Biofloc technology (BFT) has gained increased interest as a potential low-cost and environmentally friendly method for sustainable aquaculture development. However, the addition of external organic carbon source and the increased aeration needed to promote biofloc formation could be a limitation of this technology. The integration of microalgae on the BFT has the potential to improve the overall efficiency of the method, since microalgae can photosynthetically assimilate nitrogen (ammonia, nitrate, and nitrite) without the requirement of external organic carbon addition. In this work, BFT was supplemented with microalgae Nannochloris sp. isolated and adapted in BFT, while four different C/N ratios (0:1, 5:1, 10:1, and 15:1) were applied in artificial aquaculture wastewater to investigate the nitrogen removal efficiency under reduced external organic carbon addition and to assess the nutritional value of the produced biomass. The present study demonstrated that microalgal integration into a biofloc system led to an efficient ammonia removal and repression of nitrite formation at a lower C/N ratio of 5:1 compared to optimum 15:1 of the series consisting of biofloc only. At the optimum levels of C/N, protein content was not changed (around 37%); however, the incorporation of microalgae into the biofloc resulted in a significant decrease of protein digestibility. Lipid content was also decreased (from approx. 16 to 11%) in the series with incorporated microalgae with less unsaturated fatty acids composition. Microbial diversity was altered in the different series examined, while bacterial and eukaryotic communities participating in the nitrogen cycle and degradation of complex organic compounds were discussed.
