the cells to acclimate. A better understanding of those parameters and the ability to rapidly monitor those conditions will provide the growers with a better knowledge on how to optimize growth and productivity. Induction of accumulation of high value products is associated with stress conditions. Understanding the physiological response may help in providing a better production system for the desired product and, at a later stage, give an insight of the potential for genetic modification of desired strains. The potential use of microalgae as part of a biological system for bioremediation/detoxification and wastewater treatment is also associated with growing the cells under stress conditions. Important developments in monitoring and feedback control of the culture behavior through application of on-line chlorophyll fluorescence technique are in progress. Understanding the process associated with those unique environmental conditions may help in choosing the right culture conditions as well as selecting strains in order to improve the efficiency of the biological process.     

Productivity and photosynthetic efficiency ofSpirulina platensis as affected by light intensity,algal density and rate of mixing in a flat plate photobioreactor

The effect of the rate of mixing on productivity of algal mass in relation to photon flux density and algal concentration was quantitatively evaluated in cultures ofSpirulina platensis grown in a newly designed flat-plate photobioreactor. Special emphasis was placed on elucidating the principles underlying efficient utilization of high photon flux density for maximal productivity of algal-mass. Whereas the rate of mixing exerted little influence on productivity and photosynthetic efficiency in cultures of relatively low algal density, its effect became ever more significant as algal concentration was increased. Maximal mixing-enhanced cell concentrations and productivity of biomass were obtained at the highest light intensity used. At each level of incident light intensity, maximum productivity and photosynthetic efficiency could be achieved only when algal concentration and mixing rates were optimized. The higher the intensity of the light source, the higher became the optimal culture density, highest algal concentrations and productivity of biomass being obtained at the highest light intensity used. The rate of mixing required careful optimization: when too low, maximal productivity resulting from the most efficient utilization of light could not be obtained. Too high a rate of mixing resulted in cell damage and reduced output rate.Author for correspondence     

Effects of shear stress on microalgae – A review

Cultivation of microalgae requires consideration of shear stress, which is generated by operations such as mixing, circulation, aeration and pumping that are designed to facilitate mass and heat transfer as well as light distribution in cultures. Excessive shear stress can cause increased cell mortality, decreased growth rate and cell viability, or even cell lysis. This review examines the sources of shear stress in different cultivation systems, shear stress tolerance of different microalgal species and the physiological factors and environmental conditions that may affect shear sensitivity, and potential approaches to mitigate the detrimental effects of shear stress. In general, green algae have the greatest tolerance to shear stress, followed by cyanobacteria, haptophytes, red algae, and diatoms, with dinoflagellates comprising the most shear-sensitive species. The shear-sensitivity of microalgae is determined primarily by cell wall strength, cell morphology and the presence of flagella. Turbulence, eddy size, and viscosity are the most prominent parameters affecting shear stress to microalgal cells during cultivation.     

A cryptic intracellular green alga in <Emphasis Type="Italic">Ginkgo biloba</Emphasis>: ribosomal DNA markers reveal worldwide distribution

Intracellular symbioses involving eukaryotic microalgae and a variety of heterotrophic protists and invertebrates are widespread, but are unknown in higher plants. Recently, we reported the isolation and molecular identification of a Coccomyxa-like green alga from in vitro cell cultures of Ginkgo biloba L. This alga resides intracellularly in an immature “precursor” form with a nonfunctional chloroplast, implying that algal photosynthetic activity has no role in this endosymbiosis. In necrotizing Ginkgo cells, precursors evolved into mature algae, proliferated, and were liberated into the culture medium after host cell bursting. In the present paper we demonstrate by molecular methods a worldwide distribution of the alga in planta. Endosymbiont-specific sequences of ribosomal DNA could be traced in Ginkgo tissues of each specimen examined from different geographic locations in Europe, North America, and Asia. The Ginkgo/Coccomyca association represents a new kind of intracellular, vertically inherited symbiosis. Storage bodies, probably of lipid nature, present in the cytoplasm of each partner suggest a possible involvement of the endosymbiont in metabolic pathways of its host.     

Modification of an algal culture medium for sustained growth of a saxitoxin-producing isolate of Alexandrium minutum

In order to study saxitoxin (STX) production bymicro-algae in the laboratory, a defined algal culture medium which supports optimum growth over a longtime-period is a requirement. In the development of such a medium, a number of modifications were made to a standard algal culture medium (GP) and growth of a STX-producing isolate of Alexandrium minutum in the different formulations was assessed by measuring maximum cell densities and mean generation times (MGT). All experiments were carried out under controlled conditions in an aerobic atmosphere with increased CO₂. Whilst maximum cell densities in the different modifications were similar, the MGT was significantly shortened by the addition of Tris buffer and the trace metals strontium, selenium and molybdenum. Replacement of natural with artificial seawater and removal of soil extract did not adversely affect algal growth. Five of the six media formulations supported the growth of A. minutumover a 9-month period. This revised version was published online in August 2006 with corrections to the Cover Date.     

A laboratory-scale system for mass culture of freshwater microalgae in polyethylene bags

A laboratory-scale system for mass culture of microalgae in 8-, 20- and 40-L polyethylene bags, was designed. Bags are 16.8 cm diameter and 52 cm (8-L bags), 112 cm (20-L) or 224 cm (40-L) length. The system was tested successfully with two freshwater microalgae,Ankistrodesmus falcatus andScenedesmus incrassatulus, cultured in Bold's Basal medium (prepared with either deionized or tap water). The procedure described is simple, reliable and practical, and enables a very cost-effective production of freshwater microalgae to satisfy any laboratory requirements, and when quantities demanded for special applications can not be met by the standard laboratory culture procedures.     

