Page 292 - Algae

Algae and Men 275

and some fish species, and for zooplankton used in aquaculture food chains. Over the last four
decades, several hundred microalgae species have been tested as food, but probably less than 20
have gained widespread use in aquaculture. Microalgae must possess a number of key attributes
to be useful aquaculture species. They must be of an appropriate size for ingestion, for example,
from 1 to 15 mm for filter feeders; 10 to 100 mm for grazers and readily digested. They must
have rapid growth rates, be amenable to mass culture, and also be stable in culture to any fluctu-
ations in temperature, light, and nutrients as may occur in hatchery systems. Finally, they must
have a good nutrient composition, including an absence of toxins that might be transferred up
the food chain.
Successful strains for bivalve culture included Isochrysis galbana, Isochrysis sp. (T.ISO),
Pavlova lutheri, Tetraselmis suecica, Pseudoisochrysis paradoxa, Chaetoceros calcitrans, and
Skeletonema costatum.
Isochrysis sp. (T.ISO), P. lutheri, and C. calcitrans are the most common species used
to feed the larval, early juvenile, and broodstock (during hatchery conditioning) stages of
bivalve molluscs; these are usually fed together as a mixed diet. Many of the strains successfully
used for bivalves are also used as direct feed for crustaceans (especially shrimp) during the early
larval stages, especially diatoms such as Skeletonema spp. and Chaetoceros spp. Benthic
diatoms such as Navicula spp. and Nitzschia are commonly mass-cultured and then settled onto
plates as a diet for grazing juvenile abalone. Isochrysis sp. (T.ISO), P. lutheri, T. suecica,or
Nannochloropsis spp. are commonly fed to Artemia or rotifers, which are then fed on to later
larval stages of crustacean and fish larvae.
Microalgal species can vary significantly in their nutritional value, and this may also change
under different culture conditions. Nevertheless, a carefully selected mixture of microalgae can
offer an excellent nutritional package for larval animals, either directly or indirectly (through
enrichment of zooplankton). Microalgae that have been found to have good nutritional properties
— either as monospecies or within a mixed diet — include C. calcitrans, C. muelleri, P. lutheri,
Isochrysis sp. (T.ISO), T. suecica, S. costatum, and Thalassiosira pseudonana. Several factors
can contribute to the nutritional value of a microalga, including its size and shape, digestibility
(related to cell wall structure and composition), biochemical composition (e.g., nutrients,
enzymes, and toxins if present), and the requirements of the animal feeding on the alga. As the
early reports demonstrated biochemical differences in gross composition between microalgae
and fatty acids, many studies have attempted to correlate the nutritional value of microalgae
with their biochemical profile. However, results from feeding experiments that have tested micro-
algae differing in a specific nutrient are often difficult to interpret because of the confounding
effects of other microalgal nutrients. Nevertheless, from examining all the literature data, including
experiments where algal diets have been supplemented with compounded diets or emulsions, some
general conclusions can be reached.
Microalgae grown to late-logarithmic growth phase typically contain 30–40% proteins, 10–
20% lipids and 5–15% carbohydrates. When cultured through to stationary phase, the proximate
composition of microalgae can change significantly; for example, when nitrate is limiting, carbo-
hydrate levels can double at the expense of protein. There does not appear to be a strong correlation
between the proximate composition of microalgae and nutritional value, though algal diets with
high levels of carbohydrate are reported to produce the best growth for juvenile oysters (Ostrea
edulis) and larval scallops (Patinopecten yessoensis), provided polyunsaturated fatty acids
(PUFAs) are also present in adequate proportions. In contrast, high dietary protein provided best
growth for juvenile mussels (Mytilus trossulus) and Pacific oysters (Crassostrea gigas). PUFAs
derived from microalgae, that is, docosahexanoic acid (DHA), eicosapentanoic acid (EPA) and
arachidonic acid (AA) are known to be essential for various larvae.
The fatty acid content showed systematic differences according to taxonomic group, although
there were examples of significant differences between microalgae from the same class.
Most microalgal species have moderate to high percentages of EPA (7–34%). Prymnesiophytes