Regular articleEnhanced biomass and lutein production by mixotrophic cultivation of Scenedesmus sp. using crude glycerol in an airlift photobioreactor
Introduction
Two important aspects – limited fossil resources and increased CO2 emission – motivate the search for alternate fuel and techniques to produce food supplements without utilizing irrigational land. Possibly, an exquisite option to address this issue is the cultivation of microalgae, which are rich in proteins, carbohydrates and lipids. Algae are feedstock for production of biofuel as well as numerous high value products such as carotenoids and omega-3-fatty acids [1]. Algal carotenoids are used as pigment, vitamin supplement and feed additive. Carotenoids are responsible for red, orange and yellow color in plant leaves and flowers [2]. They are derived from 40-carbon polyene chain, which impart distinct molecular structure, chemical properties, and light absorbing characteristics. The commercially important carotenoids are beta carotene, xanthophylls, astaxanthin, fucoxanthin and lutein. Among these, lutein is a commercially important carotenoid. It is widely used as food grade dye as well as feed additive in aquaculture/poultry farming [3]. Lutein acts against mascular degeneration and plays a pivotal role in maintaining visual ability. Lutein is the most ubiquitous carotenoid in the human retina and prevents certain types of cancers besides protecting against UV induced damage. High consumption of lutein lowers the risk of developing cardiovascular diseases [4].
Though lutein is currently produced from petals of marigold plants, the presence of significant amount lutein in microalgae makes it as an attractive source. It should be noted that lutein extracted from plant sources is in esterified form whereas microalgae lutein is in non-esterified form. The byproducts from microalgae are protein hydrolysates, lipids and other pigments [5]. Among different lutein accumulating microalgae, Scenedesmus sp. is mostly opted for lutein production. Importantly, Scenedesmus sp. has high lutein content and is well suited for commercial production. It has very tough cell wall that protects against mechanical stress and hence, homogenous and high quality biomass can be obtained during industrial production [6]. However, commercial system for lutein production from microalgae is in progress [7,8].
Internal loop airlift photo-bioreactor (ALR) is an attractive option for Scenedesmus sp. cultivation and is superior to traditional stirred tank fermentors. However, the drawbacks of photoautotrophic cultivation include unfavorable environmental conditions, slow growth rate, and limitation of light due to mutual shading [9]. An alternative is the mixotrophic/heterotrophic cultivation in closed reactors. An important aspect of this mode of cultivation is the utilization of organic substances as the sole energy source with minimal light requirements [10,11]. Notably, organic carbon sources contribute for 80 % of the overall cost of the cultivation. But the utilization of low cost organic sources may eliminate this issue. Crude glycerol (CG) is a major byproduct obtained during biodiesel production and is a possible source for value added compounds production through mixotrophic microalgae cultivation [12].
The objective of this study was to evaluate the fed-batch mixotrophic cultivation of Scenedesmus sp. in a 3 L ALR containing synthetic wastewater (SW) supplemented with stepwise addition of CG for enhanced production of algal biomass and lutein. Prior to reactor studies, shake flask experiments were designed by following Taguchi orthogonal array (OA) experimental design technique [[13], [14], [15]] to ascertain the effect of N/P ratio, CG, methanol, and potassium hydroxide concentration on algal growth. The obtained data were analyzed through analysis of variance (ANOVA) to determine the statistically significant factors and their optimum levels [16].
Section snippets
Algal culture conditions and growth
Scenedesmus sp. (CAS-173) obtained from Algae Culture Collection Centre, University of Madras, Chennai, India was maintained in Bold´s Basal Medium (BBM) at 30 °C under continuous illumination of fluorescence light. The stock cultures were maintained on liquid as well as agar slants of BBM through regular sub culturing. The cultures were maintained at 23 ± 10₀C at 2000 lx light intensity under 16:8 light-dark cycles.
Synthetic wastewater (SW)
Synthetic wastewater was prepared by dissolving: MgSO4.7H2O(0.1 g/l), CaCl2
Characteristics of crude glycerol
CG was found to contain glycerol (743.6 ± 0.014 mg/l), soap (62.6 ± 0.36 mg/l), and low amount of total nitrogen (TN) and phosphorus (TP). The composition of CG is presented in Table 3. The constituents of CG confirm that it can serve as a suitable source for the mixotrophic cultivation of Scenedesmus sp.
Biomass productivity
The mean effects of selected factors at assigned levels are shown in Table 4. The difference between the average value of each factor at level 2 and level 1 (L2-L1) represents the relative
Conclusions
This study demonstrated the fed-batch mixotrophic cultivation of Scenedesmus sp. in a 3 L ALR containing SW supplemented with CG. Taguchi optimization of shake flask experiments was carried to determine the influence of operating parameters. The relative influence of the factors on biomass productivity was found to be in the order of N/P ratio > CG > Methanol concentration > KOH concentration. Among the four factors investigated, CG showed a positive effect on lutein productivity. The results
CRediT authorship contribution statement
Rajendran Lakshmidevi: Conceptualization, Methodology, Formal analysis, Writing - original draft. Nagarajan Nagendra Gandhi: Supervision. Karuppan Muthukumar: Conceptualization, Methodology, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
One of the authors (R Lakshmidevi) gratefully acknowledges the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for granting her the Senior Research Fellowship (CSIR-SRF).
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2022, Applied EnergyCitation Excerpt :Higher growth rates along with CO2 fixation were observed in mixotrophic cultivation [26]. Biomass and lutein productivity of Scenedesmus sp. in a mixotrophic cultivation using crude glycerol and found that when the crude glycerol was supplied at a rate of 6 g/L, a high biomass productivity of 768 mg/L/day and lutein productivity of 3.59 mg/L/day [27]. Also, Growing Chlorella vulgaris in those conditions yielded a CO2 fixation of 290 mg/L/day [25].
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2022, Renewable and Sustainable Energy ReviewsCitation Excerpt :Organic carbon sources up-regulated carotenogenesis genes in general, and lutein synthesis genes in particular (see Section 2.1). Also, mixotrophic cultures employ low-moderate light intensities due to the presence of both organic and inorganic carbon sources, which are most suitable for lutein production [99,105,137]. This overcomes the limitations in phototrophic cultures, where optimal light supply is challenging both technically and economically [138].