IN-VITRO DIGESTIBILITY OF TWO MICROALGAE BY-PRODUCTS AND WHOLE CELL MICROALGAE IN AQUATIC DIETS

Aquaculture continues to be the most rapidly growing segment of agriculture and now provides over 50% of the seafood destined for human consumption.  Many of the aquacultured fish and shrimp species consume prepared feeds that contain large amounts of fish meal and oil.  The growing demand for these limited marine feedstuffs has not only increased their cost dramatically, but has also made the worlds fisheries unsustainable.  This demand has prompted the need to develop alternatives to fish meal and oil to lessen dependence on these very nutritious but finite resources.  Previous research has demonstrated that whole microalgae used as feed grade ingredients can replace a significant amount of fish meal in fish and shrimp feeds.  A major limitation with large scale use of microalgae in aquatic feeds is related to its cost relative to fish meal and current fish meal alternatives.  Economic data suggests microalgae use in aquatic feeds will initially be limited to by-products produced during the generation of biofuels.  Current by-products in the biofuel field includes microalgae in which the lipid and phosphorus has been removed for production of biofuel and nutrient recycling.

The current trial compares the proximate composition and in-vitro digestibility of whole algal biomass to that of solventless lipid extracted algae (LEA) and LEA which also has been extracted to remove phosphorus (LEA-p).  Microalgae were grown in outdoor raceways under controlled conditions at the AgriLife Mariculture facility.  Harvested microalgae biomass was lysed using a pulse field electric generator and then passed through a membrane to remove the lipid from the algal biomass.  A portion of the LEA was further processed by osmotic shock and digestion to remove phosphorus for recycle back into the algal production raceways.  In this study, fish digestibility was assessed as a percentage of nitrogen in the soluble fraction after treatment of the protein source with proteases and subsequent precipitation of undigested protein by trichloroacetic acid (TCA).  Shrimp digestibility was assessed by in vitro pH-stat degree of protein hydrolysis with species-specific enzymes for juvenile Pacific white shrimp Litopenaeus vannamei.