Dietary administration of microalgae Navicula sp. affects immune status and gene expression of gilthead seabream (Sparus aurata)
Introduction
The growth of aquaculture, in association with the intensification of production systems, has increased the demand for high-quality feedstuff, as well as economically viable, and environmentally-friendly, diets [1]. In recent years, microalgae have emerged as a very interesting natural source of new compounds with biological activity that may be used as functional ingredients [2], [3]. In aquaculture, microalgae have diverse uses, due to their rich nutritional properties [4], which include a high protein content; the capacity to synthesize all amino acids, the presence of carbohydrates and non-digestible polysaccharides (agar, carrageenan and alginate); lipids in the form of glycerol and fatty acids of the ϖ3 and ϖ6 families; and the valuable content of many essential vitamins (A, B1, B2, B6, B12, C, E, biotin, folic acid and pantothenic acid), minerals (phosphorous, zinc, iron, calcium, selenium, magnesium) and antioxidant substances [5], [6]. In addition, microalgae isolate components have immunostimulating properties in fish and other animals [7], [8], [9], [10], [11].
Marine diatoms producing various types of natural oils are very interesting subjects of study [12]. In addition, they may have superior lipid stability, compared with traditional polyunsaturated fatty acids (PUFA), because they are naturally rich in antioxidant carotenoids and vitamins, and because lipids are bioencapsulated by the algal cell wall [13]. These characteristics have led to an increase in research into potential new functional ingredients from microalgae, with the aim of providing an additional health benefit alongside the energetic and nutritional aspects of food [2], [14], [15]. There is considerable evidence showing the importance of nutritional factors, such as proteins, essential fatty acids, polysaccharides, vitamins C and E and some trace minerals, in the maintenance of normal immune functions in fish [16]. Several factors may contribute to the nutritional value of a microalga, including its size and shape and its digestibility, as well as its biochemical composition, enzymes and toxins [4].
In recent years, dietary silages in aquaculture have been studied from the point of view of offering a nutritional benefit in digestibility [17], [18]. In the ensilage process, proteins are hydrolysed by naturally present and/or added enzymes (enzymatic silage), as the process favoured by the adjustment of pH. The final result is a paste-like product, rich in protein, short-chain peptides and free amino acids [19]. Our research group recently investigated the effects of marine silages, enriched with Lactobacillus sakei 5-4, on the immune system and growth response of pacific red snapper [20]. In previous studies, a silage diet increased the body weight, and stimulated the physiological and humoral immune parameters, of fish. To our knowledge, there are few available data on the effect on fish of administration of whole (lyophilized) or processed microalgae (as silage), with regard to generating a general benefit to fish defence. In consideration of these previous data, the present study was conducted to evaluate the potential modulatory effect of the marine diatom Navicula sp. (lyophilized and silage forms) on the immune response and gene expression in gilthead seabream (Sparus aurata L.), through dietary administration. The data we obtained will contribute to the knowledge of the efficacy of using Navicula sp. as an additive to the diets of farmed fish.
Section snippets
Navicula sp.
The benthic diatom, Navicula sp. used in this study was generously provided by Dr. Manuel Pacheco from Pichilingue Microalgae Culture Center for Autonomous University of Baja California Sur (UABCS), La Paz B.C.S., México. Diatom was maintained in standard F/2 Guillard's medium [21]. The harvested microalgae were stored at 4 °C until the production was finished and then concentrated by centrifugation.
Silages
Sterile 1 l-volume glass bottles with screw cap were used for the replicate preparation of
Innate immune parameters
Leucocyte peroxidase activity was significantly higher (57.6 units 10−7 leucocytes) (P < 0.05) in fish fed SM group at week 2 compared with this found in fish from C or LM groups; no significant variation was observed at week 4. Peroxidase activity in serum not showed a significant variation with any of treatments used in this experiment. Conversely, head-kidney phagocytic cells in gilthead seabream fed SM diet for 4 weeks showed a significant increase compared with those present in fish fed C
Discussion
Over the last four decades, several microalgae species have been tested as food, but probably fewer than 20 have gained widespread use in aquaculture [28]. Microalgae must possess a number of key attributes to be useful aquaculture species; they must be of an appropriate size for ingestion, be readily digested and have a good nutrient composition [28]. Among the microalgae, diatoms are indisputably the major component of many food webs, and estimation of their seasonal abundance, fluctuations
Acknowledgements
We thank Dr. Manuel Pacheco for microalgae culture and Milton Spanopoulos for technical support. The project was funded under SAGARPA-CONACYT grant 2004-44 and Ministerio de Economía y Competitividad of Spain (AGL-2011-30381-C03-01). Dr. MA Esteban is a member of a Grupo de Excelencia de la Región de Murcia(04538/GERM/06).
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