Figure 3: Comparative TEM micrographs from the anterior (A) and posterior (B) intestinal regions of juvenile S. aurata at the end of the feeding trial. Scenedesmusfed fish (SC20 and SC39) showed longer microvilli than control-fed fish. Dietary codes: CT: control, SC12: 12 percent Scenedesmus meal inclusion, SC20: 20 percent Scenedesmus meal inclusion, SC25: 25 percent Scenedesmus meal inclusion, C39: 39 percent Scenedesmus meal inclusion (TEM bar: 2 μm). after electrophoretic separation of proteins are shown in Fig 2. The profile of the intestinal proteases seems not to be influenced by the inclusion of Scenedesmus biomass, given that all the animals showed the same number and distribution of active fractions as control-fed fish, characterised by five groups of active bands. Another important aspect to consider when using ingredients alternative to fishmeal is the presence of anti-nutritive factors that might interfere with nutrient digestion and absorption (Alarcón et al., 1999). Among the wide range of such factors, protease inhibitors are well known
as substances that can affect dietary protein utilisation. In this study, neither Scenedesmus meal (unlikely other protein sources) nor experimental feeds contained substances able to inhibit the digestive proteases of gilthead sea bream juveniles, given that inhibition never reached more than 5 percent (data not shown).
Checking effects by histological study of the intestine
In addition to digestive enzyme activities, the structure and morphology of the intestinal mucosa play a key role in nutrient absorption. TEM analysis of anterior
and posterior intestine (Fig. 3a and 3b, respectively) revealed that the inclusion of microalgae reduced microvilli length (ML), except for the SC20 group (with higher and similar ML values compared to those of the CT group in the anterior and posterior intestine, respectively) and the SC39 group (with similar and higher ML values compared to those of control fish in the anterior and posterior intestine, respectively), and that microvilli diameter in the anterior intestine was greater in Scenedesmus-fed fish. Thus, the overall effects of both results were increased absorptive surface and improved contribution of the intestinal mucosa as a physical barrier.
The inclusion of S. almeriensis increased the level of intestinal enzyme activities as well as the intestinal absorptive surface. Therefore, the combination of these effects, together with the lack of anti-nutritional factors, confirms that Scenedesmus biomass can be used as a dietary ingredient for juvenile sea bream diets, and furthermore, that an inclusion level of 20 percent is recommended according to the positive effects observed on gut functionality at such a ratio.
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The January February 2015 edition of International Aquafeed magazine