Production of Daphnia zooplankton on wastewater-grown algae for sustainable conversion of waste nutrients to fish feed

This study investigates the upcycling of nutrients in anaerobic digestate via algal biomass to zooplankton which is a natural fish feed. There are no published studies, to the authors' knowledge, on the viability of growing zooplankton on digestate-grown algae. Here, the viability of digestate-grown Chlorella sorokiniana as a feed for the large-bodied generalist zooplankter, Daphnia, was tested. It was found that Daphnia fed with digestate-grown C. sorokiniana led to 1.5- to 14-fold greater Daphnia population growth than Daphnia fed with Ankistrodesmus sp., an established feed. A sterol analysis of C. sorokiniana found 4-6 mg/g of the sterol, ergosterol, and nearly double the alpha-linolenic acid content of Ankistrodesmus. Sterols and alpha-linolenic acid are often-limiting nutrients in Daphnia diets. Other factors hypothesized to influence nutrient transfer from algae to Daphnia were also tested, including algal feed concentration, sterol supplementation, and the presence of digestate bacteria in the algal feed. The presence of bacteria and exogenous cholesterol had no significant impacts on Daphnia growth. The higher feed concentration (5 mg C/L) led to 3 times higher Daphnia growth than the low feed concentration (1.5 mg C/L) even though the latter concentration has frequently been used by other researchers. Finally, it was determined that the feed conversion ratio of algae to Daphnia fell in the range of 0.19-0.31 and that trophic transfer of carbon was 25-28% while that of nitrogen was 29-34% in this un-optimized system. These values compare favorably to livestock feed conversion efficiency but additional losses will occur when Daphnia are fed to fish. These results show that cultivation of Daphnia on digestate-grown algae is technically feasible.