Microalgae have already been evaluated in several studies for their application as feed ingredient in aquaculture and due to their high contents of polyunsaturated fatty acids (PUFA) they are considered as valuable lipid source in fish feed (Haas et al., 2016; Sarker et al., 2016). However, a factor limiting their application in aquaculture feeds is their species-specific cell wall which could hinder nutrient accessibility. Processing methods for disruption can improve digestibility and optimize microalgae application (Teuling et al., 2019; Tibbetts et al., 2017). The purpose of this study was to obtain digestibility data for the microalgae species Isochrysis galbana and Tetraselmis chui treated with different processing methods and to evaluate their potential as a lipid source in feeds for rainbow trout.
Material and Methods
The two microalgae species Isochrysis galbana (25.6 % lipid) and Tetraselmis chui (13.3 % lipid) were tested in five test diets which consisted of 70 % reference diet and 30 % algae meal as pelleted feed. For each algae species two meals were produced via freeze-drying after harvesting and via freeze-drying and additional homogenization via a centrifugal mill (300 µm). For T. chui a third test meal was produced through supplementation of enzymes (mannanase, xylanase and glucanase) before freeze-drying. Titanium dioxide was used as inert marker in the diets for the determination of the apparent digestibility coefficients (ADC). Rainbow trout with ~ 500 g were held in a recirculating aquaculture system in triplicates of trial diets. Faeces were collected via manual stripping 28 h after each feeding event for two weeks during which the fish were fed with 1.5 % of their body weight per day.
Results and Discussion
The ADCs of dry matter and protein were not significantly different between the diets. The crude lipid ADC was lower for the I. galbana diets (77.2 %) compared to the T. chui diets (87.3 %) and the control diet (91 %). Regarding the digestible lipid content of the diets the control diet showed the highest value (15.3 %) followed by I. galbana diets (14.8 %) and T. chui diets with the lowest values (12 %). The lipid digestibility of Isochrysis was found to be negatively influenced by higher dietary saturated fatty acid (SFA) contents (Caballero et al., 2002) but showed no impairment of feed intake and growth performance in European sea bass (Tibaldi et al., 2015). T. chui showed a higher digestibility for nutrients and fatty acids compared to I. galbana. Examining total fatty acid groups within algae species, a higher digestibility for monounsaturated fatty acids (MUFA) was observed for T. chui followed by PUFA and SFA whereas in I. galbana PUFA showed the highest digestibility verifying an efficient accessibility and the potential as applicable lipid source. The homogenization after freeze-drying had no beneficial effect on the digestibility of any nutrient or fatty acid of the microalgae.
Both algae species showed a promising applicability as a multiple nutrient source in aquafeeds for rainbow trout providing important fatty acids and high protein availability. Mechanical or enzyme-treating processing however did not increase the nutrient digestibility of the here used microalgae and sole freeze-drying sufficiently provided nutrient accessibility in this trial. Further research in applying other processing methods to improve digestibility of microalgae is necessary to fully exploit the microalgae potential as well as research in nutrient usability as growth performance in fish.
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