2.4. Nutritional value of micro-algae

The nutritional value of any algal species for a particular organism depends on its cell size, digestibility, production of toxic compounds, and biochemical composition. The gross composition of 16 species of micro-algae is compared in Table 2.12. Although there are marked differences in the compositions of the micro-algal classes and species, protein is always the major organic constituent, followed usually by lipid and then by carbohydrate. Expressed as percentage of dry weight, the range for the level of protein, lipid, and carbohydrate are 12-35%, 7.2-23%, and 4.6-23%, respectively.

The content of highly unsaturated fatty acids (HUFA), in particular eicosapentaenoic acid (20:5n-3, EPA), arachidonic acid (20:4n-6, ARA), and docosahexaenoic acid (22:6n-3, DHA), is of major importance in the evaluation of the nutritional composition of an algal species to be used as food for marine organisms. The fatty acid composition of 10 species of micro-algae grown under defined conditions and harvested during the log phase is presented in Fig. 2.14. Significant concentrations of EPA are present in the diatom species (Chaetoceros calcitrans, C. gracilis, S. costatum, T. pseudonana) and the prymnesiophyte Platymonas lutheri, whereas high concentrations of DHA are found in the prymnesiophytes (P. lutheri, Isochrysis sp.) and Chroomonas salina.

Micro-algae can also be considered as a rich source of ascorbic acid (0.11-1.62% of dry weight, Fig. 2.15.).

The nutritional value of micro-algae can vary considerably according to the culture conditions. For example the effect of the composition of the culture medium on the proximate composition of various species of micro-algae is demonstrated in Table 2.13. The protein content per cell, which is considered as one of the most important factors determining the nutritional value of micro-algae as feed in aquaculture, was found to be more susceptible to medium-induced variation than the other cellular constituents.

Table 2.12. Concentrations of chlorophyl a, protein, carbohydrate and lipid in 16 species of micro-algae commonly used in aquaculture (modified from Brown, 1991).

Algal class Species		Dry weight (pg.cell-1)	Chl a	Protein	Carbohydrate	Lipid
		Weight of constituent (pg.cell-1)
Bacillariophyceae
	Chaetoceros calcitrans	11.3	0.34	3.8	0.68	1.8
	Chaetoceros gracilis	74.8	0.78	9.0	2.0	5.2
	Nitzchia closterium	-	-	-	-	-
	Phaeodactylum tricornutum	76.7	0.41	23.0	6.4	10.7
	Skeletonema costatum	52.2	0.63	13.1	2.4	5.0
	Thalassiosira pseudonana	28.4	0.27	9.7	2.5	5.5
Chlorophyceae
	Dunaliella tertiolecta	99.9	1.73	20.0	12.2	15.0
	Nannochloris atomus	21.4	0.080	6.4	5.0	4.5
Cryptophyceae
	Chroomonas salina	122.5	0.98	35.5	11.0	14.5
Eustigmatophyceae
	Nannochloropsis oculata	6.1	0.054	2.1	0.48	1.1
Prasinophyceae
	Tetraselmis chui	269.0	3.83	83.4	32.5	45.7
	Tetraselmis suecica	168.2	1.63	52.1	20.2	16.8
Prymnesiophyceae
	Isochrysis galbana	30.5	0.30	8.8	3.9	7.0
	Isochrysis aff. Galbana (T-iso)	29.7	0.29	6.8	1.8	5.9
	Pavlova lutheri	102.3	0.86	29.7	9.1	12.3
	Pavlova salina	93.1	0.34	24.2	6.9	11.2
		Percentage of dry weight
Bacillariophyceae
	Chaetoceros calcitrans	11.3	3.01	34	6.0	16
	Chaetoceros gracilis	74.8	1.04	12	4.7	7.2
	Nitzchia closterium	-	-	26	9.8	13
	Phaeodactylumtricornutum	76.7	0.53	30	8.4	14
	Skeletonema costatum	52.2	1.21	25	4.6	10
	Thalassiosira pseudonana	28.4	0.95	34	8.8	19
Chlorophyceae
	Dunaliella tertiolecta	99.9	1.73	20	12.2	15
	Nannochloris atomus	21.4	0.37	30	23.0	21
Cryptophyceae
	Chroomonas salina	122.5	0.80	29	9.1	12
Eustigmatophyceae
	Nannochloropsis oculata	6.1	0.89	35	7.8	18
Prasinophyceae
	Tetraselmis chui	269.0	1.42	31	12.1	17
	Tetraselmis suecica	168.2	0.97	31	12.0	10
Prymnesiophyceae
	Isochrysis galbana	30.5	0.98	29	12.9	23
	Isochrysis aff. Galbana (T-iso)	29.7	0.98	23	6.0	20
	Pavlova lutheri	102.3	0.84	29	9.0	12
	Pavlova salina	93.1	0.98	26	7.4	12

Moreoever, the growth of animals fed a mixture of several algal species is often superior to that obtained when feeding only one algal species. A particular alga may lack a nutrient, while another alga may contain that nutrient and lack a different one. In this way, a mixture of both algal species supplies the animals with an adequate amount of both nutrients. An extensive review of the nutritional aspects of micro-algae used in mariculture of bivalve molluscs, crustaceans, and fish is presented in Brown et al. (1989).

Table 2.13. Cellular density (10⁶ cells.ml^-1) and proximate composition (pg.cell^-1) of four marine micro-algae grown in different culture media (Algal-1 is a commercial nutrient) (modified from Herrero et al., 1991)

		Cellular density	Protein	Carbohydrates	Lipids
T. suecica
	Walne	2.29	13.31	6.20	7.04
	ES	2.58	16.98	6.93	7.22
	F/2	2.38	21.75	8.37	7.92
	Algal-1	4.11	32.22	8.83	8.65
D. tertiolecta
	Walne	4.04	13.37	13.22	22.28
	ES	4.24	14.88	15.73	23.94
	F/2	4.97	13.26	17.91	23.67
	Algal-1	8.45	18.82	11.08	18.18
I. galbana
	Walne	10.11	5.17	4.28	25.95
	ES	12.09	7.23	5.21	28.38
	F/2	10.81	8.13	5.59	26.82
	Algal-1	16.15	9.57	4.28	20.68
P. tricornutum
	Walne	19.01	2.65	6.42	6.51
	ES	16.23	5.21	9.20	6.45
	F/2	24.65	3.34	6.90	5.52
	Algal-1	39.04	4.20	5.98	5.79

Figure 2.14. Fatty acid composition of 10 species of micro-algae. Relative amounts of (a) C16- and C18-polyunsaturated fatty acids (PUFA); (b) 20:5n-3 and 22:6n-3; (c) (n-3) and (n-6) PUFA. Species abbreviations are: C. CAL: Chaetoceros calcitrans; C.GRA: C. gracilis; SKEL: Skeletonema costatum; THAL: Thalassiosira pseudonana; ISO: Isochrysis sp. (Tahitian); PAV: Pavlova lutheri; DUN: Dunaliella tertiolecta; NAN: Nannochloris atomus; TET: Tetraselmis suecica; CHRO: Chroomonas salina (Volkman et al., 1989).

Figure 2.15. Ascorbic acid in microalgae harvested from logarithmic (grey filling) and stationary phase (black filling) cultures, expressed as (a) cellular levels (fg.cell^-1), (b) % dry weight, (c) concentrations (fg. mm^-3) (Brown and Miller, 1992).