Introduction
Microalgae are photosynthetic microorganisms capable of converting carbon dioxide and water into organic macromolecules such as lipids, polysaccharides and proteins under light conditions (Cho et al., 2013). They can be produced year-round, depending on the climate and solar radiation, grow at very high rates, and in a wide variety of water sources (fresh, brackish water, seawater and wastewater) (Souza Silva et al., 2014). They can also be easily harvested and can be cultivated under various conditions (Ju et al., 2020). Therefore, the microalgae have an extraordinary biotechnological potential as an alternative to non-renewable resources due to production of natural substances and biomaterials that can be used in various industrial applications (Neto et al., 2013). The lipid content of microalgal cells range from 2% to 77% depending on the species’ environmental and growth conditions (Souza Silva et al., 2014).
Microalgae have high and good quality lipids, especially long-chain unsaturated fatty acids such as linolenic acid, arachidonic acid, eicosapentaenoic acid (EPA) and docosahexaenoic (DHA), has been a remarkable issue for many years. Adequate DHA intake protects against many noncommunicable diseases such as diabetes, cardiovascular and neurodegenerative diseases, cancer and schizophrenia (Wang et al., 2020). Microalgal lipids can be produced safely and stably using pure cultures, and the obtained products have potential to use in high-value-added industries, such as medicine and cosmetics (Ju et al., 2020).
Heterotrophic Crypthecodinium, Schizochytrium and Ulkeniaspecies are considered as the first commecialized species that has been used for food, feed and biodiesel production, due to their high fat content. Schizochytrium sp. is a marine algae, which is important in terms of DHA, closely related to diatoms, single-celled, thraustochytrid and commonly found in sea waters, estuaries and sediments (Borowitzka, 2013). Schizochytrium sp. is a globose and pale-yellow marine microalga that possesses a thallus thin wall. This is of great industrial interest because of producing metabolites (Ortega‑Berlanga et al., 2018). This marine organism has a relatively high fat content. Schizochytrium oil that is rich in DHA and EPA, also took its place in the market (Borowitzka, 2013).Schizochytrium is considered to be a promising alternative and excellent DHA source (Wang et al., 2020) and it is accumulated intracellular DHA accounts for 30%-40% of its total lipid content (Chang et al., 2020). There are no reports of this organism producing toxic chemicals or being pathogenic (Fedorova-Dahms et al., 2011).
Direct lipid extraction from wet biomass is not easy because algal cell walls, are mainly composed of cellulose, are very durable and difficult to break (Lee and Han, 2015). The lipid yield is mainly affected by the rigidity of the microalgal cell wall. Due to their complex structure, the tensile strength of microalgal cell wall is estimated to be higher than plant’s cell wall (Nagappan et al., 2019). Since the thick cell wall of microalgae, which prevents to obtain lipids, it is necessary to use different methods other than the traditional mechanical press to obtain high efficiency in lipid extraction. An ideal solvent plays an important role in lipid extraction (Neto et al., 2013). Solvents including hexane, chloroform, butanol, ethanol, methanol and diethyl ether have been widely reported for lipid extraction from microalgae (Nagappan et al., 2019). Fragmentation methods aim to increase the efficiency of obtaining lipids from microalgae by using mechanical and non-mechanical techniques. Mechanical techniques include compression, high-pressure homogenization, ultrasonic bath, autoclave, bead mill, microwave and magnetic stirring, while non-mechanical techniques include chemical fragmentation and osmotic shock and enzymatic hydrolysis (Neto et al., 2013; Zhang et al., 2018). To get higher lipid yield and quality with lower production costs from microbial cells, a suitable cell disruption method is required.
The objective of the present study was to increase the lipid yield ofSchizochytrium sp ., which has high amounts of valuable fat and fatty acids, but obtained low lipid yield due to the fact that the cell walls are not broken down by conventional lipid extraction methods. In addition to the classical Bligh and Dyer and Soxhlet methods; treatment with HCl as a solvent, osmotic shock as a non-mechanical process, ultrasonic homogenizer as a mechanical process and enzyme applications as a biological process were compared.