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.