Evaluación de la calidad dietética de microalgas mediante el estudio del balance energético de artemia franciscana

Nutritional values of microalgal diets employed in aquaculture have traditionally been evaluated by comparing some biological parameters related to survival, growth, reproduction, and also comparing the energy budget of the organisms fed such diets calculated from food consumption and metabolic costs. There are few examples of comparative studies of these two different approaches to the problem. One objective of this study was to evaluate four microalgal diets employing, as the test organism, the brine shrimp Artemia franciscana fed a diet of one of four microalgae, by comparing the growth rate calculated from the results of the energy budget with the results measured during the first 5-7 days of post-embryonic development with the same diets. The diets were: Chlorella capsulata (used only in the growth experiments), Chaetoceros muelleri, Isochrysis sp. , or Nannochloropsis oculata, which were grown in semi-continuous culture. Growth was measured daily during the growth experiments and the scope for growth or expected growth was calculated from the energy budget for days 1, 3, 5, and 7, using specimens grown with each experimental diet. Each experiment served to obtain food consumption rates, rates of oxygen consumption, and ammonia-nitrogen excretion, as well as food absorption efficiency with each diet. In the growth experiments, survival was 85-90% with C. muelleri, Isochrysis sp. , and C. capsulata, but only 31-45% with N. oculata. The larval development index was a better indicator of food quality than survival, and showed that the four diets can be ranked according to the larval stage reached with the respective diets, as: C. muelleri (L-16) > Isochrysis sp. (L-15) > C. capsulata (L-8) > N. oculata (L-4). Growth in size and weight gave the same ranking and showed that the final weight of the organisms fed C. muelleri is close to 200% of that obtained with Isochrysis sp. The other two diets gave weight increments one order of magnitude lower or close to zero. Scope for growth gave the same ranking for the three diets used in the experiments, showing that both methods gave comparable results and may be used to define the most adequate diet for the subject organism. However, the results showed that the energy budget cannot be used to model scope for growth because of the wide discrepancies between real and scope for growth. For N. oculata and Isochrysis sp. , the expected growth rate was higher than that measured in the growth experiments, and the discrepancy was particularly high with the first of these diets, whereas, with C. muelleri, the scope for growth was 20% lower than that measured in culture. These differences cannot be ascribed to energy costs of molting, which were not used in the calculations of the energy budget, because the most important discrepancy was related to the diet producing the lowest growth and development. The difference between energy budget calculations and real growth was evident even for the C. muelleri diet, which gave the best results. Back-calculations showed that food absorption efficiency may be the source of these discrepancies.