A novel approach for microalgal cell disruption and bioproducts extraction using non-thermal atmospheric plasma (NTAP) technology and chitosan flocculation
Graphical abstract
Introduction
Microalgae are well known as a sustainable source of high value bioproducts such as proteins, carbohydrates, pigments and lipids [1]. Microalgal cells contain lipid volumes up to 200 times greater than other plants [2]. The proteins extracted from microalgae can be utilized in food, feed, health and chemical industries [3], whereas microalgae lipids and carbohydrates are promising feedstocks for production of biofuel and degradable bio-plastics [4], [5], [6]. However, the rigid cell wall and membrane surrounding microalgal cells act as a barrier in the way of reaching the intracellular compounds which makes cell disruption process a critical step in the downstream bioprocessing [7].
Until now, several cell disruption methods have been developed which can be divided into physical processes such as high pressure homogenization [8], ultrasonication [9], electric flotation [10] and microwave heating [11] and, chemical processes such as using solvents, salts, nanoparticles, surfactants, ozone bubbles, and enzymes [12], [13], [14], [15]. Recently, Kadir et al. [12] applied ozone bubbles for simultaneous harvesting and cell disruption of microalgae. They were able to increase microalgae harvesting efficiency up to 55% (from 12,5% to 68%) through charge neutralization and successfully improved cell disruption (1.9 times higher than the control sample). However, these conventional methods are facing major drawbacks which significantly limit their large-scale applicability. For example, physical processes are generally operated under extreme temperature and pressure conditions which makes them highly energy intensive and costly [16], [17]. On the other hand, chemical processes suffer from high selectivity towards cell-wall composition and therefore are not applicable to all microalgae species [14]. Moreover, most of the chemicals used in these processes are harmful to the environment and hardly biodegradable [17]. In light of these drawbacks, there is an urgent need to develop a new technology for microalgal cell disruption that enables fast and efficient extraction of high value compounds.
Non-thermal atmospheric pressure plasma (NTAP) which is an advanced oxidation process (AOP) could be considered as a potential alternative [18]. Plasma is a partially ionized gas produced by applying a high-voltage electrical discharge to a neutral gas that leads to the generation of a variety of oxidative and chemically active species, such as radicals (e.g., O, H, and OH) reactive molecules (e.g., O3 and H2O2), ultraviolet radiation, shockwaves, and electrohydraulic cavitation [18], [19], [20]. To date, most of the research concerning the application of plasma on microalgae has been conducted for the purpose of algal bloom prevention [20], [21], [22]. However, NTAPs could contribute to efficient disruption of microalgal cell wall and result in the release of high value bioproducts. It is hypothesized that the strong electric field, shockwaves and reactive species generated in the discharge region together with various other physical and chemical effects of. Recently, Zhang et al. [23] used high voltage electrical discharge (HVPD) for bio-molecules extraction. However, their research has a serious limitation which is the utilization of dried microalgal biomass (5% dry matter) that requires a time and energy consuming preliminary process to harvest microalgal biomass from its culture medium and dry it up. Therefore, in this research for the first time we applied NTAP directly to the culture medium for microalgal cell disruption without any pretreatment. However, after the release of extracted compounds as a result of NTAP treatment, another step is required to recover these bioproducts from culture medium for further processing.
Coagulation/flocculation process which is a well-known technique for microalgal biomass harvesting [24], [25], [26] can be considered for bioproducts recovery from the culture medium as one of the most effective and economical methods [27]. So far, several flocculants have been applied for microalgae harvesting including inorganic metal salts, synthetic polymers and natural polymers [28], [29], [30], [31]. Among different flocculants, chitosan as a natural polymer and a biodegradable cationic flocculant has been proven to be an effective and practical substitute for chemical flocculants in the microalgae harvesting process [31], [32], [33]. According to Chen et al. [27], chitosan polymers possess a positive charge which attracts the negatively charged compounds extracted from microalgae such as carbohydrate, protein and lipid and destabilizes them through a phenomena called charge neutralization and inter-particle bridging mechanism [27]. Considering the non-toxicity and bio-degradability features of chitosan, these extracted compounds could be easily collected for further purification [32], [33], [34], [35]. Therefore, in this study natural chitosan has been selected as the flocculant for recovery of the released bioproducts such as protein, carbohydrate and lipid.
In this work, we applied non-thermal atmospheric pressure plasma (NTAP) discharge to disrupt the cell wall of Cholorella Sorokiniana sp. and to extract high value intracellular bioproducts such as proteins, carbohydrates and lipids. The effect of different plasma voltages on microalgal cell disruption was investigated. Moreover, chitosan was employed to recover the released high value compounds from culture medium. Fourier transform infrared (FTIR) spectra, scanning electron microscopy (SEM) and confocal laser scanning microscope (CLSM) images were used to analyze the flocculation process and disruption of microalgae cell walls. Finally, the consumed energy of the plasma process for cell disruption and its energy efficiency for protein, carbohydrate and lipid extraction were analyzed.
Section snippets
Microalgae cultivation and chemicals
In this study, Chlorella Sorokiniana was chosen as model microalga. The microalgae were cultivated in an 8-L hexagonal airlift flat panel photobioreactor (HAFP-PBR) described in our previous work [36] utilizing municipal wastewater as culture medium at 28 ◦C and was exposed to white fluorescent with a light intensity of 74 µmol m−2 s−1 [37]. The characterization of the wastewater medium is reported in our previous study [37]. The cell size of Chlorella Sorokiniana ranged from 10 to 15 μm.
Effect of NTAP treatment on microalgal cell disruption
The effect of non-thermal atmospheric pressure plasma (NTAP) on microalgal cell disruption and release of high value bioproducts is shown in Fig. 2. According to Fig. 2a, after applying NTAP to the microalgal suspension the concentration of microalga cells significantly decreased from 1 to 0.42, 0.30 and 0.26 mg/L under applied voltages of 8, 12 and 16, respectively after 3 h treatment. The results showed that the plasma discharge could successfully disrupt the cells of C. sorokiniana. This
Conclusions
The performance of non-thermal atmospheric plasma (NTAP) treatment on C. Sorokiniana cell wall disruption for high value bioproducts extraction was evaluated in this study. The results revealed that the plasma approach is an effective cell disruption method for extraction of 36, 24 and 41 %CDW of protein, carbohydrate and lipid respectively. NTAP was also proved to be an economical alternative to the conventional microalgal cell disruption methods, owing to its low energy consumption (less than
CRediT authorship contribution statement
Mohsen Taghavijeloudar: Conceptualization, Formal analysis, Writing – review & editing, Supervision. Behrad Farzinfar: Investigation, Writing – original draft, Validation. Poone Yaqoubnejad: Conceptualization, Writing – original draft, Methodology, Investigation. Alireza Khaleghzadeh Ahangar: Data curation, Methodology.
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
The authors would like to show gratitude to all whom shared their pearls of wisdom with us during this research. Particularly Mr. Mohammad Amin Oliaee who supported us during preparation of this article.
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