Page 299 - Algae

282 Algae: Anatomy, Biochemistry, and Biotechnology

quenching action on reactive oxygen species carry intrinsic anti-inflammatory properties, PUFA
exhibit antioxidant activity and polysaccharides act as immunostimulators.
A major bottleneck in the explotation of microalgal biomass for the production of high-value
compounds is low productivity of the culture, both in terms of biomass and product formation. A
fundamental reason for this is slow cell growth rates owing to inefficient use of strong light. Fur-
thermore, most microalgal products are secondary metabolites that are produced when growth is
limited. A solution to this bottleneck could be to milk the secondary metabolites from the micro-
algae. This involves continuous removal of secondary metabolites from cells, thereby enabling the
biomass to be reused for the continuous production of high-value compounds.
Recently, a new method was developed for milking b-carotene from Dunaliella salina in a two-
phase bioreactor. In this technique, cells are first grown under normal growth conditions and then
stressed by excess light to produce larger amounts of b-carotene. At this stage, the second, biocom-
patible organic phase is added and the b-carotene is extracted selectively via continuous recircula-
tion of a biocompatible organic solvent (lipophilic compound) through the aqueous phase
containing the cells. Because the cells continue to produce b-carotene, the extracted product is con-
tinuously replaced by newly produced molecules. Therefore, the cells are continuously reused and
do not need to be grown again. In contrast to existing commercial processes, this method does not
require the harvesting, concentrating, and destroying of cells for extraction of the desired product.
Furthermore, purification of the product is simple owing to the selectivity of the extraction process.
The general application of this process would facilitate the commercialization of microalgal bio-
technology and development of microalgal products.
The properties of the cell membrane play an important role in the contact between biocompatible
lipophilic solvents and hydrophobic parts of the cell membrane might be prevented by presence of a
cell wall or hydrophilic parts of the outer membrane. Physiological properties of the cells, such as
their capacity for continuous endo- and exocytosis, might also play a role in the milking process.
Other considerations are the location and way in which the product accumulates inside the cells
and the function of that product inside the cells. A product like chlorophyll would be difficult to
extract owing to its location in thylakoid membranes and because it is bound strongly to other
cell components. The extraction of a product with a protective effect on the cells (e.g., b-carotene)
will enhance its synthesis. The milking process can be applied also to other algae and other products
besides D. salina, for example in Haematococcus pluvialis for the recovery astaxanthin, and marine
microalgae for PUFA.
In addition to its use in aquaculture (e.g., to give salmon a pink color), astaxanthin has
also been described as having nutraceutical importance related to free-radical scavenging, immu-
nomodulation, and cancer prevention. H. pluvialis can produce and accumulate astaxanthin to con-
centrations of 1–8% of the dry weight. This concentration within the cell would make milking of H.
pluvialis more successful compared with D. salina. However, cultivation of H. pluvialis is more
complex than D. salina, and productivity is lower. Furthermore, extraction, purification, and con-
centration are a heavy burden on the production cost. As the final product cost is also sensitive to
algal productivity and duration of the growth period, this process is not economically feasible at
present.
PUFAs are gaining increasing importance as valuable pharmaceutical products and ingredients
of food owing to their beneficial effect on human health. DHA (22 : 6 v3) and EPA, (20 : 5 v3), in
particular, are important in the development and functioning of brain, retina, and reproductive
tissues both in adults and infants. They can also be used in the treatment of various diseases and
disorders, including cardiovascular problems, a variety of cancers, and inflammatory disease. At
present, PUFAs are produced commercially from fish oil, but this is an insufficient source of
these products and microalgae provide an optimal lipid source of PUFAs.
The heterotrophic marine dinoflagellate Crypthecodinium cohnii has a lipid content greater
than 20% dry weight and is known for its ability to accumulate fatty acids that have a high fraction
(30–50%) of DHA. Lipids are important components of algal cell membranes but also accumulate