Protein content, amino acid composition and nitrogen-to-protein conversion factors of Ulva rigida and Ulva capensis from natural populations and Ulva lactuca from an aquaculture system, in South Africa

The South African abalone aquaculture industry is expanding and there is a lack of information on the nutritional profiles of the seaweeds that are used as feed. The current study quantified the protein contents of Ulva rigida and Ulva capensis from natural populations and Ulva lactuca from a commercial, integrated seaweed/abalone aquaculture system. Three methods of protein quantification were used: the Bradford method, crude protein, and specific nitrogen to protein (N-Prot) conversion factors, and results were compared to each other. The results showed that values obtained with the traditional conversion factor of 6.25 were on average higher than Bradford values by factors of 64.1 % in U. capensis, 77.1 % in U. rigida and 58.9 % in U. lactuca. This pattern is in line with other published work on seaweed and microalgae. Analyses of amino acid composition showed that aspartic acid was the most abundant amino acid and that these species were also rich in glycine and alanine but poor in histidine, methionine and cysteine. N-Prot factors were as follows: U. capensis, 5.58; U. rigida, 5.12 and U. lactuca, 5.65. An average N-Prot factor of 5.45 provides a more accurate estimate of the protein content of the Ulva species studied than the traditional factor of N x 6.25.