Successful strains for bivalve culture included Isochrysis galbana, Isochrysis sp. (T.ISO), Pavlova lutheri, Tetraselmis suecica, Pseudoisochrysis paradoxa, Chaetoceros calcitrans, and Skeletonema costatum.
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 microalgae 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, carbohydrate 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 (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 C20 (Tetraselmis spp.) or C22 (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-1 d. w.). Concentrations of other vitamins typically show a two- to fourfold difference between species that is, β-carotene 0.5–1.1 mg g-1, niacin 0.11–0.47 mg g-1, α-tocopherol 0.07–0.29 mg g-1, thiamin 29 to 109 µg g-1, riboflavin 25–50 µg g-1, pantothenic acid 14–38 µg g-1, folates 17–24 µg g-1, pyridoxine 3.6–17 µg g-1, cobalamin 1.8–7.4 µg g-1, biotin 1.1–1.9 µg g-1, retinol ≤2.2 µg g-1 and vitamin D <0.45 µg g-1. To put the vitamin 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 nutritional value of microalgal species. Sterols, minerals, and pigments also may contribute to nutritional 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 possibilities. Maintenance of NH3 and O2 balance has also been proposed, though this has not been supported by experimental evidence. The most popular algae species used for green water applications are N. oculata and T. suecica. More research is needed on the application of other microalgae, 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 macroalga 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.
Because Ascophyllum is so accessible, it is the main raw material for macroalgae meal and most experimental work to measure the effectiveness of macroalgae meal has been done on this macroalgae. The macroalgae used for meal must be freshly cut, as drift macroalgae is low in minerals and usually becomes infected with mould. The wet macroalgae is passed through hammer mills with progressively smaller screens to reduce it to fine particles. These are passed through a drum dryer starting at 700–800°C and exiting at no more than 70°C. It should have a moisture level of about 15%. It is milled and stored in sealed bags because it picks up moisture if exposed to air. It can be stored for about a year.
Analysis shows that it contains useful amounts of minerals (potassium, phosphorus, magnesium, calcium, sodium, chlorine, and sulfur), trace elements, and vitamins. Trace elements are essential elements needed by humans and other mammals in smaller quantities than iron (approximately 50 mg/kg body weight), and include zinc, cobalt, chromium, molybdenum, nickel, tin, vanadium, fluorine, and iodine. Because most of the carbohydrates and proteins are not digestible, the nutritional value of macroalgae has traditionally been assumed to be in its contribution of minerals, trace elements, and vitamins to the diet of animals. In Norway, it has been assessed as having only 30% of the feeding value of grains.
Ascophyllum is a very dark macroalga due to a high content of phenolic compounds. It is likely that the protein is bound to the phenols, giving insoluble compounds that are not attacked by bacteria in the stomach or enzymes in the intestine. Alaria esculenta is another large brown macroalgae, but much lighter in color and in some experimental trials it has been found to be more effective than Ascophyllum meal. It is this lack of protein digestibility that is a distinct drawback to Ascophyllum meal providing useful energy content. In preparing compound feedstuffs, farmers may be less concerned about the price per kilogram of an additive; the decisive factor is more likely to be the digestibility and nutritive value of the additive.
In feeding trials with poultry, adding Ascophyllum meal had no benefit except to increase the iodine content of the eggs. With pigs, addition of 3% Ascophyllum meal had no effect on the meat yield. However, there have been some positive results reported with cattle and sheep. An experiment for 7 yr with dairy cows (seven pairs of identical twins) showed an average increase in milk production of 6.8% that lead to 13% more income. A trial involving two groups each of 900 ewes showed that those fed macroalgae meal over a 2 yr period maintained their weight much better during winter feeding and also gave greater wool production.
The results of trials reported above and in the suggested reading below leave the impression that macroalgae meal is probably only really beneficial to sheep and cattle. Certainly the size of the industry has diminished since the late 1960s and early 1970s, when Norway alone was producing about 15,000 tons of macroalgae meal annually. Nevertheless, a Web search for “macroalgae meal” shows that there are companies in at least Australia, Canada, Ireland, Norway, the U.K. and the U.S. advocating the use of macroalgae meal as a feed additive for sheep, cattle, horses, poultry, goats, dogs, cats, emus, and alpacas. The horse racing industry seems to be especially targeted. An interesting report from a U.S. university states that the immune system of some animals is boosted by feeding a particular Canadian macroalgae meal. Obviously the industry is still active, pursuing niche markets and fostering research that might lead back to further expansion.
In fish farming, wet feed usually consists of meat waste and fish waste mixed with dry additives containing extra nutrients, all formed together in a doughy mass. When thrown into the fish ponds or cages it must hold together and not disintegrate or dissolve in the water. A binder is needed; sometimes a technical grade of alginate is used. It has also been used to bind formulated feeds for shrimp and abalone. However, cheaper still is the use of finely ground macroalgae meal made from brown macroalgae; the alginate in the macroalgae acts as the binder. The binder may be a significant proportion of the price of the feed so macroalgae meal is a much better choice. However, as the trend is to move to dry feed rather than wet, this market is not expected to expand. There is also a market for fresh macroalgae as a feed for abalone. In Australia, the brown macroalgae Macrocystis pyrifera and the red macroalgae Gracilaria edulis have been used. In South Africa, Porphyra is in demand for abalone feed and recommendations have been made for the management of the wild population of the macroalgae. Pacific dulse (Palmaria mollis) has been found to be a valuable food for the red abalone, Haliotis rufescens, and development of land-based cultivation has been undertaken with a view to producing commercial quantities of the macroalgae. The green macroalgae, Ulva lactuca, has been fed to Haliotis tuberculata and Haliotis discus. Feeding trials showed that abalone growth is greatly improved by high protein content, and this is attained by culturing the macroalgae with high levels of ammonia present.
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