Protein content and amino acid composition of the live feed rotifer ( Brachionus plicatilis): With emphasis on the water soluble fraction

Rotifers are a commonly used live feed in fish larvae cultures. Two experiments were conducted on the rotifer Brachionus plicatilis to investigate the protein and amino acid composition. Based on the idea that soluble protein is more digestible to fish larvae, special emphasis was put on the rotifer soluble protein fraction. In experiment 1, the nitrogen to protein factor and the amino acid composition of the rotifer crude fraction and the rotifer water soluble fraction were determined in rotifers fed with yeast, oil and live algae Chlorella (65:25:15 dry weight). The rotifer soluble protein constituted 50.6% of crude protein. The nitrogen to protein factor was different in the crude fraction and in the soluble fraction, 4.46 and 3.52, respectively. The amino acid compositions of the crude and soluble fractions, however, were almost equal, despite of small but statistical significant differences in some amino acids. In experiment 2, rotifers were grown in five different diet systems: baker's yeast with cod liver oil (3.3:1 dry weight/volume, DYO), baker's yeast with Algamac 2000™ (3.5:1 dry weight, DYA), baker's yeast with live algae Chlorella (4.1:1 dry weight, DYC), Culture Selco 3000™ (DCS); baker's yeast with cod liver oil (10:1, weight/volume) with vitamin supplement and live algae Isochrysis, DCNT. On dry weight basis, total protein content was significantly ( P < 0.05) lower in rotifers from the DCNT diet system (34.4%) as compared to rotifers from the DYO diet system (41.2%). Rotifers from the other diet systems were intermediate. On wet weight basis, however, total protein content was equal. Differences, thus, probably rely on differences in lipid accumulation rather than in different protein content per individual. The absolute contents of soluble protein in rotifers were almost equal between the different diet systems, however, in terms of percent of crude protein the soluble fractions were more different, ranging from 44.28% in rotifers from the DYC diet system to 52.32% in rotifers from the DCS diet system. The rotifers from experiment 1 contained the largest free amino acid pool (5.4% of dry weight), significantly larger than in all other diet systems. The rotifers from the DCNT diet system contained a significant larger free amino acid pool (3.2% of dry weight) as compared to rotifers from the other four diet systems tested in experiment 2 (2.2–2.4% of dry weight). The amino acid compositions of the free pool and the composition of the total