Recycling Nannochloropsis oceanica culture media and growth inhibitors characterization

The sequential recycling of culture medium that had different initial nitrogen concentrations and was treated with an ultrafiltration membrane was investigated to determine the effects of recycled media on the growth of Nannochloropsis oceanica. Algae growth and lipid accumulation were inhibited by the recycled media. The final average ash-free dry weight (AFDW) values of N. oceanica after 12 days of cultivation in the recycled media at initial nitrogen concentrations of 64 and 32 mg L^− 1 were 2.3 and 1.6 g L^− 1, respectively, and these values were 14.8% and 23.8% lower than those of the AFDW of N. oceanica cultured in fresh media (2.7 and 2.1 g L^− 1, respectively). The total lipid contents of the algae cultured in the recycled media for 12 days (37.2 ± 2.9% and 39.7 ± 4.3%, at initial nitrogen concentrations of 64 and 32 mg L^− 1, respectively) were lower than those of the algae cultivated in the fresh media (43.7 ± 3.3% and 47.2 ± 1.2%, respectively). A greater number of cell aggregates were observed in the recycled media culture at low initial nitrogen concentrations than in the fresh media. The removal of potential growth inhibitors in the recycled media by activated carbon resulted in a moderate increase in the AFDW and total lipid content. The recycled media were characterized using Fluorescence Excitation-Emission Matrix (FEEM), Fourier Transform Infrared (FTIR) and size-exclusion HPLC analyses, and the results indicated that polysaccharides humic acid-like and fulvic acid-like substances occurred in the recycled media and likely caused the growth inhibition. Further studies are necessary to confirm the inhibitory effects of these components, explore the mechanisms of inhibition, and identify more efficient methods of removing growth inhibitors from recycled culture media.