Aging forming process of Chlorella vulgaris growing medium and its cultivation inhibition mechanism

To investigate the possibility of culture medium reuse in large-scale industrial microalgae cultivation for the alleviation of the massive water requirement pressure, the aging forming process of Chlorella vulgaris growing medium was explored and the aged medium's inhibition mechanisms on cell growth were inspected. The results demonstrated that when the medium was continuously reused, the collected maximal C. vulgaris biomass decreased. After the fourth medium reuse, the maximal biomass concentration was only 55 ± 1.1% of that in the fresh medium, which indicated the gradual aging of the medium. Furthermore, the composition variation of the released organic secretions during the culture medium reuse was monitored and the results showed that high concentrations of fatty acids (FAs), including palmitic acid, stearic acid, and small amounts of polysaccharides, were accumulated. Further investigation indicated that the obtained maximal biomass of C. vulgaris has a negative relationship with the manually added initial FA concentrations in the medium which suggested that the accumulated FAs in the medium probably were the main C. vulgaris growth inhibition factor. The inhibition effect of FAs on C. vulgaris was mainly achieved via impacting the cells' photosynthesis efficiencies to destroy the intracellular antioxidant system.