Microalgae and cyanobacteria are potential foods, feeds, sources of high-value bioactive molecules and biofuels, and find tremendous applications in bioremediation and agriculture. Although few efforts have been undertaken to index the microalgal germplasm available in terms of lipid content, information on suitability of strains for mass multiplication and advances in development of methods for extraction and generating biofuel are scarce. Our review summarizes the potential of microalgae, latest developments in the field and analyzes the "pitfalls" in oversimplification of their promise in the years to come. Microalgae represent "green gold mines" for generating energy; however, the path to success is long and winding and needs tremendous and concerted efforts from science and industry, besides political will and social acceptance for overcoming the limitations. The major advantages of second generation biofuels based on microalgal systems, include their higher photon conversion efficiency, growth all around the year, even in wastewaters, and production of environment friendly biodegradable biofuels.
Biofuels derived from marine algae are a potential source of sustainable energy that can contribute to future global demands. The realisation of this potential will require manipulation of the fundamental biology of algal physiology to increase the efficiency with which solar energy is ultimately converted into usable biomass. This 'photosynthetic solar energy conversion efficiency' sets an upper limit on the potential of algal-derived biofuels. In this review, we outline photosynthetic molecular targets that could be manipulated to increase the efficiency and yield of algal biofuel production. We also highlight modern 'omic' and high-throughput technologies that might enable identification, selection and improvement of algal cell lines on timescales relevant for achieving significant contributions to future energy solutions.
Microalgae have the potential to deliver biofuels without the associated competition for land resources. In order to realise the rates and titres necessary for commercial production, however, system-level metabolic engineering will be required. Genome scale metabolic reconstructions have revolutionized microbial metabolic engineering and are used routinely for in silico analysis and design. While genome scale metabolic reconstructions have been developed for many prokaryotes and model eukaryotes, the application to less well characterized eukaryotes such as algae is challenging not at least due to a lack of compartmentalization data.
Optimization of the light conditions for biofuel production by the microalga Botryococcus braunii BOT-22 (race B) was performed using monochromatic red light. The lipid and sugar contents were approximately 40% and 20–30% of the cell dry weight, respectively, and about half of the lipids were liquid hydrocarbons. The half-saturation intensities for the production rate of lipids, hydrocarbons, and sugars were 63, 49, and 44 μmol m−2 s−1, respectively. Fluorescence microscopic images of Nile Red-stained cells showed an increased number of intracellular neutral lipid granules due to increased light intensity. After 16 days of incubation in the dark, lipid and sugar, but not hydrocarbon content decreased. Growth, metabolite production, and photosynthesis were saturated at 100, 200 and 1000 μmol m−2 s−1, respectively. These results indicate that photosynthetically captured energy is not used efficiently for metabolite production; thus, improvements in metabolic regulation may increase hydrocarbon production.
There is increasing interest in the use of microalgae as a renewable source for the production of fuels and chemicals, but improvements are needed in all steps of this process, including harvesting. A continuous microalgae harvest system was developed based on electrolysis, referred to here as a continuous electrolytic microalgae (CEM) harvest system. This innovative system combines cultivation and harvesting and enables continuous and efficient concentration of microalgae. The electrodes were subject to a polarity exchange (PE) in the middle of the operation to further improve the harvest efficiency. Use of PE, rather than conventional electro-coagulation-flotation (ECF), led to more efficient cell recovery and more uniform recovery over the entire harvest chamber. In addition, PE increased the cell growth rate and the circulated cells remained intact after harvesting.
