Page 293 - Algae

276 Algae: Anatomy, Biochemistry, and Biotechnology

(e.g., Pavlova spp. and Isochrysis sp. [T.ISO]) and cryptomonads are relatively rich in DHA (0.2–
11%), whereas eustigmatophytes (Nannochloropsis spp.) and diatoms have the highest percentages
of AA (0–4%). Chlorophytes (Dunaliella spp. and Chlorella spp.) are deficient in both C20 and
C22 PUFAs, although some species have small amounts of EPA (up to 3.2%). Because of this
PUFA deficiency, chlorophytes generally have low nutritional value and are not suitable as a
single species diet. Prasinophyte species contain significant proportions of C 20 (Tetraselmis spp.)
or C 22 (Micromonas spp.) — but rarely both. In late-logarithmic phase, prymnesiophytes, on
average, contain the highest percentages of saturated fats (33% of total fatty acids), followed by
diatoms and eustigmatophytes (27%), prasinophytes and chlorophytes (23%), and cryptomonads
(18%). The content of saturated fats in microalgae can also be improved by culturing under high
light conditions.
The content of vitamins can vary between microalgae. Ascorbic acid shows the greatest variation,
that is, 16-fold (1–16 mg g 21 d. w.). Concentrations of other vitamins typically show a two- to four-
21
21
fold difference between species that is, b-carotene 0.5–1.1 mg g , niacin 0.11–0.47 mg g ,
21 21 21
a-tocopherol 0.07–0.29 mg g , thiamin 29 to 109 mgg , riboflavin 25–50 mgg , pantothenic
21 21 21 21
acid 14–38 mgg , folates 17–24 mgg , pyridoxine 3.6–17 mgg , cobalamin 1.8–7.4 mgg ,
21 21 21
biotin 1.1–1.9 mgg ,retinol 2.2 mgg and vitamin D ,0.45 mgg .To put thevitamin
content of the microalgae into context, data should be compared with the nutritional requirements
of the consuming animal. Unfortunately, nutritional requirements of larval or juvenile animals that
feed directly on microalgae are, at best, poorly understood. However, the requirements of the adult
are far better known and, in the absence of information to the contrary, will have to serve as a
guide for the larval animal. These data suggest that a carefully selected, mixed-algal diet should
provide adequate concentrations of the vitamins for aquaculture food chains.
The amino acid composition of the protein of microalgae is very similar between species and
relatively unaffected by the growth phase and light conditions. Further, the composition of essential
amino acids in microalgae is very similar to that of protein from oyster larvae (Crassostrea gigas).
This indicates that it is unlikely the protein quality is a factor contributing to the differences in nutri-
tional value of microalgal species. Sterols, minerals, and pigments also may contribute to nutri-
tional differences of microalgae.
A common procedure during the culture of both larval fish and prawns is to add microalgae
(i.e., “green water”) to intensive culture systems together with the zooplankton prey. Addition of
the microalgae to larval tanks can improve the production of larvae, though the exact mechanism
of action is unclear. Theories advanced include (a) light attenuation (i.e., shading effects), which
have a beneficial effect on larvae, (b) maintenance of the nutritional quality of the zooplankton
(c) an excretion of vitamins or other growth-promoting substances by algae, and (d) a probiotic
effect of the algae. Most likely, the mechanism may be a combination of several of these pos-
sibilities. Maintenance of NH 3 and O 2 balance has also been proposed, though this has not been
supported by experimental evidence. The most popular algae species used for green water appli-
cations are N. oculata and T. suecica. More research is needed on the application of other micro-
algae, especially those species rich in DHA, to green water systems. Green water may also be
applied to extensive outdoor production systems by fertilizing ponds to stimulate microalgal
growth, and correspondingly, zooplankton production, as food for larvae introduced into the
ponds.
For a long time, animals such as sheep, cattle, and horses that lived in coastal areas have eaten
macroalgae, especially in those European countries where large brown macroalgae were washed
ashore. Today the availability of macroalgae for animals has been increased with the production
of macroalgae meal: dried macroalgae that has been milled to a fine powder. In the early 1960s,
Norway was among the early producers of macroalgae meal, using Ascophyllum nodosum, a macro-
alga that grows in the eulittoral zone so that it can be cut and collected when exposed at low tide.
France has used Laminaria digitata, Iceland both Ascophyllum and Laminaria species, and the
U.K., Ascophyllum.