Simple growth chambers for culturing microalgae with precision at different temperatures and irradiance

Profiles describing the relationships between growth, irradiance and temperature are important in the evaluation of microalgae and cyanobacteria for biomass production, as well as for their general characterization. To get correct results culturing chambers with plane parallel optical faces are needed. Only these give a defined light climate in the culture, but such devices are not found commercially. We here describe the testing of thermostated growth chambers with 200-mL culture volumes, easily constructed from disposable 144-mm diameter plastic Petri dishes. Their properties were examined by growing Spirulina platensis concurrently in 15 parallel cultures under identical conditions of illumination and temperature, showing efficient and reproducible growth between them. The chambers are naturally also very suitable for growing microalgae in general. This revised version was published online in August 2006 with corrections to the Cover Date.     

Removal of nitrogen and phosphorus from wastewater using microalgae immobilized on twin layers: an experimental study

Removal of nitrogen and phosphorus from wastewater by two green microalgae (Chlorella vulgaris and Scenedesmus rubescens) was investigated using a novel method of algal cell immobilization, the twin-layer system. In the twin-layer system, microalgae are immobilized by self-adhesion on a wet, microporous, ultrathin substrate (the substrate layer). Subtending the substrate layer, a second layer, consisting of a macroporous fibrous tissue (the source layer), provides the growth medium. Twin-layers effectively separate microalgae from the bulk of their growth medium, yet allow diffusion of nutrients. In the twin-layer system, algae remain 100% immobilized, which compares favourably with gel entrapment methods for cell immobilization. Both microalgae removed nitrate efficiently from municipal wastewater. Using secondary, synthetic wastewater, the two algae also removed phosphate, ammonium and nitrate to less than 10% of their initial concentration within 9 days. It is concluded that immobilization of C. vulgaris and S. rubescens on twin-layers is an effective means to reduce nitrogen and phosphorus levels in wastewater.     

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.     

Comparing the response of Antarctic,tropical and temperate microalgae to ultraviolet radiation (UVR) stress

The response of Antarctic, tropical and temperate microalgae of similar taxonomic grouping to ultraviolet radiation (UVR) stress was compared based on their growth and fatty acid profiles. Microalgae of similar taxa from the Antarctic (Chlamydomonas UMACC 229, Chlorella UMACC 237 and Navicula UMACC 231), tropical (Chlamydomonas augustae UMACC 246, Chlorella vulgaris UMACC 001 and Amphiprora UMACC 259) and temperate (Chlamydomonas augustae UMACC 247, Chlorella vulgaris UMACC 248 and Navicula incerta UMACC 249) regions were exposed to different UVR conditions. The cultures were exposed to the following conditions: PAR (42 μmol photons m⁻² s⁻¹), PAR + UVA (854 μW cm⁻²) and PAR + UVA + UVB (117 μW cm⁻²). The cultures were subjected to UVA doses of 46.1, 92.2 and 184.4 J cm⁻² and UVB doses of 6.3, 12.6 and 25.2 J cm⁻² by varying the duration of their exposure (1.5, 3 and 6 h) to UVR during the light period (12:12 h light-dark cycle). UVA did not affect the growth of the microalgae, even at the highest dose. In contrast, growth was adversely affected by UVB, especially at the highest dose. The dose that caused 50% inhibition (ID₅₀) in growth was used to assess the sensitivity of the microalgae to UVB. Sensitivity of the microalgae to UVB was species-dependent and also dependent on their biogeographic origin. Of the nine microalgae, the Antarctic Chlorella was most tolerant to UVB stress (ID₅₀ = 21.0 J cm⁻²). Except for this Chlorella, the percentage of polyunsaturated fatty acids of the microalgae decreased in response to high doses of UVB. Fatty acid profile is a useful biomarker for UVB stress for some microalgae. Presented at the 6th Meeting of the Asian Pacific Society of Applied Phycology, Manila, Philippines.     

Commercial production of microalgae: ponds, tanks, tubes and fermenters

The commercial culture of microalgae is now over 30 years old with the main microalgal species grown being Chlorella and Spirulina for health food, Dunaliella salina for β-carotene, Haematococcus pluvialis for astaxanthin and several species for aquaculture. The culture systems currently used to grow these algae are generally fairly unsophisticated. For example, Dunaliella salina is cultured in large (up to approx. 250 ha) shallow open-air ponds with no artificial mixing. Similarly, Chlorella and Spirulina also are grown outdoors in either paddle-wheel mixed ponds or circular ponds with a rotating mixing arm of up to about 1 ha in area per pond. The production of microalgae for aquaculture is generally on a much smaller scale, and in many cases is carried out indoors in 20–40 l carboys or in large plastic bags of up to approximately 1000 l in volume. More recently, a helical tubular photobioreactor system, the BIOCOIL™, has been developed which allows these algae to be grown reliably outdoors at high cell densities in semi-continuous culture. Other closed photobioreactors such as flat panels are also being developed. The main problem facing the commercialisation of new microalgae and microalgal products is the need for closed culture systems and the fact that these are very capital intensive. The high cost of microalgal culture systems relates to the need for light and the relatively slow growth rate of the algae. Although this problem has been avoided in some instances by growing the algae heterotrophically, not all algae or algal products can be produced this way.