There are two major energy and cost constraints to bulk production of single cell microalgae for biofuels or feed: expensive culture systems with high capital costs and high energy requirements for mixing and gas exchange; and the cost of harvesting using high-speed continuous centrifugation for dewatering. This report deals with the latter; harvesting by flocculation where theory states that alkaline flocculants neutralize the repelling surface charge of algal cells, allowing them to coalesce into a floc. It had been assumed that with such electrostatic flocculation, the more cells to be flocculated, the more flocculant needed, in a linear stoichiometric fashion, rendering flocculation overly expensive. Counter to theory of electrostatic flocculation, we find that the amount of alkaline flocculant needed is a function of the logarithm of cell density, with dense cultures requiring an order of magnitude less base than dilute suspensions, with flocculation occurring at a lower pH. Various other theories abound that flocculation can be due to multi-valent cross-linking, or co-precipitation with phosphate or with magnesium and calcium, but are clearly not relevant with the flocculants we used. Monovalent bases that cannot cross-link or precipitate phosphate work with the same log-linear stoichiometry as the divalent bases, obviating those theories, leaving electrostatic flocculation as the only tenable theory of flocculation with the materials used. The cost of flocculation of dense cultures with this procedure should be below $1.00/T algae for mixed calcium:magnesium hydroxides.
Ca alginate polymer fibers were developed to effectively disperse and stabilize an efficient photocatalyst such as AEROXIDE® TiO2 P25 in their matrix. The biopolymer/TiO2 fibers were prepared and tested either in the hydrogel non-porous form or in the highly porous aerogel form prepared by sc-CO2 drying. Batch photocatalytic experiments showed that the porous, Ca alginate/TiO2 fibers, exhibited high efficiency for the removal of methyl orange (MO) from polluted water. In addition, their high porosity and surface area led to high MO degradation rate which was faster than that observed not only for their non-porous analogs but also of the bulk P25 TiO2 powder. Specifically, 90% removal for 20 μM MO was achieved within 220 min for the porous sc-CO2 dried fibers while for their non-porous analogs at 325 min. The corresponding value (at 60 μM MO) for the porous sc-CO2 dried fibers was 140 min over 240 min for the AEROXIDE® TiO2 P25 as documented in the literature. Furthermore the composite alginate/photocatalyst porous fibers were combined with TiO2 membranes in a continuous flow, hybrid photocatalytic/ultrafiltration water treatment process that led to a three fold enhancement of the MO removal efficiency at 400 ml of 20 μM MO total treated volume and to dilution rather than condensation in the membrane retentate as commonly observed in filtration processes. Furthermore the permeability of the photocatalytic membrane was enhanced in the presence of the fibers by almost 20%. This performance is achieved with 26 cm2 and 31 cm2 of membrane and stabilized photocatalyst surfaces respectively and in this context there is plenty of room for the up-scaling of both membranes and fibers and the achievement of much higher water yields since the methods applied for the development of the involved materials (CVD and dry-wet phase inversion in a spinning set-up) are easily up-scalable and are not expected to add significant cost to the proposed water treatment process.
Ultrasound at 20 Hz was applied at different energy levels (Es) to treat Scenedesmus biomass, and organic matter solubilization, particle size distribution, cell disruption and biochemical methane potential were evaluated. An Es of 35.5 and 47.2 MJ/kg resulted in floc deagglomeration but no improvement in methane production compared to untreated biomass. At an Es of 128.9, cell wall disruption was observed together with a 3.1-fold organic matter solubilization and an approximately 2-fold methane production in comparison with untreated biomass. Thermal pretreatment at 80 °C caused cell wall disruption and improved anaerobic biodegradability 1.6-fold compared to untreated biomass. Since sonication caused a temperature increase in samples to as high as 85 °C, it is likely that thermal effects accounted for much of the observed changes in the biomass. Given that ultrasound treatment at the highest Es studied only increased methane production by 1.2-fold over thermal treatment at 80 °C, the higher energy requirement of sonication might not justify the use of this approach over thermal treatment.
Une expérimentation pilote est menée à l’usine Ciments Calcia de Gargenville (Yvelines) : afin de réduire les émissions de CO2, des micro-algues sont cultivées et chargées de capter une partie de ce gaz à effet de serre. Une façon de valoriser biologiquement le gaz carbonique qui est plus intéressante que le simple stockage dans le sol.
