Characteristics of extracellular hydrocarbon-rich microalga Botryococcus braunii for biofuels production: Recent advances and opportunities
Introduction
Facing the challenge of fossil hydrocarbon fuels replacing with more sustainable liquid fuels produced from renewable resources, biofuels attract more and more attention. At present, biofuels, namely methane, ethanol and diesel, are characterized by lower energy content than conventional fossil fuels and higher oxygen content, detrimental to the combustion engine [1]. The hydrocarbon biofuels would be essentially the same as those currently obtained from petroleum; it would not be necessary to modify engines, pumps, or distribution networks to accommodate the new renewable liquids in the transportation sector [2]. Therefore, drop-in jet fuels need hydrocarbons similar to those obtained naturally from the fractional distillation of crude oil, characterized by un-oxygenated structures and high energy per volume. The need for hydrocarbon bio-production has also been linked to the use of an eco-friendly process, for example, by combining it with carbon dioxide (CO2) capture system [3]. Hydrocarbon-producing crops have been studied by the Nobel laureate Melvin Calvin on Euphorbia lathyris some decades ago [4]. However, energy acquired from land-based biomass productions could compete for agricultural productions. By contrast, microalgae, which may be a source of water and nutrients, can be cultivated on non arable lands and adapted for using wastewater [5].
Hydrocarbons are widely distributed in all organisms, including animals, plants, bacteria, and fungi. The share of hydrocarbons in the body of an organism is generally less than 3% of the dry weight [6], [7], [8], [9], [10]. The hydrocarbon fractions are composed of short-chain hydrocarbons, often semi-volatile organic compounds; medium-chain hydrocarbons, generally in liquid form; and long-chain hydrocarbons, the waxy fractions. These last ones are often linked to cuticle components to protect against predation or viral attacks. Whatever the organisms are, the contents of these solid hydrocarbons are generally in the range of 0.001–0.4% of the dry weight [6], [7], [8], [9], [10]. Microorganisms such as bacteria or fungi are generally hydrocarbon hypo-producers. Low contents of hydrocarbons are also found in microalgae, such as Chlorella vulgaris with 0.045% [11].
Barely few organisms produce high concentrations of liquid hydrocarbons. Among heterotrophs, the halotolerant bacteria Vibrio furnissii produce up to 60% of dry biomass as extracellular hydrocarbons, mainly alkanes from C15H32 to C24H50 [12]. This biosynthesis was shown to be associated with the culture growth [12]. However, the potential toxicity of the species hampers industrial exploitation [13], the heterotrophic micro-eukaryote, which belongs to the family Thraustochytriaceae of the class Labyrinthulomycetes, accumulates squalene up to 19.8% of dry biomass [14], [15]. However, the optimized medium of the strain 18W-13a contains peptone, yeast extract, and glucose [15], and therefore very expensive for commercial production of squalene [16]. The hyperhalophilic microalga Dunaliella salina with its high β-carotene content, up to 14% dry weight, could be looked upon as a hydrocarbon source; however, the high melting point (183 °C), unless an appropriate chemical hydrolysis, and the high value added for nutraceutical applications of this carotenoid do not argue for jetfuel applications. Another well-known hydrocarbon-rich microorganism is the microalga Botryococcus braunii, in which the hydrocarbon content could exceptionally reach up to 61% of dry biomass or more [17], [18]. To the best of our knowledge, humans do not face the risk of disease, although there might be some indirect toxic effect on fish and zooplankton in the proximity of B. braunii blooms, mainly associated with either the well-known cytotoxic effects of free fatty acids or likewise with toxic effect induced by a decrease in dissolved oxygen [19]. Then, as shown by Mendes and Vermelho [20], improvement of B. braunii cultures could take advantage of this potential allelopathy. The majority of the hydrocarbons produced by B. braunii are accumulated in an extracellular matrix, but not in the culture medium. At present, this characteristic is studied to improve hydrocarbon productivity. The extracellular location of the hydrocarbons could support a milking-like process and then the reuse of the microalgal biomass, an advantage in comparison to other microalgae that accumulate triacyglycerol in the intracellular lipid body [21], [22]. In addition, recent studies have shown that the biomass residues, obtained after hydrocarbons extraction, are compatible with co-valorization processes for either residual energy recovery or production of different kinds of valuable bioproducts according to the B. braunii strains [23], [24], [25]. Therefore, B. braunii is regarded as one of the most outstanding candidate for biohydrocarbon production through cultivation of microalgae. Here, the main characteristics of the microalga used or could be used to improve the efficiency of the hydrocarbon bioproduction are reviewed.
Section snippets
Liquid hydrocarbon fraction from B. braunii biomass
Pressure-released extracellular oil droplets [22] can be observed easily under the microscope. These hydrocarbon droplets are excreted from the matrix, where cells of B. braunii are embedded. The medium-chain hydrocarbons constitute this liquid fraction of the B. braunii hydrocarbons [26].
Hydrocarbon accumulation in an extracellular matrix
In microbial cultures, lipid secretion is not a common process in the absence of detectable cellular lysis. This attribute is shared by few microorganisms, such as the hydrocarbon bacteria V. furnissii or the microalga B. braunii; these two species differ in terms of hydrocarbons accumulating in the culture medium or in an extracellular matrix, respectively.
They are also found in intracellular lipid bodies of the microalga, which have no storage function, in contrast to triacylglycerol bodies
Biomass and hydrocarbon productivities
Recent studies are opposed to the fact that B. braunii is a very slow-growing microalga [82]. Indeed, some strains were shown to grow at specific rates up to 0.5 d−1 [46]. With biomass concentrations up to 20 g L−1, these experiments have demonstrated that this hydrocarbon-accumulating microalga could be moderately cultivated in semi-continuous PBR efficiently [72]. But these performances could still be improved to a great extent as regards those obtained with other microalgae.
The high generation
Perspectives
Recent studies have highlighted some techniques, which could use some B. braunii specific characteristics to develop a cost-effective bioproduction of hydrocarbons; indeed, one of the main bottlenecks of applying microalgae for producing the biofuel is their relatively low biomass productivities [103].
Conclusion
Reaching either a positive energy balance or a cost-competitive process for B. braunii exploitation could be anticipated on the basis of some specific characteristics [143]. Thanks to its chimiodiversity some strain selections on the basis of the hydrocarbon content and composition, with more than 60 already isolated, have already improved the proportion of some particular hydrocarbons. Thanks to the high calorific value biomass and physical properties of B. braunii oil. B. braunii oil can be
Acknowledgments
This work was supported by Carburants Alternatifs pour l’AERonautique (CAER)—Alternative fuels for the aviation industry and China Scholarship Council (CSC) by presenting a scholarship to Jian JIN (CSC number 201308430233). The authors appreciate the assistance of Emi Kusuda and Carole Broussard.
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