Microalgae vary significantly in the composition of their constituents. These differences may reflect genetic differences as well as culture conditions and the growth stage at harvest (Brown et al., 1997; Becker, 2013b). In the case of marine or freshwater species, it has been noted that the growth of those fed with mixtures of several algal species is often superior to that obtained by feeding only one species, probably because a particular alga may lack a nutrient that another may contain (Yamaguchi, 1996). Thus, it has been recommended to feed animals with mixtures of carefully selected microalgae for optimal growth (Becker, 2013b).
Microalgae are usually rich in protein, which may amount to more than 60% of DM. Spirulina (Arthospira) contains a high amount of protein, between 55 and 77% of DM (Garofalo, 2011). As noted above, there are considerable variations due to genetic or culture conditions. For instance, some Chlorella strains contain less than 30% protein in the DM, whereas regular Chlorella exceed 50% protein. The protein content was found to be more susceptible to medium-induced variation than the other cellular constituents (Becker, 2013b).
The amino acid composition of microalgal proteins is quite similar between species, and relatively unaffected by intrinsic and extrinsic factors (Becker, 2013b). For example, cultures grown over a range of light intensities were identical in amino acid composition, as were those grown in stationary phase and logarithmic-phase cultures (Brown et al., 1993). In general, aspartic acid and glutamic acid occur in the highest concentrations, and cysteine, methionine, tryptophan, and histidine occur in the lowest concentrations (Becker, 2013b). Spirulina protein contains all essential (for humans) amino acids, though reduced amounts of methionine, cysteine, and lysine when compared to animal proteins. It is superior to a typical plant protein, including that of legumes (Garofalo, 2011).
The quality of the algal lipids is of prime importance to the nutritional value of microalgae in aquaculture. Fatty acids from microalgae may be efficiently transferred to higher trophic levels (e.g. to fish larvae) via zooplankton. Polyunsaturated fatty acids (PUFAs), in particular eicosapentaenoic acid (EPA, C20:5 n-3), arachidonic acid (AA, C20:4 n-6), and docosahexaenoic acid (DHA, C22:6 n-3) are of major importance in the evaluation of the nutritional composition of an algal species. C18:2 n-6 and/or C18:3 n-3 (α-linolenic acid, ALA) are essential for many freshwater fish. C20 PUFAs can form longer chains more efficiently in freshwater fish than in marine fish (Becker, 2013b). Generally, the fatty acid content in microalgae shows systematic differences according to the taxonomic group, although there are always differences between species from the same algal class (Becker, 2013b).
Most microalgal species have moderate to high percentages of EPA. Diatoms, eustigmatophytes, cryptomonads, rhodophytes, and some prymnesiophytes (Pavlova spp.) are all rich sources of EPA (7-34%) (Volkman et al., 1993). Many representatives from these classes have been used successfully as feed in larval culture (Brown et al., 1989).
Prymnesiophytes (e.g., Pavlova spp. and Isochrysis sp.) and cryptomonads are relatively rich in DHA, whereas eustigmatophytes (Nannochloropsis spp.) and diatoms have the highest percentages of AA.
Chlorophytes (Dunaliella spp. and Chlorella spp.) are deficient in both C20 and C22 PUFAs, although some species have small amounts of EPA. Because of this PUFA deficiency, chlorophytes generally have low nutritional value and are not suitable when used alone in the diet (Brown et al., 1989).
Cryptomonads and prymnesiophytes are relatively rich in DHA, whereas eustigmatophytes, rhodophytes, and diatoms are highest in AA.
Prasinophyte species contain significant proportions of C20 and C22 PUFAs.
Prasinophyte species such as Tetraselmis spp. have been used successfully for prawn and mollusc culture (Brown et al., 1992b).
Significant levels of C18:2 n-6 and C18:3 n-3 are found in most microalgal groups, except diatoms and eustigmatophytes which contain very low levels (Volkman et al., 1993).
Prymnesiophytes contain the highest proportions of saturated fats (33%), followed by diatoms and eustigmatophytes (27%), chlorophytes and prasinophytes (23%), and cryptomonads (18%) (Volkman et al., 1993).
Spirulina is rich in C18:3 n-6 (γ-Linolenic acid), and also contains C18:2 n-6 (α-linoleic acid), and C18:4 n-3 (stearidonic acid).
Large variations do occur in fatty acid composition. For instance, in spirulina of different origins, palmitic acid, C16:0, ranged from 18 to 39%; oleic acid, C18:1, from 3 to 20%; linoleic acid, C18:2 n-6, from 6 to 16%; and γ-linolenic acid from 4 to 23% (Diraman et al., 2009).
