Bioprospecting for oil producing microalgal strains: evaluation of oil and biomass production for ten microalgal strains

Microalgae have the ability to grow rapidly, synthesize and accumulate large amounts (approximately 20-50% of dry weight) of lipids. A successful and economically viable algae based oil industry depends on the selection of appropriate algal strains. In this study ten species of microalgae were prospected to determine their suitability for oil production: Chaetoceros gracilis, Chaetoceros mulleri, Chlorella vulgaris, Dunaliella sp., Isochrysis sp., Nannochloropsis oculata, Tetraselmis sp., Tetraselmis chui, Tetraselmis tetrathele and Thalassiosira weissflogii. The study was carried out in 3 L glass flasks subjected to constant aeration and controlled artificial illumination and temperature at two different salinities. After harvesting, the extraction of oil was carried out using the Bligh and Dyer method assisted by ultrasound. Results showed that C. gracilis presented the highest oil content and that C. vulgaris presented the highest oil production.     

Closed photobioreactors for production of microalgal biomasses

Microalgal biomasses have been produced industrially for a long history for application in a variety of different fields. Most recently, microalgae are established as the most promising species for biofuel production and CO(2) bio-sequestration owing to their high photosynthesis efficiency. Nevertheless, design of photobioreactors that maximize solar energy capture and conversion has been one of the major challenges in commercial microalga biomass production. In this review, we systematically survey the recent developments in this field.     

Anaerobic digestion of microalgal biomass with ultrasonic disintegration

The use of photosynthetic microalgae for nutrient removal and biofuel production has been widely discussed. Anaerobic digestion of waste microalgal biomass to produce biogas is a promising technology for bioenergy production. However, the methane yield from this anaerobic process was limited because of the hard cell wall of Chlorella vulgaris. The use of ultrasound has proven to be successful at improving the disintegration and anaerobic biodegradability of Chlorella vulgaris. Ultrasonic pretreatment in the range of 5–200 J ml⁻¹ was applied to waste microalgal biomass, which was then used for batch digestion. Ultrasound techniques were successful and showed higher soluble COD at higher applied energy. During batch digestion, cell disintegration due to ultrasound increased in terms of specific biogas production and the degradation rate. Compared to the untreated sample, the specific biogas production was increased in the ultrasound-treated sample by 90% at an energy dose of 200 J ml⁻¹. For the disintegrated samples, volatile solids reduction was also increased according to the energy input and degradation. These results indicate that the hydrolysis of microalgal cells is the rate-limiting step in the anaerobic digestion of microalgal biomass.     

Effect of ethylene and 1-methylcyclopropene on chlorophyll catabolism of broccoli florets

Branchlets of broccoli (Brassica oleracea L.) were used to examine ethylene-stimulated chlorophyll catabolism. Branchlets treated with: 1) air (CK); 2) 1 µL·L^–1 1-methylcyclopropene (1-MCP) for 14 hr at 20 °C; 3) 1000 µL·L^–1 ethylene (C₂H₄) for 5 hr at 20 °C; or 4) 1-MCP then C₂H₄, were stored in the dark at 20 °C for up to 3 d. Chlorophyll (Chl) content and branchlet hue angle decreased during the storage period and 1-MCP treatment delayed this change. Chl degradation in broccoli was accelerated by exposure to C₂H₄, especially for Chl a. Prior treatment with 1-MCP prevented degreening stimulated by C₂H₄. Lipoxygenase activity was not altered by any of the treatments, however, 1-MCP with or without ethylene resulted in reduced activity of chlorophyllase (Chlase) and peroxidase (POD). Exposure to C₂H₄ stimulated Chlase activity and extended the duration of high POD activity. Treatment with 1-MCP followed by C₂H₄ resulted in reduced POD activity and delayed the increase in Chlase activity. The results suggest chlorophyll in broccoli can be degraded via the POD – hydrogen peroxide system. Exposure to C₂H₄ enhances activity of Chlase and extends the duration of high POD activity, and these responses may accelerate degreening. Treatment with 1-MCP delays yellowing of broccoli, an effect that may be due to the 1-MCP-induced reduction in POD and Chlase activities.     

