A question of digestion: How microalgae species affects lipid and fatty acid digestibility in rainbow trout (Oncorhynchus mykiss)

Alternative nutrient sources for fish feed are currently of high interest. Feed ingredients with suitable lipid contents and valuable amounts of essential fatty acids are especially needed to relieve the usage of fish oil and maintain high product quality of fish from aquaculture. Microalgae contain promising nutrient contents and suitable fatty acid profiles that cover dietary requirements of fish. Nutrient digestibility, however, can be hindered by their rigid cell wall. Therefore, in the presented study, the microalgae species Isochrysis galbana and Tetraselmis chui were processed by freeze-drying and centrifugal milling to evaluate their nutrient and fatty acid digestibility in fish nutrition. For T. chui, an additional treatment with enzyme supplementation was tested. Test diets contained 70% of a fish meal based reference diet and 30% of the respective microalgae meal, as well as titanium dioxide as an inert marker to calculate apparent digestibility coefficients (ADC). Rainbow trout (Oncorhynchus mykiss) were fed for 14 days and faeces were collected via manual stripping. The ADCs of Kjeldahl nitrogen (~ 64-79%), nitrogen-free extracts (~ 40-54%) and energy (~ 52-66%) were not significantly different between the algae species, but I. galbana inclusion resulted in reduced lipid digestibility (~ 57% to 77%), which was linked to the high content of saturated fatty acids. The fatty acid ADCs were higher for T. chui except for arachidonic acid (20:4 n-6) and lauric acid (12:0). Mechanical or enzyme-treating processing did not increase nutrient and fatty acid digestibility for the microalgae used in this trial, and sole freeze-drying sufficiently ensured nutrient availability. The high digestibility of nutrients and, particularly, polyunsaturated fatty acids of I. galbana and T. chui underscore their potential as fatty acid and nutrient sources in fish feed.