Le projet est mené par plusieurs institutions dont la prestigieuse université de Tsukuba. L’équipe a développé une plante aquatique qui absorbe la lumière du soleil et réduit la radioactivité de son environnement. La technologie utilise une propriété des phytoplanctons. Elle permet d’absorber le césium capable de décontaminer les réservoirs, les océans, les lacs ou les champs de riz. Soit toutes étendues d’eau touchées par la fusion des réacteurs de la centrale de Fukushima.
Alors que l’Ademe Bretagne lance un appel à projets «méthanisation», le conseil régional persiste à vouloir éradiquer la pollution. Pourtant le ramassage s’organise, le traitement par compostage se met en place et l’Etat pousse à la méthanisation des effluents agricoles. Bien malgré elle, la Bretagne va-t-elle développer une industrie de l’algue verte?
Une dépêche de Reuters du 5 mars dernier de N Chestney annonce que l'entreprise espagnole de gestion de l'eau Aqualia (ICI) envisage de lancer un projet de démonstrateur à l'échelle commerciale en utilisant des eaux usées pour cultiver des algues pour la production de biocarburants, ce qui pourrait alimenter 400 véhicules.
The marine microalga Isochrysis galbana was cultured under different light regimes to examine the changes in growth and fatty acid profile. We have obtained preliminary results that I. galbana cultured under white intermittent light for 24 h day− 1 shows better growth than continuous white light with light/dark (L/D) cycles of 12 h/12 h. In this study, we searched for an effective intermittent light color for the growth of I. galbana. Control cultures were grown under white continuous light, with a photon flux density at 104 μmol m− 2 s− 1 with L/D cycles of 12 h/12 h. The other cultures were grown under a photon flux density of 52 μmol m− 2 s− 1 of 24 h flashing per day, and white, red, and blue intermittent light at 10,000 Hz as L/D cycles of 50 μs/50 μs. After 6 days of cultivation, the cell density of the sample cultured under blue intermittent light was significantly higher than those of the others. The lipid contents in I. galbana were 98 mg L− 1 from the culture under constant white light and 155 mg L− 1 from the culture under blue intermittent light. Total lipids from I. galbana were separated into neutral lipids (29–35%), glycolipids (38–47%), and phospholipids (20–28%). The light condition did not affect the ratio of lipid classes or the fatty acid composition of total lipids, neutral lipids, glycolipids, or phospholipids from I. galbana. The amounts of neutral lipids, glycolipids, and phospholipids obtained from culture medium were the highest under blue intermittent light (3.27, 4.71, and 2.48 mg L− 1, respectively). The highest amounts of phospholipids and DHA were recovered from I. galbana cultured under blue intermittent light.
Conventional microbiology methods used to monitor microbial biofuels production are based on off-line analyses. The analyses are, unfortunately, insufficient for bioprocess optimization. Real time process control strategies, such as flow cytometry (FC), can be used to monitor bioprocess development (at-line) by providing single cell information that improves process model formulation and validation. This paper reviews the current uses and potential applications of FC in biodiesel, bioethanol, biomethane, biohydrogen and fuel cell processes. By highlighting the inherent accuracy and robustness of the technique for a range of biofuel processing parameters, more robust monitoring and control may be implemented to enhance process efficiency.
Excess nutrients, particularly nitrogen and phosphorus remaining in anaerobically digested liquid manure (AD) effluent, have major impacts on the environment if disposed of inappropriately. Algal cultivation, with the advantage of a faster uptake of nutrients in effluent streams, represents one of the best processes for the removal of excessive nutrients. Meanwhile, algae have also been proved as one of the most promising non-food-crop-based feedstock for biofuels production. This study applying ecological approach on an open algal cultivation system elucidated that non-filamentous green algae, especially Chlorella, were able to tolerate high nutrient loadings in a five-month cultivation; a chemically pretreated AD effluent which contained 200 g m−3 of total nitrogen and 2.5 g m−3 of total dissolved phosphorus (TDP) provided an optimal nutrient concentration for the cultivation of selected algae. Additionally, the cultivation of selected algae with optimal pretreated AD effluent in a pilot-scale semi-continuously fed raceway pond revealed a stable algal biomass productivity of 6.83 g m−2 d−1.