Different strategies are applied to improve the PUFA content in microalgae. Manipulation of processing conditions such as light intensity, nutrient status, or temperature allows the modulation of the lipid composition and consequent optimization of their overall yield and productivity (Becker, 2013b).
With the exception of the cyanobacteria spirulina (Arthrospira) and Aphanizomenon flos-aquae, most microalgae possess a relatively thick cellulosic cell wall, which poses a problem in digesting algal biomass by monogastric species. Treatments are necessary to disrupt the cell wall, and make the algal protein nutritionally accessible (see Processes in the "Description" tab). The cell wall of spirulina does not represent a barrier to proteolytic enzymes, and this alga can be digested by monogastrics without previous physical or chemical rupture of the cell wall (Becker, 2013a).
Algae generally provide excess or adequate amounts of vitamins to support normal growth in aquacultural species (Brown et al., 1999). However, a comparison of the vitamin content of 5 microalgae species (Tetraselmis suecica, Isochrysis galbana, Pavlova lutheri, Skeletonema costatum and Chaetoceros calcitran) showed that though all five were rich in most vitamins, they also had low concentrations of at least one or more (De Roeck-Holtzhauer et al., 1991). Therefore, mixed algal diets may be necessary to meet the vitamin requirements of maricultured species or zooplankton. Transfer of vitamins between trophic levels is important for fish larvae and late larval/early juvenile crustaceans that are reared on algal-fed zooplankton (Becker, 2013b).
The levels of ascorbic acid (vitamin C) in 11 microalgal species during logarithmic and stationary growth phases were found to range 15-fold and to be unrelated to algal class. Values ranged from 1.1 g/kg DM (Thalassiosira pseudonana) to 16 g/kg (Chaetoceros muelleri). Many of the species had different levels of ascorbic acid between logarithmic and stationary phases (Brown et al., 1992a). These values were above the requirement of mariculture species (Becker, 2013b). A similar study on the riboflavin content of 6 species found that the concentrations at the logarithmic phase ranged from 20 mg/kg DM (Thalassiosira pseudonana) to 40 mg/kg (Isochrysis sp.). Riboflavin increased in all species in the stationary growth phase, sometimes double or threefold. Chaetoceros gracilis contained more riboflavin (106 mg/kg) than all other species (48-61 mg/kg) in the stationary phase (Brown et al., 1994). Levels in all species were in excess of the dietary requirements of maricultured species (Becker, 2013b).
Microalgae are an important source of pigments, notably carotenoids such as ß-carotene, lutein and astaxanthin. Spirulina (Arthrospira) contains many pigments, including chlorophyll a, ß-carotene, echinenone, myxoxanthophyll, zeaxanthin, canthaxanthin, diatoxanthin, 3'-hydroxyechinenone, ß-cryptoxanthin, oscillaxanthin, plus the phycobiliproteins C-phycocyanin and allophycocyanin (Leema et al., 2010). A survey of the carotenoid content of cultures of 15 Chlorophycean microalgae found that lutein was the most abundant carotenoid in all strains except one. The highest lutein levels were found in Chlorella fusca SAG 211-8b, Chlorococcum citriforme, Neospongiococcum gelatinosum, and Muriellopsis sp. Violaxanthin and ß-carotene were found in virtually all the strains tested, although at lower levels than those of lutein. The most important factors that affect lutein content in microalgae are temperature, irradiance, pH, availability and source of nitrogen, salinity (or ionic strength), and the presence of oxidizing substances (or redox potential) (Becker, 2013b).
The green microalga Haematococcus pluvialis represents the richest biological source of astaxanthin, and is the only source for microalgal astaxanthin. Haematococcus hematocysts contain 1.5-3% of the dry biomass after the reddening phase. After harvesting, the biomass is dried and cracked to fracture the thick, hard cell wall of the cysts, thus ensuring maximum bioavailability (Becker, 2013b). Astaxanthin is used as a pigmentation source in aquaculture, as a vitamin A precursor in fish, as well as an enhancer of the immune system of fish and shrimp, for maximum survival and growth. Natural micro-algal astaxanthin has shown superior bio-efficacy over the synthetic form (Ravishankar et al., 2012).
Microalgae can contain substantial amounts of minerals. Spirulina, for example, is a rich source of potassium, and also contains calcium, chromium, copper, iron, magnesium, manganese, phosphorus, selenium, sodium, and zinc (Garofalo, 2011).