Page 300 - Algae

Algae and Men 283

in globules in other parts of the cells. Microalgal growth and fatty acid formation is affected by
medium composition and environmental conditions (e.g., carbon sources). Lipid production
occurs under growth-limiting conditions; during linear growth, the cells are stressed owing to nutri-
ent limitation and therefore produce more lipids. Also the concentration of DHA, hence the lipid
quality, is negatively affected by increases in lipid concentration. The highest quality lipid it
obtained when glucose is used as the carbon source, and when the cell concentration and lipid
content of the cells are the lowest.
Milking can be used also for DHA production by C. cohnii. In this process, cells are first grown
under the correct conditions for growth, after which they are stressed to produce higher concen-
trations of DHA. A biocompatible organic solvent is added during the DHA production stage to
extract the product. This process enables the production of high-quality lipid, thereby reducing
extraction and purification costs. Furthermore, higher amounts of DHA are produced by substi-
tution of extracted lipids by newly synthesized lipid, increasing the productivity of the system.
Microalgae represent also one of the most promising EPA producers, as the purification of this
PUFA from fish oil, which remains the main commercial source of EPA, involves many drawbacks.
Many Eustigmatophyceae, such as Nannochloropsis sp. and Monodus subterraneus, and Bacillar-
iophyceae species contain a considerable amount of EPA. An EPA production potential has been
found in the genus Nitzschia (especially N. alba and N. laevis). It was reported that the oil
content of N. alba was as high as 50% of cell dry weight and the EPA comprises 4–5% of the
oil. N. laevis could utilize glucose or glutamate as single substrate for heterotrophic growth, and
the cellular EPA content of the alga in heterotrophic conditions was also higher than that in photo-
autotrophic conditions suggesting that this diatom is a good heterotrophic EPA producer.
The bioprocess engineering aspect of heterotrophic EPA production by N. laevis has been
extensively studied. Major achievements include:

. Optimization of various medium components (including silicate, glucose, nitrogen sources,
salts and trace elements) and environmental factors (including pH, temperature) for the alga
culture
. Investigation of detailed physiological behavior of the alga (cell growth, nutrient consump-
tion, fatty acid compositions, etc.) by a continuous culture (the dilution rate and glucose
concentration in the feed medium were optimized in terms of EPA productivity and
glucose utilization efficiency)
. Development of high cell density and high productivity techniques, which led to an EPA
yield of 1112 mg l 21 and an EPA productivity of 174 mg l 21 day 21 , both of which are
the highest ever reported in microalgal cultures.

Microalgae produce many different types of polysaccharides, which may be a costituent of the
cell wall as in unicellular red algae as Porphyridium sp. and Rhodella sp., or be present inside the
cell, as in the Euglenophyceae. The polysaccharides of Rhodophyta are highly sulfated and consist
mainly of xylose, glucose, and galactose. These compounds selectively inhibit reverse transcriptase
(RT) enzyme of human immunodeficiency virus (HIV) and its replication in vitro.
Rodents fed with a diet supplemented with biomass and polysaccharides derived from Porphyr-
idium results in a decrease in blood cholesterol concentration (by 22% and 29%, respectively) and
triglyceride levels, increased feces weight (by 130% and 196%, respectively) and bile acid
excretion (5.1- and 3.2-fold or more). Moreover, algal biomass or polysaccharide increased the
length of both the small intestine (by 17% and 30%, respectively) and the colon (by 8.5% and
32%, respectively).
Paramylon is the term used for the reserve polysaccharide of Euglena and euglenoids in
general. Its granules appearing in various locations inside the cell; in many species they are scat-
tered throughout the cytoplasm, but others can be massed together or few, but large, and located in a
fairly constant position. Their shape and size differ markedly, and together with their distribution