Evaluation of Chlamydomonas reinhardtii Microalgae as a Sustainable Feed Supplement and Fishmeal Substitute in Aquaculture with a Positive Impact on Human Nutrition

Currently, there is an urgent need for the growing aquaculture sector to rely on sustainable ingredients which can achieve optimal growth while maintaining fish’s nutritional value (especially omega-3 fatty acid content) for human consumption. Here, C. reinhardtii biomass was substituted for fishmeal in zebrafish (Danio rerio) diets in wild-type and mutant (Casper) strains. Four isonitrogenous (46% cp), isocaloric (19–21 MJ/kg DW) diets were prepared with C. reinhardtii replacing 10% (C10), 20% (C20), and 50% (C50) of the fishmeal component of the diet formulation. Over 8 weeks of feeding trials, the zebrafish showed a significant growth improvement when fed C10, C20, and C50 compared with the control (no C. reinhardtii), with C20 giving the best performance in terms of growth, feed conversion ratio (FCR), and specific growth rate (SGR). Interestingly, C. reinhardtii in the diet increased the levels of linolenic acid (C18:3 n-3) and hexadecatrienoic acid (C16: 4-n-3) (p ≤ 0.05) in the zebrafish. Yellow pigmentation, which was shown to be lutein, was observed in eggs and zebrafish flesh for fish fed a diet containing C. reinhardtii. Moreover, the zebrafish assimilated β-carotene from C. reinhardtii and converted it to vitamin A. Overall, while replacing 20% of fishmen in the zebrafish’s diet with C. reinhardtii biomass offers the best results, replacement with only 10% showed a significant benefit for the zebrafish. Furthermore, replacing fishmeal with 50% C. reinhardtii is still possible and beneficial, and C. reinhardtii whole cells are digestible by zebrafish, thus demonstrating that C. reinhardtii not only has the potential to serve as a feed supplement but that it can also act as a feed substitute once the production cost of microalgae becomes competitive.