The isolation and identification of new microalgal strains producing oil and carotenoid simultaneously with biofuel potential
Introduction
Microalgae, widely distributed, and with a longer evolutionary history than terrestrial plants, show a rich diversity among their more than 200,000 species (Guiry and Guiry, 2014). However, 30,000 species have been studied but so far not fully exploited (Mata et al., 2010). Bioprospecting of microalgae has been carried out from ecologically diverse habitats such as the deep seas (Boeuf and Kornprobst, 2009) and polluted waters (Sterrenburg et al., 2007). Such diverse habitats may harbour distinct isolates with unique properties and multiple applications. During the past few years, microalgae have been extensively explored for biofuel and for bioactive compounds. In recent years, unique strains have been isolated from a range of habitats in the tropics including both aquatic (lakes, streams, and backwaters) and terrestrial habitats. Such extremes as very high temperatures and prolonged exposure to intense light found in the tropics have conferred on the extremophiles some distinctive physiological properties (Gouveia et al., 2009). Since the early 1970s, bioprospecting of microalgae (such as green algae, cyanobacteria, and diatoms) has been attempted for various uses, and their ability to consume carbon dioxide during photosynthesis and to act as a sustainable source of biofuels has led to increasing attention in recent years (Jones and Mayfield, 2011). Both lipid and biomass content are equally important for achieving higher lipid productivity (Yen et al., 2013). Microalgae lipids are divided in two main categories, those used as biofuel (with 14–20 carbon chains) and those used as food (containing 20 carbon chains) because they are rich in essential nutrients that are not synthesized by higher eukaryotic organisms (Jacob-Lopes et al., 2015). Deriving multiple products such as lipids and high-value by-products from the same biomass in one growth cycle is one way to make process economically sustainable (Nobre et al., 2013). Biologically active compounds derived from microalgae have attracted increasing industrial attention (De Morais et al., 2015). The diverse gene pool of microalgae still remains to be fully explored for production of bioactive compounds, particularly using the biorefinery approach. So far, the market for carotenoids is primarily based on β-carotene and astaxanthin whereas lutein have not been similarly exploited. The importance of lutein and astaxanthin from microalgae has grown significantly in recent years. Lutein has multiple applications: it prevents some degenerative diseases of the eye; it is used as a food dye and as a food additive in aquaculture; and it can help prevent cardiovascular diseases as well as age-related macular degeneration. Astaxanthin is used in the pigmentation of salmonids, ornamental fish, and in the poultry industry (Lorenz and Cysewski, 2000). The market for these bioactive compounds continues to expand. Thus, new microalgal species that can produce these or other useful compounds is a continuing area of research.
It is against this background that the present study sought to compare twenty two microalgal isolates for identifying the strains most suitable for biofuel and carotenoids production. The approach was to isolate a number of microalga from a range of diverse habitats and to characterise the isolates using 18S rDNA analysis. The selection criteria were the ability to produce (1) lipids with fatty acid methyl ester (FAME) profiles suitable for biofuels and (2) carotenoids, which are valuable compounds that can be readily separated from oils and would potentially offset the cost of biofuel production.
Section snippets
Collection, isolation, and purification of microalgal isolates
A total of two hundred isolates were collected from different habitats – aquatic and terrestrial rocky– in Asia, North America, and the Middle East (Table 1). The aquatic habitats included freshwater, chemically polluted river water, and backwater. Twenty two microalgal isolates from Chlorellaceae and Scenedesmaceae were selected based on their molecular taxonomy and assessed for their potential to produce biofuel and carotenoids. The isolates from these two families were priorities for
Isolation and identification of microalgae
The study aimed at to find species with the potential to produce a range of useful product, particularly carotenoids and biofuel. Two hundred samples were collected from aquatic and terrestrial rocky habitats from Asia, North America, and the Middle East (Table 1). Scanning electron microscopy imaging have been conducted in the selected species which showed the lipids and biofuel potential as depicted in Supplementary Fig. 1A–D. To further characterise, phylogenetic analysis using 18S rDNA
Conclusions
The overall results from the present study confirm that some of the isolates S. bijugus, Coelastrella sp., Chlorella sp., and Auxenochlorella protothecoides showed the potential to produce significant quantities of multiple products. The designed primer sets provided better insights into biodiversity and helped in finding new species that would have been hard to find through microscopic observations alone. S. bijugus was the most productive among the four promising microalgal isolates selected.
Acknowledgement
The first author acknowledges fellowship provided by Deakin University and infrastructure support extended by The Energy and Resources Institute. The assistance provided Ms. Deeprajni for GC–MS, and by Ms. Shikha and Mr. Chandrakant for SEM is duly acknowledged. Mr Pavan Sunkireddy and Mr Sachin Rastogi contribution for assisting in collections is duly acknowledged. The support extended by Dr R K Pachauri (Director-General, TERI) and by Prof Jane Dan Hollander (Vice Chancellor, Deakin
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