Bioprospecting for hyper-lipid producing microalgal strains for sustainable biofuel production

Global petroleum reserves are shrinking at a fast pace, increasing the demand for alternate fuels. Microalgae have the ability to grow rapidly, and synthesize and accumulate large amounts (approximately 20-50% of dry weight) of neutral lipid stored in cytosolic lipid bodies. A successful and economically viable algae based biofuel industry mainly depends on the selection of appropriate algal strains. The main focus of bioprospecting for microalgae is to identify unique high lipid producing microalgae from different habitats. Indigenous species of microalgae with high lipid yields are especially valuable in the biofuel industry. Isolation, purification and identification of natural microalgal assemblages using conventional techniques is generally time consuming. However, the recent use of micromanipulation as a rapid isolating tool allows for a higher screening throughput. The appropriate media and growth conditions are also important for successful microalgal proliferation. Environmental parameters recorded at the sampling site are necessary to optimize in vitro growth. Identification of species generally requires a combination of morphological and genetic characterization. The selected microalgal strains are grown in upscale systems such as raceway ponds or photobireactors for biomass and lipid production. This paper reviews the recent methodologies adopted for site selection, sampling, strain selection and identification, optimization of cultural conditions for superior lipid yield for biofuel production. Energy generation routes of microalgal lipids and biomass are discussed in detail.     

Engineered fatty acid catabolism for fuel and chemical production

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Recovery of microalgal biomass and metabolites: process options and economics

Commercial production of intracellular microalgal metabolites requires the following: (1) large-scale monoseptic production of the appropriate microalgal biomass; (2) recovery of the biomass from a relatively dilute broth; (3) extraction of the metabolite from the biomass; and (4) purification of the crude extract. This review examines the options available for recovery of the biomass and the intracellular metabolites from the biomass. Economics of monoseptic production of microalgae in photobioreactors and the downstream recovery of metabolites are discussed using eicosapentaenoic acid (EPA) recovery as a representative case study.     

絮凝微生物采收微藻的作用机制研究进展

微藻广泛分布于自然界,其易培养,生长快且应用价值高,普遍用于生物燃料、医学原料、优质食品源及畜牧养殖业等。近年来,通过对光生物反应器改造设计、高产藻株筛选、代谢通路基因改造等方法实现微藻产量的提高,而在微藻处理的下游过程的研究与创新不足,特别是微藻采收已经成为其产业发展的瓶颈。本文综述了絮凝法在微藻采收中的作用,重点讨论了絮凝微生物在微藻采收中的作用,并对絮凝微生物对微藻的絮凝机制进行广泛探讨,为絮凝微生物采收微藻提供理论依据。     

Advances in direct transesterification of microalgal biomass for biodiesel production

Microalgae biomass is becoming an interesting raw material to produce biodiesel, where several approaches in the transesterification process have been applied such as different catalysts, different acyl acceptors, incorporation of co-solvents and the different operational conditions. However there are some drawbacks that must be solved before any industrial application could be intended. The main problems are related with the high water content of the biomass (over 80 %) and the several process steps involved in biodiesel production such as: drying, cell disruption, oils extraction, transesterification and biodiesel refining. In comparison to other alternatives, the use of direct transesterification could be a suitable alternative since cell disruption, lipids extraction and transesterification are carried out in one step, with a direct reaction of oil-bearing biomass to biodiesel. This process could be applied even using biomass with high water content, and its efficiency could be improved by the incorporation of promising technologies such as microwave or ultrasonication that can enhance the mass transfer rate between immiscible phases, simultaneously diminishing the reaction time. However, it is still necessary to decrease the costs of these technologies so they can be suitable alternatives in future industrial applications.     

Using chlorophyll fluorescence to monitor yields of microalgal production

A monitoring system for microalgal production was developed over a 12:12-h light:dark cycle at a steady state of growth to test the feasibility of estimating carbon production using the fluorescence method. An empirical linear relationship between the electron transport rate, based on the fluorescence method, and carbon assimilation, based on the conventional carbon method, was successfully obtained. The results demonstrate the relevance of the electron transport rate in determining carbon production of microalgae under steady-state growth conditions.     