The rapid increase of CO2 concentration in the atmosphere combined with depleted supplies of fossil fuels has led to an increased commercial interest in renewable fuels. Due to their high biomass productivity, rapid lipid accumulation, and ability to survive in saline water, microalgae have been identified as promising feedstocks for industrial-scale production of carbon-neutral biodiesel. This study examines the principles involved in lipid extraction from microalgal cells, a crucial downstream processing step in the production of microalgal biodiesel. We analyze the different technological options currently available for laboratory-scale microalgal lipid extraction, with a primary focus on the prospect of organic solvent and supercritical fluid extraction. The study also provides an assessment of recent breakthroughs in this rapidly developing field and reports on the suitability of microalgal lipid compositions for biodiesel conversion.
Thermochemical conversion is a promising route for recovering energy from algal biomass. Two thermochemical processes, hydrothermal liquefaction (HTL: 300 °C and 10–12 MPa) and slow pyrolysis (heated to 450 °C at a rate of 50 °C/min), were used to produce bio-oils from Scenedesmus (raw and defatted) and Spirulina biomass that were compared against Illinois shale oil. Although both thermochemical conversion routes produced energy dense bio-oil (35–37 MJ/kg) that approached shale oil (41 MJ/kg), bio-oil yields (24–45%) and physico-chemical characteristics were highly influenced by conversion route and feedstock selection. Sharp differences were observed in the mean bio-oil molecular weight (pyrolysis 280–360 Da; HTL 700–1330 Da) and the percentage of low boiling compounds (bp < 400 °C) (pyrolysis 62–66%; HTL 45–54%). Analysis of the energy consumption ratio (ECR) also revealed that for wet algal biomass (80% moisture content), HTL is more favorable (ECR 0.44–0.63) than pyrolysis (ECR 0.92–1.24) due to required water volatilization in the latter technique.
An indigenous microalga Chlorella vulgaris ESP-31 grown in an outdoor tubular photobioreactor with CO2 aeration obtained a high oil content of up to 63.2%. The microalgal oil was then converted to biodiesel by enzymatic transesterification using an immobilized lipase originating from Burkholderia sp. C20. The conversion of the microalgae oil to biodiesel was conducted by transesterification of the extracted microalgal oil (M-I) and by transesterification directly using disrupted microalgal biomass (M-II). The results show that M-II achieved higher biodiesel conversion (97.3 wt% oil) than M-I (72.1 wt% oil). The immobilized lipase worked well when using wet microalgal biomass (up to 71% water content) as the oil substrate. The immobilized lipase also tolerated a high methanol to oil molar ratio (>67.93) when using the M-II approach, and can be repeatedly used for six cycles (or 288 h) without significant loss of its original activity.
A Scenedesmus sp. was cultivated in a 23-L airlift-driven raceway reactor under artificial lighting and laboratory conditions, in batch and continuous modes. In batch mode, a maximum volumetric biomass productivity of 0.085 dry g L−1 day−1 was achieved under sparging at a CO2-to-air ratio of 1%, and a maximum CO2 utilization efficiency of 33% was achieved at a CO2-to-air ratio of 0.25%. In continuous mode, the maximum volumetric biomass productivity was 0.19 dry g L−1 day−1. Biomass productivities per unit power input achieved in this reactor configuration (0.60–0.69 dry g W−1 day−1) were comparable to or better than those reported in the literature for different photobioreactor designs (0.10–0.51 dry g W−1 day−1). Based on the energy-efficient productivity and the high CO2 utilization efficiency demonstrated in this study, the proposed airlift-driven raceway design holds promise for cost-effective algal cultivation.