The potential of microalgal biomass production for biotechnological purposes using wastewater resources

The utilization of microalgae for wastewater treatment represents an attractive opportunity for wastewater valorization through the use of the produced biomass. Five strains of microalgae were isolated from municipal wastewater and grown in autoclaved and non-autoclaved effluent at 30 °C and 150 μmol photons m^?2 s^?1 to study biomass production, nutrient removal, and the biochemical composition of the biomass. All strains reached high biomass productivity (35.6 to 54.2 mg dry weight L^?1 day^?1) within 4 days of batch culturing. In this period, ammonium-N and phosphate were reduced by more than 60 and 90 %, respectively. The high growth rate (0.57 to 1.06 day^?1) ensured a rapid removal of nutrients and thereby a short retention time. By the fourth day of cultivation, the algal biomass contained 32 % protein, but only 11 % lipids and 18 % carbohydrates. It was found that the biomass was a suitable raw material for biogas production by anaerobic digestion. Biodigestion of obtained biomass was simulated by employing the Aspen HYSYS modeling software, resulting in methane yields comparable to those found in the literature. The elemental analysis of the algal biomass showed very low concentrations of pollutants, demonstrating the potential of use of the digestate from biodigestion as a bio-fertilizer.     

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.     

Integration of microalgal cultivation system for wastewater remediation and sustainable biomass production

Untreated wastewaters have been a great concern and can cause major pollution problems for environment. Conventional approaches for treating wastewater involve tremendous capital cost, have major short comings and are not sustainable. Microalgae culture offers an interesting step for wastewater treatment. Microalgae serve the dual purpose of phycoremediation along with the production of potentially valuable biomass, which can be used for several purposes. The ability of microalgae to accumulate nitrogen, phosphorus, heavy metals and other toxic compounds can be integrated with wastewater treatment system to offer an elegant solution towards tertiary and quaternary treatment. The current review explores possible role of microalgal based wastewater treatment and explores the current progress, key challenges, limitations and future prospects with special emphasis on strategies involved in harvesting, boosting biomass and lipid yield.     

Nutrient removal and biomass production: advances in microalgal biotechnology for wastewater treatment

Owing to certain drawbacks, such as energy-intensive operations in conventional modes of wastewater treatment (WWT), there has been an extensive search for alternative strategies in treatment technology. Biological modes for treating wastewaters are one of the finest technologies in terms of economy and efficiency. An integrated biological approach with chemical flocculation is being conventionally practiced in several-sewage and effluent treatment plants around the world. Overwhelming responsiveness to treat wastewaters especially by using microalgae is due to their simplest photosynthetic mechanism and ease of acclimation to various habitats. Microalgal technology, also known as phycoremediation, has been in use for WWT since 1950s. Various strategies for the cultivation of microalgae in WWT systems are evolving faster. However, the availability of innovative approaches for maximizing the treatment efficiency, coupled with biomass productivity, remains the major bottleneck for commercialization of microalgal technology. Investment costs and invasive parameters also delimit the use of microalgae in WWT. This review critically discusses the merits and demerits of microalgal cultivation strategies recently developed for maximum pollutant removal as well as biomass productivity. Also, the potential of algal biofilm technology in pollutant removal, and harvesting the microalgal biomass using different techniques have been highlighted. Finally, an economic assessment of the currently available methods has been made to validate microalgal cultivation in wastewater at the commercial level.     

Simultaneous flue gas bioremediation and reduction of microalgal biomass production costs

A flue gas originating from a municipal waste incinerator was used as a source of CO₂ for the cultivation of the microalga Chlorella vulgaris, in order to decrease the biomass production costs and to bioremediate CO₂ simultaneously. The utilization of the flue gas containing 10–13% (v/v) CO₂ and 8–10% (v/v) O₂ for the photobioreactor agitation and CO₂ supply was proven to be convenient. The growth rate of algal cultures on the flue gas was even higher when compared with the control culture supplied by a mixture of pure CO₂ and air (11% (v/v) CO₂). Correspondingly, the CO₂ fixation rate was also higher when using the flue gas (4.4 g CO₂ l⁻¹ 24 h⁻¹) than using the control gas (3.0 g CO₂ l⁻¹ 24 h⁻¹). The toxicological analysis of the biomass produced using untreated flue gas showed only a slight excess of mercury while all the other compounds (other heavy metals, polycyclic aromatic hydrocarbons, polychlorinated dibenzodioxins and dibenzofurans, and polychlorinated biphenyls) were below the limits required by the European Union foodstuff legislation. Fortunately, extending the flue gas treatment prior to the cultivation unit by a simple granulated activated carbon column led to an efficient absorption of gaseous mercury and to the algal biomass composition compliant with all the foodstuff legislation requirements.     

Two-phase systems: potential for in situ extraction of microalgal products

Algae are currently used for production of niche products and are becoming increasingly interesting for the production of bulk commodities, such as biodiesel. For the production of these goods to become economically feasible, production costs will have to be lowered by one order of magnitude. The application of two-phase systems could be used to lower production costs. These systems circumvent the costly step of cell harvesting, whilst the product is extracted and prepared for downstream processing. The mechanism of extraction is a fundamental aspect of the practical question whether two-phase systems can be applied for in situ extraction, viz, simultaneous growth, product formation and extraction, or as a separate downstream processing step. Three possible mechanisms are discussed; 1) product excretion 2) cell permeabilization, and 3) cell death. It was shown that in the case of product excretion, the application of two-phase systems for in situ extraction can be very valuable. With permeabilization and cell death, in situ extraction is not ideal, but the application of two-phase systems as downstream extraction steps can be part of a well-designed biorefinery process. In this way, processing costs can be decreased while the product is mildly and selectively extracted.Thus far none of the algal strains used in two-phase systems have been shown to excrete their product; the output has always been the result of cell death. Two-phase systems can be a good approach as a downstream processing step for these species. For future applications of two-phase in situ extraction in algal production processes, either new species that show product excretion should be discovered, or existing species should be modified to induce product excretion.     

Photoautotrophic cultivating options of freshwater green microalgal Chlorococcum humicola for biomass and carotenoid production

Photoautotrophic cultivation of Chlorococcum humicola was performed in batch and continuous modes in different cultivating system arrangements to compare biomass and carotenoids’ concentration and their productivities. Batch result from stirred tank and airlift photobioreactors indicated the positive effect of increasing light intensity on growth and carotenoid production, whereas the finding from continuous cultivation indicated that carotenoid enhancement preferred high light intensity and nitrogen-deficient environment. The highest biomass (1.31?±?0.04?g?L^?1) and carotenoid (4.59?±?0.06?mg?L^?1) concentration as well as the highest productivities, 0.46?g?L^?1 d^?1 for biomass and 1.61?mg?L^?1 d^?1 for carotenoids, were obtained when maintaining high light intensity of 10 klx, BG-11 medium and 2% (v/v) CO₂ simultaneously, while the highest carotenoid content (4.84?mg?g^?1) was associated with high light intensity and nitrogen-deficient environment, which was induced by feed-modified BG-11 growth medium containing nitrate 20 folds lower than the original medium. Finally, the cultivating system arranged into smaller stirred tank photobioreactors in series yielded approximately 2.5 folds increase in both biomass and carotenoid productivities relative to using single airlift photobioreactor with equivalent working volume and similar operating condition.     

Evaluation of microalgal consortia for treatment of primary treated sewage effluent and biomass production

The present investigation was aimed towards analyzing the potential of consortia of native filamentous microalgal strains (MC2), native unicellular microalgal strains (MC3), and selected microalgae from germplasm (MC1) in terms of nutrient removal, water quality improvement, and biomass production using primary treated sewage water. Highest NO₃-N (90 %) and PO₄-P (97.8 %) removal was obtained with MC2-inoculated sewage water. Highest decrease in total dissolved solids to 806 from 1,120 mg L^?1 and highest increase in dissolved oxygen of 9.0 from 0.4 mg L^?1 were obtained using MC2-inoculated sewage water on the sixth day. The biomass production was also highest in MC2 (1.07 g L^?1) followed by MC1 and MC3 (0.90 and 0.94 g L^?1, respectively) on the sixth day. The consortium of filamentous strains from native environment not only proved promising in nutrient removal efficiency but also led to enhanced biomass. The present study highlighted the utility of such a consortium for sewage wastewater treatment and the promise of sewage water as a growth medium for biomass production.     

Two-stage heterotrophic and phototrophic culture strategy for algal biomass and lipid production

A two-stage heterotrophic and phototrophic culture strategy for algal biomass and lipid production was studied, wherein high density heterotrophic cultures of Chlorellasorokiniana serve as seed for subsequent phototrophic growth. The data showed growth rate, cell density and productivity of heterotrophic C.sorokiniana were 3.0, 3.3 and 7.4 times higher than phototrophic counterpart, respectively. Hetero- and phototrophic algal seeds had similar biomass/lipid production and fatty acid profile when inoculated into phototrophic culture system. To expand the application, food waste and wastewater were tested as feedstock for heterotrophic growth, and supported cell growth successfully. These results demonstrated the advantages of using heterotrophic algae cells as seeds for open algae culture system. Additionally, high inoculation rate of heterotrophic algal seed can be utilized as an effective method for contamination control. This two-stage heterotrophic phototrophic process is promising to provide a more efficient way for large scale production of algal biomass and biofuels.